1
Introduction and Scope
The present advances in biology are related to progress in genomics,
proteomics, and other “-omics”, including glycomics,
working with a large amount of data regarding human and other genomes,
protein expression, post-translational modifications of proteins,
as well as a great diversity of glycan composition in glycoproteins,
etc. Genomics, proteomics, and glycomics are intimately connected
with each other at various levels. In their exponential growth, they
require new integrative technologies for highly parallel analysis
of genes and proteins of entire organisms.
1
The current experimental techniques applied in these fields have
been reviewed in a number of articles.
1−13
Among them, finding a review on electrochemical (EC) techniques
in proteomics is rather difficult.
14
This
is in contrast to a very large amount of reviews on EC analysis of
nucleic acids and particularly on sensors and arrays applicable in
genomics, which appeared in the recent decade.
15−36
Also, reviews on EC analysis of glycoproteins are rather scarce,
limited mostly to promising EC impedance spectroscopic detection of
lectin-captured glycoproteins.
37−42
Wider application of EC analysis in proteomics and biomedicine was
hindered until recently by the absence of a sensitive EC reaction
applicable to thousands of proteins existing in nature. However, interfacial
electrochemistry of conjugated proteins containing nonprotein redox
centers (such as some metalloproteins) allowing direct (i.e., unmediated)
and reversible electron transfer between electrode and nonprotein
component greatly advanced in recent decades.
43−48
The number of metalloproteins in nature is very large; unfortunately,
only a very small fraction among them was shown to yield such reversible
electrochemistry (see section 3 for details).
To make methods of EC analysis more convenient for application in
biomedicine and in the above “-omics”, advances in both
label-free and label-based EC methods of proteins and carbohydrate
components of glycoproteins analysis are desirable.
In this
Review, we wish to show that in recent years significant
progress was done in the EC analysis of practically all proteins,
based on the electroactivity of amino acid (aa) residues in proteins.
Also, electrochemistry of polysaccharides, oligosaccharides, and glycoproteins
greatly advanced in creating important steps for its larger application
in the glycoprotein research. In recent decades, a great effort was
devoted to the discovery and application of biomarkers for analysis
of different diseases, including cancer.
49−53
In the following paragraphs, special attention will
be paid (i) to intrinsic electroactivity of peptides and proteins,
including the sensitivity to changes in protein 3D structures (sections 4–6), as well
as to
recent advances in EC investigations of DNA–protein interactions
(section 7), (ii) to intrinsic electroactivity
of glycans and polysaccharides, advances in EC detection of lectin–glycoprotein
interactions, and introduction of electroactive labels to polysaccharides
and glycans (section 8), and finally (iii)
to EC detection of protein biomarkers, based predominantly on application
of antibodies in immunoassays, nucleic acid and peptide aptamers for
construction of aptasensors, and lectin biosensors for detection of
glycoprotein biomarkers (section 9).
1.1
Intrinsic Electroactivity of Proteins
Since the beginning
of the 1970s, EC analysis of proteins focused
on reversible processes of nonprotein components in conjugated proteins.
This very interesting electrochemistry was reviewed in numerous articles
43−48
and will be here only briefly mentioned in connection to proteins
involved in the DNA repair (section 7). At
the beginning of the 1980s, it was shown that tyrosine (Tyr) and tryptophan
(Trp) residues in proteins produced voltammetric oxidation signals
at carbon electrodes.
54−56
In the first decade after this discovery, the oxidation
signals of proteins exhibited only low sensitivity, but later by using
different carbon electrodes and EC techniques, these signals became
more useful tools in electrochemical protein analysis (section 4) and were applied
in biomedical research. Recently,
a simple label-free chronopotentiometric stripping (CPS) electrocatalytic
method has been introduced (section 5), allowing
the determination of practically any protein at low concentration,
as well as recognition of changes in the protein structures (section 5.3), including
those resulting from a single aa
exchange (point mutations). The protein structure-sensitive analysis
requires very fast potential changes (taking place at highly negative
current densities), which can be hardly obtained using the usual voltammetric
techniques. Special properties of CPS in relation to protein analysis
are discussed in sections 5.1–5.3. For protein structure-sensitive analysis, thiol-modified
liquid mercury or solid amalgam electrodes are convenient (section 5.4). CPS appeared
particularly useful in the analysis
of proteins important in biomedicine (section 6), including tumor suppressor p53 protein
(section 6.2) and its sequence-specific interaction with DNA (section 7.5).
1.2
DNA–Protein Interactions
Until
recently, EC methods were little used in DNA–protein interaction
studies and were not included among the methods listed in handbooks
on DNA–protein interaction analysis.
57
In section 7, we review new EC methods dealing
with DNA–protein interactions, which play significant roles
in nature (e.g., sequence-specific transcription factors binding to
DNA). These methods are based on different principles, and some of
them show significant advantages over the methods commonly used in
DNA–protein interaction studies. To our knowledge, section 7 represents the first
comprehensive review on EC
analysis of DNA–protein interactions. This section does not
cover literature on DNA or RNA aptamers, which are mentioned only
in relation to biomarkers in section 9.
1.3
Analysis of Glycoproteins
It is estimated
that 70% of cytosolic proteins and 80% of membrane-bound proteins
are glycosylated. It is thus very important to analyze glycans for
better understanding of their role in the cell physiology and pathology
and to develop novel and robust methods applicable in diagnostics.
EC analysis of glycans is gaining increasing attention, providing
an exceptionally low limit of detections (LODs) and in some cases
also a label-free format of analysis. Moreover, EC analysis of glycans
can be performed on various intact cell lines. Section 8 begins with a short historical
overview on the development
of EC methods of glycan analysis. It then continues with methods relying
on a glycan release from glycoproteins. The most recent schemes, where
EC detection platform can be applied for a direct glycoprofiling of
glycoproteins and even intact cells (section 8.3.4), are discussed. A novel method
of catalytic hydrogen evolution
reaction in glucosamine-containing carbohydrates and selective modification
of saccharides by osmium Os(VI) complexes has a potential for detection
of glycoproteins in the future. Lectins, natural interacting partners
of glycans, after being integrated into EC platforms of detection
could analyze intact glycoproteins down to a single molecule level.
Two EC detection platforms are discussed including label-free and
label-based approaches, and some applications of such devices are
provided.
1.4
Detection of Protein Biomarkers
Many
diseases, including cancer, could be efficiently cured if early diagnosed.
The diagnostics, among others, relies on detection of protein biomarkers,
which circulate at elevated concentrations in body fluids sometimes
even before any other symptoms appear. This makes early diagnostics
extremely important not only because it saves and increases the quality
of life, but also because it greatly decreases financial cost associated
with the treatment of diseases in advanced stages.
Protein biomarkers
can be detected using EC methods, offering such advantages as low
cost, short time of analysis, and excellent sensitivity. In section 9, we describe
a recent progress in development of
strategies for ultrasensitive determination of protein biomarkers,
including protein labeling. Many authors still use purified, commercially
available samples, which can be important to demonstrate a proof of
concept or to develop a novel strategy, but it is very important not
to forget about application of real samples obtained from patients’
sera and other body fluids, where protein biomarkers can be often
found at very low concentrations present in excess of a huge amount
of interfering species.
The most current papers report on the
development of immunoassays
based on surface-immobilized antibodies (in most cases directly at
the electrode) for binding of target biomarker. This binding is usually
followed by introduction of labeled secondary antibody and by monitoring
the resulting signal, which is greatly amplified through action of
enzymes or nanoparticles (section 9.2). In
the last years, synthetic aptamers (either nucleic acid- or peptide-based)
with high affinity to various proteins are being more often used,
having the same function as an immobilized antibody. In such a case,
the aptamer-based platform is referred to as the aptasensor, instead
of antibody-based immunoassays (or immunosensors) (sections 9.3 and 9.4). Last, if
the
protein biomarker is a glycoprotein (section 9.5), it can be advantageous to specifically
capture its glycan part
using lectins. Such lectin biosensors were constructed not only for
detection of protein molecules, but also for identification of whole
cancer cells.
Because we have touched on the topic of biosensors,
we should make
a short comment regarding correct terminology. Considering today’s
literature on DNA or protein biosensors, as compared to the literature
on sensors in general,
58
one should, strictly
speaking, refer to them more correctly as “sensing systems”
or “bioelectrochemical assays”, because a true biosensor
acquires information continuously, while a sensing system may do that
in discrete steps.
58
In this Review, we
will use the more widespread, although not terminologically perfect,
term “biosensor”, with its more relaxed definition stating
that biosensors are devices that combine
59
or integrate
60
a biochemical recognition
element with a signal conversion unit (transducer). This confusion
in terminology was discussed in detail in our previous review.
34
2
History of Protein Electrochemistry
Proteins were the first biomacromolecules that were analyzed by
EC methods. In 1930, that is, only 8 years after J. Heyrovský’s
invention of polarography, Heyrovský and Babička published
their paper showing that albumins, in the presence of ammonium ions,
produced the direct current (dc) polarographic “presodium wave”
(Figure 1), for which catalytic evolution of
hydrogen was responsible.
61
Two years later,
Herles and Vančura
62
showed that
the presodium wave was produced by various human body fluids, including
blood serum and urine. Their work started several years earlier when
J. Heyrovský gave a chance to young medical doctors to study
polarographic activity of various human tissue liquids using the polarographic
instrument in his laboratory. At that time, polarographs were not
commercially available, and the young M.D.’s utilized their
unique chance very efficiently. They described the body fluid-produced
“presodium wave” as a cathodic wave occurring at potentials
little more positive than the polarographic reduction wave of sodium
ions and tentatively assigned this wave to proteins.
Later,
the “presodium wave” was characterized by
Brdička
63
in greater detail. He
showed that the presence of ammonium ion was not essential for the
reaction and concluded that the “presodium wave” was
due to −SH groups in proteins similarly to his previously discovered
electrocatalytic “double wave”, produced by thiols in
the presence of cobalt ions.
64
Comparison
of the two polarographic responses of proteins showed that the “presodium
wave” is much higher than the Brdička’s “double
wave” and the former wave does not require the presence of
cobalt ions in the electrolyte.
65
It was
concluded from a number of later studies (e.g., refs (66−69)) that the “presodium
wave” did not depend specifically
on the −SH or another group in the catalyst molecule, but rather
on its structure, adsorbability, and other factors including solution
composition, the electrode potential, and the rate of its change.
In its 80 year history, the polarographic presodium wave was only
occasionally utilized in the analysis of proteins. The dc polarographic
version of this wave was too close to the background discharge and
difficult to measure (Figure 1). The indentations
of proteins obtained with oscillographic polarography (alternating
current (ac) cyclic chronopotentiometry or cyclic reciprocal derivative
chronopotentiometry
70
according to the
present nomenclature) were too close to the shining end point at negative
potentials,
71
and their evaluation was
even more difficult than that of the dc polarographic presodium waves.
Figure 1
Polarographic
catalytic waves of human serum. (1) Pure supporting
electrolyte, 0.1 M ammonia/ammonium chloride buffer; (2) the “presodium”
catalytic wave, 400-times diluted human serum (A) in 0.1 M ammonia/ammonium
chloride; (3) two-step reduction of Co(III), 1 mM Co(NH3)6Cl3 in 0.1 M ammonia/ammonium
chloride; (4)
the catalytic double-wave in Brdička solution, 1 mM Co(NH3)6Cl3 + 400-times diluted
human serum
(A′) in 0.1 M ammonia/ammonium chloride; recorded from 0 V
vs mercury pool, 200 mV/abscissa. Adapted with permission from ref (64). Copyright
1933 Collection
of Czechoslovak Chemical Communications.
In contrast, Brdička’s double wave (Figure 1) was intensively applied in biochemistry,
for several
decades around the middle of the 20th century,
72−75
particularly because of expected
application of Brdička’s catalytic response in cancer
diagnostics. After yielding some interesting data, the specificity
of Brdička’s catalytic response turned out to be insufficient
for cancer diagnostics, and the interest in Brdička’s
catalytic response declined. In the 1970s, the attention of electrochemists
turned to the electrochemistry of proteins containing a redox-active
center (reviewed in refs (76−78)), and the outlooks
for application of EC methods as tools for analysis of the majority
of proteins important in molecular biology and biomedicine appeared
grim.
3
Reversible Electrochemistry of Nonprotein Components
in Conjugated Proteins
Among conjugated proteins (such as
lipoproteins, glycoproteins,
etc.), some contain one or more nonprotein redox centers, essential
for direct electron transfer (DET) between the protein and its natural
acceptor or donor (e.g., metalloproteins). Under conditions when the
redox center is sufficiently close to the electrode, DET between this
center and the electrode may take place (reviewed in refs (79−83)). The first papers
on DET were published by the end of the 1970s.
84−86
Pioneering studies described interaction of cytochrome c with either
tin-doped indium oxide,
86
4,4′-bipyridine-modified
gold electrode,
84
or mercury electrode.
85
At present, this specialized branch of protein
electrochemistry is well established as documented by numerous book
chapters
46,76−78,87−91
and reviews
79,80,92−98
and a large number of original papers.
The DET rate from donor
to acceptor is dependent on a number of
factors, including orientation of the protein molecule at the surface,
temperature, distance from the surface, reaction Gibbs energy, reorganization
energy, etc. More efficient DET can be obtained by introducing a redox
center into more hydrophobic regions far from aqueous environment,
thus lowering the reorganization energy. For example, the reorganization
energy for the solvent-accessible copper atom in redox pair Cu(1,10-phenantroline)2
2+/+ is 1.7 eV higher than that for copper embedded
in the small protein azurin.
99
Experiments
with iron yielded similar results.
100
Strict requirements for DET in proteins greatly limit application
of DET phenomenon for analysis of proteins important in biomedicine.
For example, numerous oncoproteins, such as tumor suppressor proteins
p53,
101,102
p63,
103
and p73,
104
contain metal ions playing important biological
roles in their large molecules,
101−104
but they do not yield any DET.
However, such proteins are able to produce CPS responses based on
catalytic hydrogen evolution reaction,
105
sensitively reflecting changes in the protein structures as well
as the presence and absence of metal ions in the molecules (section 5.3).
In this section, we shall only briefly
summarize the electrochemistry
of some conjugated (DNA repair) proteins containing [4Fe–4S]
clusters, which have been utilized in EC studies of DNA–protein
interactions (section 7). Such clusters were
found in glycosylases involved in base excision repair (e.g., Endonuclease
III, Endo III) and in a nucleotide excision repair (e.g., Xeroderma pigmentosum factor
D, XPD). These enzymes
are responsible for searching the genome for damaged bases/nucleotides
and for enzymatic catalysis of their excision.
106
After the damaged site is located (in a vast amount of
intact bases), base excision repair enzymes flip their substrate into
the protein active site, and catalyze rupture of the N-glycosidic bond between the
damaged base and the DNA sugar–phosphate
backbone. The searching process was studied in detail, but some aspects
of this process in vivo have not been yet fully understood.
107−112
Among glycosylases of this type, Endo III
113
and a structurally similar MutY protein
114
were identified. Endo III removes oxidized pyrimidines
from DNA,
107,115−122
while MutY removes adenine from 8-oxo-guanine:adenine mispaired
bases.
123−133
Crystal structures are available for free and DNA-bound EndoIII
and MutY.
114,119,120,134,135
In these structures, a [4Fe–4S] cluster is ligated by a cysteine
(Cys) motif (C–X6–C–X2–C–X5–C). The [4Fe–4S] cluster is required for an
enzyme acitivity and DNA binding, but not for protein folding and
thermal stability of the protein.
136−138
Figure 2
(A) Schematic representation
of electrochemistry for endonuclease
III (Endo III) on highly oriented pyrolytic graphite electrode (HOPGE)
with and without modification with DNA. (B) Cyclic (left, 50 mV/s
scan rate) and square wave voltammograms (right, 15 Hz) of 50 μM
Endo III in 20 mM Na phosphate, 100 mM NaCl, 1 mM EDTA, 20% glycerol,
pH 7.5. The top two panels show electrochemical responses of Endo
III at a HOPGE modified with the sequence pyrene-(CH2)4-Pi-5′-AGT ACA GTC ATC GCG-3′
plus complement. Cyclic voltammograms of a HOPGE modified with DNA
featuring an abasic site are in red (top left), where the abasic position
corresponds to the complement of the italicized base. The bottom two
panels show electrochemical responses of Endo III on a bare HOPGE.
All runs were taken using the inverted drop cell electrode configuration
vs Ag/AgCl reference and Pt auxiliary electrode. (C) Illustration
of the potentials vs normal hydrogen electrode (NHE) of the couples
of Endo III in the presence and absence of DNA. These values are obtained
from SWV on a HOPGE and are averages of at least four trials each.
Adapted with permission from ref (143). Copyright 2006 American Chemical Society.
Earlier studies had shown that
the [4Fe–4S]2+ cluster is not readily electro-oxidized
nor reduced within the physiological
potential range.
113,139−142
Gorodetski et al.
143
investigated Endo
III at a highly oriented pyrolytic graphite electrode (HOPGE) using
cyclic voltammetry (CV) and square wave voltammetry (SWV). On a bare
HOPGE, they observed an irreversible anodic peak at 250 ± 30
mV against a normal hydrogen electrode, while at the DNA-modified
HOPGE, a quasi-reversible redox couple was observed at 20 ± 10
mV (Figure 2). This peak reflected a DNA-bound
protein redox process, and it did not result from Endo III interaction
with DNA containing an abasic site, which was unable to take part
in the DNA-mediated charge transport.
34
Interaction of the above base excision repair and the nucleotide
excision repair of DNA by proteins
144
will
be discussed in section 7.2.
4
Protein Oxidation at Carbon and Other Solid
Electrodes
4.1
Free Amino Acids
More than 30 years
ago, it was shown that free Cys, histidine (His), methionine (Met),
Tyr, and Trp are oxidized at carbon electrodes.
56,145
Other aa’s did not produce any oxidation signal at carbon
electrodes in a pH range 4–10.
54,55,146
Oxidation of the Tyr and Trp occurred at positive
potentials far from zero. Application of new carbon-based nanomaterials
for biosensing has attracted great attention (reviewed in refs (147−149)). Recently,
it has been shown that a glassy
carbon electrode (GCE) modified with multiwalled carbon nanotubes
and gold nanorods allowed oxidation of free l-Cys at a very
low anodic potential (0 V vs Ag/AgCl).
150
A modified GCE and boron doped diamond electrode were used also
in the analysis of Cys, Tyr, and Met.
151,152
Shortly after
the discovery of the electro-oxidation of some free aa’s,
56,145
it was found that Tyr and Trp residues yield oxidation signals also
in proteins.
54−56
4.2
Peptides and Proteins
Protein electro-oxidation
attracted greater attention in recent decades.
146,153−158
SWV or CPS
154,159
with efficient baseline correction
yielded Tyr and Trp peaks that were better developed and allowed the
determination of much lower peptide and protein concentrations than
linear sweep voltammetry.
146
Moreover,
it was found that proteins are strongly adsorbed at carbon electrodes,
which made it possible to prepare protein-modified electrodes without
covalent binding of the protein to the surface.
154
Using adsorptive transfer stripping voltammetry, microliter
volumes of proteins were sufficient for the analysis at carbon electrodes.
160
In contrast to metal electrodes, such as gold
and mercury, at which thiol self-assembled monolayers (SAMs) can be
easily formed,
161
such SAMs do not form
at carbon electrodes. However, tightly packed structures of DNA functionalized
with pyrene at HOPGE were shown,
162
and
reduction of disulfide bonds incorporated in the DNA backbone was
demonstrated.
163
This system has not been
so far widely applied in protein electrochemistry.
Figure 3
Schemes of EC oxidation
of tyrosine (Tyr) and tryptophan (Trp).
Adapted with permission from ref (164). Copyright 2013 Wiley-VCH Verlag GmbH&Co.
Oxidation schemes for Tyr and
Trp proposed about three decades
ago (Figure 3) are still used in the literature.
164
The combination of EC oxidation of peptides
and proteins with mass spectrometry (MS) recently revealed a specific
cleavage of the peptide bond at the C-terminal side of Trp and Tyr
residues.
165,166
A set of Tyr and Trp-containing
tripeptides (e.g., LYL, EYE, LWL) was studied, including the effect
of adjacent aa residues. It was found that the ratio of oxidation
and cleavage products is sequence-dependent and that the secondary
chemical reactions occurring after the initial oxidation step are
influenced by the adjacent aa residues.
167
Control of the oxidation potential appears critical for avoiding
dimer formation of Tyr and increasing hydroxylation of Trp. Working
at pH values 1.9–3.1 resulted in optimal cleavage yields, but
at basic pH’s no or a little cleavage took place. In proteomic
experiments, usually enzymatic or chemical protein cleavage is used.
Electrochemistry may offer a fast and simple instrumental alternative
to these cleavage methods. The above-mentioned studies revealed complicated
reaction schemes, but the primary step in Trp oxidation was in agreement
with that proposed a long time ago (Figure 3). Using GCE, Enache and Brett
168
investigated
the pathways of EC oxidation of Trp and other indole-containing compounds
with a substituent at C3 position. They found that oxidation of Trp
occurs at the C2 position of the pyrrole ring followed by the hydroxylation
at the C7 position of the indole benzene moiety in an irreversible
pH-dependent process.
By the end of the 20th century, it was
believed that electroactivity
of aa residues in proteins was limited to oxidation of Tyr and Trp.
Recently, oxidation of His residues in a protein was reported at highly
positive potentials on GCE.
158,169
His oxidation peak
was not observed in His-containing angiotensin peptides at a basal
plane pyrolytic graphite,
170
but it was
not excluded that His oxidation may occur in proteins and peptides
at GCE and other electrodes. Investigations of electroactivity of
His residues in proteins are particularly interesting, because His-tags
(usually short chains of six His residues) are frequently used to
facilitate recombinant protein isolation.
171
Moreover, attachment of His-tagged proteins on electrodes received
recently special attention in relation to forming well-organized protein
layers at electrified interfaces (reviewed in ref (172)). Very recently, it has
been shown that using oxidation peaks of Tyr and Trp, subpicomole
amounts of a potential cancer biomarker, protein AGR2,
173
can be detected at carbon electrodes.
174
The N-terminal His-tagged and non-His-tag forms
of this protein were studied, and it was found that only the His-tagged
form yielded a peak of histidine. Similar results were obtained with
other His-tag containing and not-containing proteins, such as α-synuclein
or cytochrom b5. It was concluded that His-tags in proteins influence
the protein adsorption and orientation at the electrode surface and
that the appearance of a His oxidation peak at carbon electrodes depends
on many factors, including the number of His residues and their accessibility
in the surface-attached protein molecule.
Figure 4
(A) Oxidation of Trp-
and Tyr-containing peptides on carbon paste
electrodes. (a) Differential pulse voltammograms and (b) chronopotentiograms
for (A1) Tyr- and Trp-containing luteinizing hormone releasing hormone,
(A2) Tyr-containing neurotensin, and (A3) Trp-containing bombesin.
10 nM peptide was adsorbed for 5 min at accumulation potential of
0.1 V followed by chronopotentiogram or DP voltammogram recording.
CPS: I
str 5 μA; DPV scan rate, 5
mV/s. Y refers to Tyr and W to Trp residues. Adapted with permission
from ref (159). Copyright
1996 Elsevier. (B) Oxidation peak of 2 μM human serum albumin
(HSA) denatured in 8 M urea at glassy carbon electrode. (C) Dependences
of square wave voltametric peak heights (−■−)
and changes in fluorescence emission at 334 nm (− –○– −)
on urea concentration. 1 μM HSA was incubated overnight with
different urea concentrations (indicated in the figure) at 4 °C.
Oxidation peak height of HSA denatured in 8 M urea was taken as 1.
In the fluorescence measurements, intensity at 334 nm produced by
1 μM HSA incubated in the absence of urea was taken as 1. Adapted
with permission from ref (218). Copyright 2012 Elsevier.
Oxidation processes of free Cys were reviewed,
153
and oxidation of Cys in short peptides was
reported.
153,175,176
To our knowledge, oxidation
of Cys residues in large proteins at carbon electrodes was however
not shown.
153
Nevertheless, Suprun et al.
164
recently considered oxidation of Cys residues
in several proteins, including bovine serum albumin (BSA) and human
serum albumin (HSA), without bringing experimental evidence about
oxidation of Cys residues in the complex protein molecules. Oxidation
of Met in dipeptides was observed at boron doped diamond electrode
at highly positive potentials, which depended on aa sequences.
177
Also, oxidation of Met residues in proteins
at carbon electrodes was reported,
169,178
but unambiguous
experimental evidence for this oxidation is still needed. With peptides,
well-separated peaks of Tyr and Trp were obtained (Figure 4A). Also, a relatively
small protein, lysozyme (containing
3 Tyr and 6 Trp residues), produced two separated peaks.
146,154
In contrast, larger proteins containing both residue types produced
mostly only a single peak (Figure 4B). Nitration
of Tyr resulted in shifting of the Tyr oxidation peak to more positive
potentials and formation of a reduction peak at ∼0.65 V. Also,
nitrated BSA produced a peak at ∼0.75 V, which made it possible
to discriminate it from unmodified native BSA.
164
Ricin (RCA-60, ∼60 kDa), a deadly toxic glycoprotein,
179,180
was recently analyzed at carbon, Au, and Pt electrodes.
181
The metal electrodes were found inconvenient
for this analysis because formation of Pt and Au oxides occurred at
potentials where oxidation of RCA-60 took place. Using CV, SWV, or
differential pulse voltammetry (DPV) and GCE, 200, or 100 μM
RCA-60 produced in a wide pH range two peaks, probably due to oxidation
of Tyr and Trp residues. Practical RCA-60 determination would require
a combination of EC analysis with some separation technique, involving,
for example, specific antibodies. Considering that RCA-60 contains
Cys, Lys, His, and a large number of Arg residues (almost 5%), CPS
electrocatalytic peak H (see section 5.2) would
probably offer better sensitivity. Using this peak, it might be also
possible to recognize reduction of the disulfide bond in RCA-60 (section 5.3), which
results in separation of chains A and
B and loss of the protein toxicity.
182
EC
analysis might be also useful for simple detection of glycan in the
RCA-60 glycoprotein, using lectins and/or chemical modification (see
sections 8.5 and 8.6).
Recently, a signal enhancing system was developed to increase
the
aa and protein irreversible oxidation signals.
183−186
This system relied on the electrocatalytic oxidation of Tyr mediated
by phenoxazine or osmium bipyridine complexes (Figure 5). Using indium tin oxide (ITO)
as the working electrode,
detection of protein oxidative damage
187
and phosphorylation of the Tyr residues in proteins,
186
as well as ligand-protein binding
184
and protein-conformation changes, were demonstrated.
183
Figure 5
(A) Oxidation of tyrosine (Tyr) with signal enhancement.
ITO, indium
tin oxide electrode; Bipy, 2,2′-bipyridine; dppz, dipyrido
[3,2-a:20,30-c] phenazine. (B) Cyclic
voltammograms of (a) 5 μM Os(bpy)2dppz, (b) 2 mM
Tyr, and (c) 5 μM Os(bpy)2dppz and 2 mM Tyr. Working
electrode: ITO. Reference: Ag/AgCl/3 M KCl. Supporting electrolyte:
100 mM sodium phosphate, pH 7.3. Scan rate: 30 mV/s. Adapted with
permission from ref (185) Copyright 2012 Elsevier.
Post-translation modification of proteins and particularly
phosphorylation
by kinases play important roles in many biological processes, such
as cell cycle, differentiation, growth, and in apoptosis.
188
During the phosphorylation, the γ-phosphoryl
group of adenosine triphosphate (ATP) can be transferred to residues
of serine, threonine, or Tyr.
189
Abnormal
protein phosphorylation is involved in many diseases, including cancer
and neurodegenerative diseases.
190
Different
methods have been developed for studies of kinase-catalyzed protein
phosphorylation (e.g., refs (191−193)), including relatively simple and inexpensive
EC methods.
164,185,186,194−200
First papers showed that phosphorylation of peptides and proteins
resulted in a decrease of Tyr oxidation peak using screen-printed
electrode.
164,201
The difference between oxidation
peaks of phosphorylated and nonphosphorylated Tyr was recently used
by Li et al.
202
to investigate activity
and inhibition of protein kinase at a graphene-modified GCE. Graphene
with its unique one atom thick structure, large specific surface area,
excellent electric and thermal conductivity, and high mobility of
charge carriers attracted great attention in recent years.
203−205
However, skeptical opinion appeared regarding graphene application
in various biosensors.
206
An enhancement
of Tyr peak Y through electrocatalytic oxidation
reaction was observed at the graphene electrode.
202
At this electrode, phosphorylated Tyr was inactive. Tyr
oxidation peak Y was thus used to assay the Src kinase activity using
peptide YIYGSFK as a substrate (phosphorylated predominantly at its
N-terminus). Peak Y decreased with the logarithm of the kinase concentration
from 0.26 to 33.79 nM, with a LOD of 0.087 nM, which was better than
that reported previously.
202
Using the
same method, the kinase inhibition by low molecular weight PP2 inhibitor
was followed, showing an increase of peak Y with increasing PP2 inhibitor
concentration. The reported results are very interesting, but a biochemist
might be not fully satisfied with the absence of control experiments
in both the dependence of EC signal on the kinase and the PP2 inhibitor
concentration. The enzymatic reaction was performed in a complex mixture
containing the protein Src kinase and, in addition, different ions
as well as 1 mM dithiothreitol and 50% glycerol. Using the same mixture
with the inactivated kinase might be a proper control. More information
on EC analysis of kinase activity can be found in section 9.1.1, describing the label-based
approach.
4.2.1
Folded and Unfolded/Denatured Proteins
For several
decades, it was unclear whether the protein oxidation
peaks can be used to distinguish native from denatured proteins. Already
in 1985 oxidation peaks of tobacco mosaic virus and its isolated protein
in native and denatured forms were reported.
207
Urea-denatured viral protein (at a concentration of 100 μg/mL)
produced higher oxidation DPV peaks than the native protein. Urea
was, however, not removed from the sample of the denatured protein
prior to the DPV measurement. Later, differences in oxidation peak
heights between native and denatured proteins were reported at various
carbon electrodes in some studies, while other studies showed that
oxidation peaks at carbon electrodes reflected poorly the changes
in protein structures resulting from protein denaturation
208
or a single aa exchange.
105
Generally, oxidation responses of proteins at carbon electrodes
were much less sensitive to changes in protein structures
105,208
and redox states
209
than reduction signals
at bare and thiol-modified mercury electrodes (see section 5.3 and 5.4). Nevertheless,
these protein oxidation signals were used in different sensors (reviewed
in ref (210)) and for
the development of various biomarker sensors (reviewed in refs (211) and (212)).
Over the recent
years, various types of carbon electrodes have emerged,
210,213−217
opening the door for a wider application of electroanalysis to diverse
targets,
76
including the biomacromolecules.
Pyrolytic graphite,
218
graphene,
202
glassy carbon,
218
as well as conductive diamond electrodes
219
can be considered as convenient electrode materials for protein
analysis.
76
Polishing with abrasives can
modify properties of carbon electrodes as it results in the formation
of oxide groups and hydrophilic surfaces at these electrodes. In edge
plane pyrolytic graphite electrodes (EPGE), edge planes are at the
electrode surface and can be easily oxidized. It appears that EPGE
may now represent an ideal material for electroanalytical purposes.
214
Recently, it has been shown that by using these
electrodes, the course of protein denaturation can be traced,
218
yielding the results in good agreement with
fluorescence emission at 334 nm (Figure 4C).
Testing other carbon electrodes for their ability to distinguish native
from denatured forms of HSA
218
or α-2-macroglobulin
220
showed GCE as a suitable material for this
purpose (albeit worse than EPGE). Similarly, using boron doped diamond
electrode, native and denatured BSA could be distinguished.
219,220
Topal et al.
220
showed oxidation signals
of native and denatured macroglobulin at a gold electrode close to
0.7 V. However, a basal plane carbon electrode displayed poor ability
to distinguish between these forms of HSA.
221
Using EPGE, a number of proteins were tested, showing much higher
oxidation peaks in denatured than in their native forms. In good agreement
with the above results, treatment of natively unfolded α-synuclein
protein
222
with denaturing agents resulted
only in a very small change in the oxidation peak.
218
Treatment of this almost structureless protein with a denaturing
agent can result only in small changes in accessibility of their aa
residues (including accessibility of the Tyr residues responsible
for the protein electro-oxidation). Oxidation peaks of proteins were
shown to be able to follow aggregation (resulting in burying of aa
residues) of amyloid proteins such as amyloid β-peptides
223−225
and α-synuclein
156,157,218,226
in relation to their roles in
Alzheimer’s or Parkinson’s diseases, respectively (see
section 6.1). Analysis of another type of post-translational
modification, glycosylation by glycan assays, is described in section 8.
5
Label-Free
Electrocatalysis in Peptides and
Proteins
According to the review by Herzog and Arrigan
153
in 2007, the electroactivity of nonconjugated
proteins
appeared rather poor. The authors deliberately omitted studies at
mercury electrodes due to their claim that these electrodes were disappearing
from laboratory benches and were generally unsuited for use in miniaturized
and/or out-of-laboratory applications. Almost at the same time, it
was shown that proteins yield chronopotentiometric signals at mercury
and solid amalgam electrodes
155,227−229
in a wide pH range.
208,209,230−233
These signals appeared at highly negative potentials but were still
well-separated from the background. They were not observed with mercury-free
electrodes, suggesting that mercury electrodes can be especially useful
in protein analysis.
5.1
Mercury-Containing Electrodes
and Chronopotentiometry
in Protein Analysis
5.1.1
Mercury Electrodes
The objection
that mercury electrodes are unsuited for use in miniaturized and/or
out-of-laboratory applications
153
does
not seem to have a solid ground because (a) miniaturization of Hg
electrode was reported
234−236
and (b) solid amalgam electrodes
(SAEs) were miniaturized and SAE chips for protein analysis were developed.
227,236
Moreover, regarding the electrode choice for protein analysis, other
criteria should be considered as well. For example, the accessible
potential window of a liquid mercury electrode and SAEs (roughly between
0 and –2 V against a saturated calomel electrode,
SCE) greatly differs from that of most of the solid electrodes, such
as carbon, gold, platinum, and silver (shifted by about 1 V in a positive
direction as compared to Hg electrodes), making thus solid electrodes
better suitable for studies of (irreversible) oxidation processes.
However, Hg electrodes are more suited for studying reduction processes
and particularly for processes involving catalytic hydrogen evolution
in proteins, which has been observed solely with Hg electrodes (see
below). An atomically smooth surface of liquid mercury makes it possible
to prepare pinhole-free monolayers of thiolated DNAs
237
and of other thiols
161
to obtain
chemically modified electrodes for protein analysis.
105,228,238−240
Smooth surfaces can be prepared also by forming a miniature liquid
Hg meniscus at SAE. Moreover, strong hydrophobicity and other properties
of Hg electrodes differ from most of the solid electrodes predominantly
used in EC analyses. With excellent reproducibility of their clean
surfaces, liquid mercury electrodes still remain attractive for research
purposes. It can be concluded from the recent development of electrochemistry
of nonconjugated proteins that mercury electrodes will not soon disappear
from laboratory benches as they open the door to new approaches in
the EC analysis of biomacromolecules and particularly of nucleic acids,
34,229,241,242
proteins,
155,229,243
and carbohydrates.
229,244−246
In the following paragraph, it will be shown that the combination
of bare and chemically modified mercury electrodes with constant current
chronopotentiometric stripping and electrocatalysis resulted in new
methods of EC protein analysis applicable in biomedicine.
5.1.2
Chronopotentiometry
Contrary to
voltammetry (in which current is recorded as a function of the potential
applied to the working electrode), in chronopotentiometry (galvanostatic
or controlled current methods) the electrode potential is measured
as a function of time upon applying a current perturbation to the
working electrode.
247
Controlled-current
methods were introduced a long time ago,
248−250
and different current versus time programs were used for studying
biomolecules.
251
At present, the stripping
mode of constant-current chronopotentiometric stripping (CPS) is gaining
ground.
155
The raw chronopotentiometric
response (E–t curve) is of
limited analytical interest, but the derivative of the E–t curve yielding a peak-shaped
plot (dE/dt)−1 as a function of the potential E (Figure 6C,D) is calculated and
usually used in protein analysis. In CPS, the applied current imposes
a rate of charge flow across the electrode|solution interface, with
total charge increasing linearly with time.
Figure 6
(A) Schematic representation
of the rate of potential changes in
chronopotentiometry (CP) at two different intensities as compared
to voltammetry. In linear sweep voltammetry (LSV), the scan rate (chosen
by an experimenter) is constant throughout the whole voltammogram
recording. However, in CP the rate of potential changes is influenced
by the current density. At constant electrode size, this density is
determined by polarizing current intensity (I, chosen
by an experimenter). In the absence of the electrode process, the
potential changes very rapidly (e.g., 390 V/s at I = −50 μA), but it gets much slower
in a narrow potential
range where the electrode process (e.g., proton reduction and hydrogen
evolution) is taking place. (B–D) In protein analysis, both
native (nat, black) and denatured (den, green or red) proteins are
firmly attached to the Hg electrode surface, and prolonged exposure
of native folded protein to negative potentials (at low scan rates
or I intensities) may result in its denaturation,
indicated by almost the same (B) LSV or (C) CP responses. (D) At high
current intensity in CP, for example, at I = −50
μA, fast potential changes (390 V/s) prevent protein from the
denaturation at the negatively charged electrode surface, as indicated
by a relatively small CP response of native (black) protein and a
very large response of the denatured (red) protein.
As compared to voltammetric methods, CPS offers
some advantages
in protein analysis.
252
For a given EC
process, such as catalytic hydrogen evolution, chronopotentiometry
yields better resolved peaks and lower background levels (baseline
correction is usually not required) (Figure 6C,D), and polarization of the electrode
may proceed in a very short
time. Some part of these advantages may be due to the differential
nature of the signal, but the main advantage lies in the way of the
current perturbation itself. As mentioned above, the rate of the overall
process in CPS is imposed by the value of the applied current and
very fast potential changes can be reached (Figure 6), while in voltammetries the
constant scan rate is used.
Using CPS peak H in protein analysis (Figure 6D), it is important that the electrode
potential shifts slower during
an electrode process than in its absence, when potential changes can
reach extremely high rates (e.g., about 390 V/s at stripping current, I
str −50 μA at 0.4 mm2 Hg electrode size, corresponding to a current density of −12.5
mA/cm2, Figure 6A). This feature
is critical in protein structure analysis (sections 5.3 and 6), as well as in DNA–protein
interaction studies (section 7.5). CPS peak
H of proteins appears at highly negative potentials, such as −1.8
V (against Ag/AgCl electrode). To reach this potential, the surface-attached
protein is exposed to the electric field effects at negative potentials
causing unfolding/denaturation of the native protein
238
or dissociation/disintegration of the DNA–protein
complex.
240
It has been shown that such
damage to the surface-attached biomacromolecules depends strongly
on the time of their exposure to negative potentials and can be avoided
at highly negative I
str intensities (sections 5.3 and 7.5), inducing very high rates of potential changes
and thus very short exposure times. Such negative I
str intensities (current densities) can
be hardly used in low electron-yield protein electrode processes (e.g.,
in oxidation of protein Tyr or Trp residues, section 4), because the CPS signals decrease
with increasing −I
str intensities, and at highly negative I
str intensities, they can disappear or become
too low. In contrast, in high electron-yield electrocatalytic processes,
such as those involving catalytic hydrogen evolution reaction, high
−I
str intensities can be applied,
and a well-developed peak H can be obtained using picomole amounts
of proteins.
230,231,238,239,253,254
In voltammetry, very short time
scales can be also obtained using high scan rates, but a voltammetric
peak of BSA analogous to CPS peak H shifts to negative potentials
with the scan rate and merges with the background discharge at relatively
low scan rates. Moreover, CPS analysis of proteins can mostly be performed
under air.
155,230,231,238,239,254
5.2
Peak
H of Peptides and Proteins
By
the end of the 1990s, CPS in combination with a hanging mercury drop
electrode (HMDE) was applied in studies of peptides.
255
Peak of vasopressin was observed at highly negative potentials
well-separated from the background electrolyte. This peak was due
to the catalytic hydrogen evolution reaction (CHER) and was denominated
as peak H, as a tribute to Jaroslav Heyrovský, as well as due
to its high sensitivity and hydrogen evolution. Peak H was much better
developed and allowed the determination of lower concentrations of
the peptides than a polarographic or a voltammetric presodium wave.
Soon, CPS was shown to be the most sensitive EC label-free method
allowing the determination of proteins at nanomolar and subnanomolar
concentrations,
155,253
suggesting a wide application
of peak H in the analysis of peptides and proteins.
105,208,209,230−233,239,254,256,257
5.2.1
Theory
The hydrogen evolution reaction
and its reverse process of the hydrogen oxidation, as well as the
CHER, have attracted great attention.
258−263
The CHER can be described by the following equations:
1
2
where PH and P– stand for
the protonated and unprotonated aa residues in the protein, respectively,
BH is the acid component of the buffer solution, and B– is its conjugate base. The
symbols in parentheses represent the
state of the molecules ((aq) stands for aqueous, (surf) for surface
confined, and (g) for gaseous). These reactions imply that the catalyst
is the protein anchored at the electrode surface. Adsorptive transfer
experiments
155
suggest that the protein
binding to the surface must be particularly strong, because the protein-modified
electrode is washed, followed by immersion into the blank protein-free
background electrolyte. CHER theories considered little the structure
and properties of the protein catalyst and positioning and accessibility
of catalytically active aa residues in the protein folded structure.
Recently, homopolymers of different aa’s and peptides were
studied to shed some light on this problem.
170,264
Figure 7
(A)
CPS peaks H of 1 μM angiotensin peptides (AT) in McIlvaine
buffer, pH 7 at HMDE; dotted line represents blank background electrolyte.
Inset: CPS peak H of hexaArg and hexaHis. (B) Amino acids sequences
of AT peptides. Adapted with permission from ref (170). Copyright 2013 Elsevier.
Arginine (Arg), lysine (Lys),
and Cys were found as catalytically
active residues in proteins close to neutral pH.
155,265,266
Under these conditions, His
residues behaved as a much weaker catalyst than Cys, Arg, or Lys.
Each of these aa types alone (bound in peptide chains of polyamino
acids or peptides) was sufficient to catalyze hydrogen evolution and
produce peak H at mercury electrodes.
265,266
CHER in four
angiotensin (AT) peptides containing Arg and His residues was studied
in detail.
170
At neutral pH, only one of
them (ATIII) produced a well-developed CPS peak H (Figure 7A). This peptide contained
one Arg and one His residue.
ATII had the same sequence, but acidic aspartic acid (Asp) residue
was added at the N-terminus (Figure 7B). ATI
differed from ATII only by additional His and leucine (Leu) residues
at the carboxyl end. ATIV was similar to ATIII but did not contain
the Arg residue. These results suggested that Arg residue played a
critical role in CHER, but the presence of the acidic Asp in its close
neighborhood canceled the Arg catalytic activity. CPS behavior of
hexaArg and hexaHis supported the importance of Arg in CHER as compared
to the much smaller contribution of His (Figure 7A, inset).
170
Studies of peptides and
proteins showed that a particular contribution of the given aa residue
to CHER depended on the electrolyte ionic conditions and accessibility
of the aa in the surface-immobilized protein. Further work will be,
however, necessary to better understand relations between the protein
composition and structure, on one hand, and its CHER responses, on
the other hand.
5.3
Protein Structure-Sensitive
Electrocatalysis
at Bare Mercury Electrodes
5.3.1
Determination of Peptide
and Protein Redox
States
Reduced state of intracellular proteins is frequently
associated with their biological activities. The EC analyses of redox
states were limited to low molecular weight thiols, such as glutathione
and its fragments.
267
These methods were
based either on direct oxidation of thiols at solid electrodes (usually
at large overpotentials close to 1 V) or on the formation of stable
mercury thiolate complexes at mercury electrodes.
268
The reduction of disulfide bonds of proteins at Hg electrodes
was intensively studied (reviewed in refs (269−271)). However, little attention was
paid to
proteins, which require the reduced state for their biological function.
Only recently have methods for the determination of the redox states
of peptides and proteins based on CPS peak H been developed.
It was shown that reduced peptides adsorbed at positively charged
HMDE produced substantially higher peak H than their oxidized forms.
257
Similar behavior was observed with thioredoxin
(a general protein disulfide reductase with a large number of biological
functions).
209
Large differences in the
CPS responses of reduced and oxidized forms of peptides and proteins
were explained by differences in chemisorption and orientation of
the reduced compounds, combined with very fast potential changes in
CPS not allowing significant changes in orientation of the species
adsorbed at positively charged Hg surface. Peak H was used not only
for the analysis of thioredoxin at nanomolar concentrations and for
the determination of the thioredoxin redox states, but also to follow
interactions of this protein with the product of lipid peroxidation
such as 4-hydroxy-2-nonenal. CPS of thioredoxin at carbon electrodes
(based on oxidation of Tyr and Trp residues) was less sensitive and
did not allow discrimination between reduced and oxidized forms of
thioredoxin and peptides.
257,272
5.3.2
Are Proteins Denatured at Mercury Electrodes?
Early
polarographic studies (with a dropping mercury electrode)
of proteins indicated different Brdička’s catalytic
responses of native and denatured (Cys-containing) proteins.
72
These results were, however, not confirmed when
metal solid electrodes
76,273
or HMDE
274
were used. Thus, any attempt to use a bare mercury electrode
for protein structure analysis appeared ridiculous.
It was believed
for a long time that routinely used metal electrodes such as gold,
platinum, mercury, and silver led to denaturation and irreversible
adsorption of the proteins at the electrode surfaces.
76
Some papers claimed (e.g., refs (269) and (274)) that proteins were denatured upon
adsorption at mercury
electrodes producing adsorbed layers of uniform thickness. Few papers
disagreed, however, with the above-mentioned conclusions.
74,275
Recently, this problem was studied in greater detail using voltammetric
and CPS methods in connection with HMDE and solid amalgam electrodes.
208,230−233
It was found that, as compared to oxidation responses of proteins
at carbon electrodes, peak H is more sensitive to local and global
changes in the protein structures.
105,208,209,230−233,239,254
For example, at weakly alkaline and neutral pH’s, large differences
between peak H heights of native and denatured forms of BSA and some
other proteins were observed.
155,208,230−232
Native proteins produced very small signals,
while their denatured forms yielded 10–50 times higher peaks
208,230−233
(Figure 6D). These results were in qualitative
agreement with the solution structure of native (ordered, folded)
and denatured (disordered, unfolded) proteins, greatly differing in
accessibility of aa residues (particularly hydrophobic aa’s
are usually buried in the interior of the native folded protein molecule)
and consequently in the protein orientation and adsorption at the
electrode surface. However, very large differences between CPS signals
of native and denatured proteins
208,230−233
(Figure 6D) were at variance with the claimed
denaturation of proteins attached to the mercury electrodes. Clearly,
if a native protein was denatured at the electrode surface, its EC
responses should not greatly differ from that of the protein, which
was denatured in solution and adsorbed in its denatured form at the
electrode surface.
5.3.3
Proteins Are Not Surface
Denatured at Potentials
Close to Potential of Zero Charge on Hg
Very small peaks
of native proteins (Figures 8 and 9) suggested that no significant denaturation of
the protein took place at the mercury electrode surface. Recently,
it has been found
208,231−233
that proteins are not denatured when adsorbed at the mercury electrode
surface at the potential of zero charge (pzc) and at potentials positive
of pzc.
233
However, denaturation of proteins
took place due to a prolonged exposure of protein at a negatively
charged mercury surface.
233
5.3.4
Ionic Strength-Dependent Protein Denaturation
at Negatively Charged Hg Electrode
Using peak H, an ionic
strength-dependent structural transition in BSA at the HMDE surface
was detected.
232
In 50 mM sodium phosphate
at pH 7.0, peak H of 100 nM denatured BSA was much higher than that
of the native protein (Figure 8A). Increasing
the phosphate concentration resulted at first only in small increase
of peak H of native BSA, followed by a steep increase of this peak
between 90 and 110 mM phosphate (not observed in the control urea-denatured
BSA). At 200 mM phosphate concentration, peaks of native and urea-denatured
BSA were almost the same (Figure 8A). Similar
responses were observed with other proteins.
Figure 8
(A) Dependence of peak
height of 100 nM native (black) and denatured
(blue) BSA on concentration of sodium phosphate, pH 7 in the presence
of 56 mM urea (black). Accumulation time t
A of 60 s, accumulation potential E
A of
−0.1 V, stirring 1500 rpm, stripping current, I
str, of −30 μA. (B) Column graph showing
peak H heights of native (stripped column) and denatured (black column)
BSA obtained in AdT (ex situ) stripping experiment. 100 nM BSA was
adsorbed at HMDE for t
A of 60 s at E
A of −0.1 V either from 50 mM or from
200 mM sodium phosphate, pH 7, and the BSA-modified electrode was
transferred to the electrolytic cell with blank 50 or 200 mM sodium
phosphate, pH 7, to record the chronopotentiogram. 50 → 200
indicates BSA adsorption from 50 mM phosphate, followed by a transfer
of BSA-modified electrode to 200 mM phosphate in the electrolytic
cell. Denaturation of 14.4 μM BSA in 0.1 M Tris-HCl, pH 7.3,
with 8 M urea was performed overnight at 4 °C. The protein solution
was then diluted by the background electrolyte to the final protein
concentration (usually about 100 nM and immediately measured). Reprinted
with permission from ref (232). Copyright 2009 Royal Society of Chemistry.
In principle, this transition could take place
already during the
adsorption of the BSA at the electrode charged to the accumulation
potential −0.1 V or later in the course of electrode polarization
to more negative potentials, subjecting the adsorbed protein to higher
electric field strengths. To clarify this point, an adsorptive transfer
analysis
271,276
was performed, in which the
BSA accumulation on the surface was separated from the electrode process
(Figure 8B). After adsorbing BSA either from
50 mM or from 200 mM phosphate, and the transfer of the BSA-modified
electrode to (blank) 50 mM phosphate, the great difference between
peak heights of native and denatured BSA was retained,
232
suggesting that no significant irreversible
surface denaturation of BSA took place during the protein 60 s accumulation
at E
A −0.1 V. However, transfer
of surface-immobilized BSA to 200 mM phosphate removed the large difference
in the peak heights of native and denatured BSA, suggesting that surface
denaturation occurred at potentials more negative than E
A in a time interval ≪1 s. The absence of denaturation
in proteins adsorbed at the Hg electrode at the pzc, and at potentials
positive of pzc, was observed in a wide pH range.
232,233
The abrupt increase of peak H (Figure 8A)
was tentatively explained by the effect of strong electric field on
the BSA immobilized at the negatively charged Hg surface,
232
resembling thus the surface denaturation of
double-stranded DNA at a negatively charged Hg and other surfaces.
34
In the case of the BSA surface denaturation,
formation of Hg–S bonds and hydrophobic interactions between
the protein and hydrophobic Hg surface could be involved. Most probably,
the electric field repulsed negatively charged protein segments from
the surface and caused alteration of the charge distribution in the
protein introduced by shifts in the acid–base equilibrium toward
the ionized forms, the hydrogen bonds polarization, alignment of the
molecular dipoles, and displacement of the charged residues.
277
It cannot be excluded that protein structure
changes controlled by ionic conditions and the electric field might
take place also at cell surfaces and eventually play some biological
role.
5.3.5
Interplay of Current Density, Ionic Conditions,
and Temperature Affects the Structure of Surface-Attached Proteins
The above results suggested that the investigated proteins attached
at Hg surfaces around the pzc did not denature under the usual experimental
conditions, that is, close to neutral pH, moderate ionic strengths,
and room temperature. However, in proteins attached to negatively
charged Hg electrodes,
238
denaturation
took place depending on the time of exposure to the negative potentials.
In CPS experiments, this exposure time depends on the current density
(i.e., on polarizing current intensity at constant electrode size,
see section 5.1). Thus, at high negative I
str intensities, the exposure to negative potentials
is very short and the damage to protein structure is negligible, as
documented by very small peak H of native protein as compared to a
very large peak H of the protein denatured, for example, by urea or
guanidinium (Figure 9). In contrast, at low
negative I
str intensities, peaks H of
native and denatured protein are either the same or differ only a
little. The particular shape of the curve shown in Figure 9 may differ depending on
temperature and ionic strength.
238
At lower temperatures, longer exposure time
periods (i.e., lower I
str intensities)
are necessary to cause denaturation of the surface-attached protein.
However, increasing the ionic strength makes the surface-attached
protein more vulnerable to the effect of the electric field, and denaturation
of the surface-attached protein may occur even at shorter time periods.
Figure 9
Schematic
representation of the effect of the stripping current
intensity (I
str) on peaks H of native
and denatured proteins. The scheme demonstrates that at low I
str intensities, the surface-attached protein
is denatured, producing almost the same peak H as the protein, which
was denatured in solution by a chemical denaturation agent. The protein
denaturation at the electrode surface is due to the prolonged effect
of the electric field at negative potentials. At higher I
str intensities, the time of exposure of the protein to
negative potentials is much shorter, causing a little harm to the
surface-attached protein as manifested by a relatively small peak
H.
It can be concluded from the above-mentioned
data that by using
CPS peak H, bare Hg electrodes can be used to study changes in the
protein structures under certain ionic conditions (e.g., at neutral
pH and relatively low ionic strengths, Figure 8), while at diferrent ionic conditions
protein surface denaturation
at a negatively charged bare Hg electrode may take place even under
very fast potential changes in CPS. For example, in 0.2 M sodium phosphate,
pH 7 at I
str of −30 μA, it
was not possible to discriminate between native and denatured BSA
(Figure 8), because of the electric field-induced
denaturation of the surface-immobilized protein.
232
Recently, attempts have been made to decrease or eliminate
protein-denaturing effects of the electric field, by using chemically
modified Hg working electrodes.
239
5.4
CPS Peak H at Thiol-Modified Mercury Electrodes
It has been shown that alkanethiols
161,278
and thiolated
DNA
237
form in short time intervals impermeable,
pinhole-free SAMs on Hg surfaces. Similar modification of gold electrodes
has been widely used in the studies of conjugated proteins, yielding
EC responses of nonprotein redox centers,
77,78
but pinhole-free SAMs similar to those found at Hg electrodes have
not been reported. Protein-catalyzed hydrogen evolution
263
(responsible for peak H) has not been, however,
observed at gold electrodes and other solid electrodes not containing
Hg but solely with mercury-containing electrodes.
155,263,271,279
To prevent direct contact of proteins with the metal surface, a
dithiothreitol (DTT) SAM was formed at the HMDE surface (DTT-HMDE)
and silver solid amalgam electrodes (DTT-AgSAE).
239
DTT was chosen because this reducing agent in millimolar
concentrations is frequently used for storage of reduced proteins.
5.4.1
Electric Field-Induced Denaturation of Surface-Immobilized
BSA Is Strongly Decreased at DTT-Modified Hg Electrodes
It
was shown that BSA and other proteins could be easily immobilized
at DTT-HMDE and that denatured proteins produced high peak H. In contrast,
native BSA (which underwent surface denaturation in 0.2 M sodium phosphate
or McIlvaine buffer, pH 7, at the bare HMDE) displayed almost no peak
at DTT-HMDE, suggesting the absence of BSA surface denaturation under
the same conditions (Figure 10). Very small
peak H in native BSA could be explained either by negligible adsorption
of native BSA at the electrode and/or by inaccessibility of the catalytic
aa residues in the surface-attached protein molecules. To solve this
question, combined CPS and CV experiments were performed, which showed
that at DTT-HMDE, native BSA was adsorbed to an extent similar to
that of denatured BSA, but in CV, surface-immobilized native BSA was
denatured during slow potential scanning to negative potentials, suggesting
that much smaller CPS peak H height in native (than in denatured)
BSA was predominantly due to inaccessibility of catalytic aa groups
in the native surface-attached protein.
Figure 10
Constant current chronopotentiometric
stripping (CPS) peak H of
100 nM native BSA (n, black), DTT-reduced BSA (r, blue), and guanidinium
chloride (GdmCl)-denatured BSA (d, red) (A) at bare and (B) at DTT-modified
HMDE (DTT-HMDE) in McIlvaine buffer, pH 7. GdmCl was present in all
samples at nondenaturing (70 mM) concentration; conventional CPS measurements
using stripping current, I
str, −70
μA. (C, D) Adsorptive transfer stripping cyclic voltammograms
of native (black) and denatured BSA (red) at DTT-modified HMDE at
scan rates of (C) 50 mV/s and (D) 1 V/s. In this experiment, 1 μM
BSA was adsorbed at DTT-HMDE from McIlvaine buffer, pH 7, for t
A 60 s. The adsorptive transfer procedure was
applied to prevent additional BSA adsorption during the potential
scanning. BSA was denatured with 6 M GdmCl. Adapted with permission
from ref (239). Copyright
2010 American Chemical Society.
5.4.2
Prolonged Exposition of the DTT-SAM to Negative
Potentials Disturbs DTT-SAM
At potential values, more positive
than the reduction potential of the DTT Hg–S bond (∼ –0.65
V against Ag/AgCl), the densely packed DTT-SAM was impermeable to
[Ru(NH3)6]3+.
228
Prolonged exposure of the DTT-SAM to more negative potentials
resulted in disturbance of the SAM, but under conditions of CPS (with
very fast potential changes), the (reductively desorbed) SAM still
protected the immobilized protein from surface-induced denaturation.
In contrast, the usual slow scan voltammetry (scan rates between 50
mV/s and 1 V/s) displayed large disturbance of the densely packed
DTT-SAM, and the adsorbed protein (in McIlvaine buffer, pH 7) was
fully or partially denatured (Figure 10C,D).
Exposure of BSA-modified DTT-HMDE to different potentials, E
B for 60 s, followed by CPS measurement revealed
three E
B regions, in which BSA remained
either native (region A, −0.1 to −0.3 V), was denatured
(B, −0.35 to −1.4 V), or underwent desorption (C, at
potentials more negative than −1.4 V).
239
5.4.3
DTT-Hg Electrodes Can
Be Used in the Analysis
of Reduced and Oxidized Proteins
Chemical reduction of disulfide
bonds in native BSA resulted in a large increase of peak H at both
bare and DTT-modified HMDE, suggesting easier denaturation of reduced
BSA at the electrode surface.
231,239
Most probably, DTT
chemisorbed at HMDE at millimolar concentrations adopted the “stand
up” position with one of two thiol groups exposed to the solution.
228
Nevertheless, adsorption of native BSA on DTT-HMDE
did not result in a detectable increase of peak H, showing that no
significant BSA reduction took place at the DTT-HMDE surface during
a short contact of the protein with the DTT-SAM. Thus, the DTT-HMDE
can be used for studies of both reduced and disulfide-containing proteins,
but it can be expected that it will be particularly useful for the
analysis of intracellular proteins in their reduced state, as documented
by the analysis of glutathione-S-transferase (GST)
239
and tumor suppressor protein p53
105
(see section 6.2). Using
DTT-AgSAE, results similar to those observed on DTT-HMDE
228
were obtained, and, recently, AgSAE arrays
were constructed.
227,236
As compared to the established
label-free optical methods for tracing the protein denaturation, such
as fluorescence and circular dichroism spectroscopy, peak H is either
more or equally sensitive to changes in the proteins structure.
239
In EC adsorptive transfer experiments, 3–5
μL volumes of protein solution can easily be used. Moreover,
miniaturization of SAEs can further decrease protein volumes necessary
for the analysis. It can be expected that thiol-modified Hg electrodes
in combination with CPS peak H will soon become an important tool
in protein analysis.
5.4.4
Thiol SAMs at Hg Electrodes
Originally,
the DTT-modified electrodes for protein analysis were prepared by
forming first the DTT-SAM followed by immobilization of the respective
protein on the SAM.
239
Later, an easier
way of attaching BSA and other proteins at HMDE or AgSAE based on
adsorbing BSA and DTT together in a single step was proposed.
228
Properties of the DTT layers, dependent on
the DTT bulk concentration, were tested. Changes in the cyclic voltamograms
of a redox couple of [Ru(NH3)6]3+/[Ru(NH3)6]2+ and in reduction of
the Hg–S bonds (peak S, Figure 11A)
suggested that DTT at lower concentrations was adsorbed as a dithiol
with both −SH groups attached to the surface.
228
When the electrode was incubated in a DTT solution with
concentration between 200 and 900 μM of DTT, a change in the
DTT-SAM occurred, and increasing the DTT concentration resulted in
a densely packed pinhole-free layer
237,280
in which
the DTT molecules were probably bound to the electrode surface by
a single −SH group, oriented perpendicularly to the surface.
The amount of BSA molecules adsorbed at DTT-HMDE or coadsorbed with
DTT at a bare HMDE appeared roughly the same, and thus a great majority
of BSA molecules were attached to the DTT layer and not to the bare
HMDE. This was explained by a faster diffusion and adsorption of DTT
at HMDE as compared to slower diffusion of much larger BSA molecules,
which were in solution at concentrations by 4 orders of magnitude
lower than that of DTT.
281,282
BSA was measured at
relative high ionic strength, that is, under the conditions inducing
BSA denaturation at bare HMDE.
232
Figure 11
(A) Chronopotentiograms
of 100 nM native BSA coadsorbed with 60
μM DTT at HMDE. (B) Dependence of peak H and peak S heights
obtained with BSA·DTT-HMDE on concentration of DTT. BSA·DTT-HMDE
was prepared by coadsorption of 100 nM BSA and 1 mM DTT at HMDE followed
by CPS peak H recording at stripping current, I
str, −70 μA. Adapted with permission from ref (228). Copyright 2013 Elsevier.
Figure 11 shows that peak H of native BSA
decreased while peak S increased with increasing DTT concentration
up to about 1 mM DTT. In contrast, peak H of urea-denatured BSA responded
only little to changes in DTT concentration (not shown). It can be
read from Figure 11, that when coadsorbing
native BSA and DTT on HMDE, the DTT concentration should be well above
1 mM. Similar results were obtained with some other thiols.
5.5
Enzyme Activity at Hg Electrodes
Various
enzymes showed a good activity at different surfaces and
enzyme electrodes,
88,92−95
enabling construction of various
enzyme sensors,
46,274
such as the well-known glucose
biosensor.
283,284
Already in 1977, it was shown
by Bards’s group
275
that urease
and alcohol dehydrogenase retained their enzymatic activity, when
adsorbed at bare mercury and solid amalgam electrodes. Later, the
opinion prevailed that, when adsorbed at bare mercury and other metal
electrodes, proteins were irreversibly denatured.
76
Bard’s group also showed that prolonged exposure
of the surface-attached urease to negative potentials resulted in
the enzyme inactivation. This inactivation was explained by electroreduction
of the disulfide groups in the protein.
275
Recently, it has been shown
238
that the
original Bard’s finding regarding the urease enzymatic activity
at Hg electrodes was correct
275
and that
exposure of the enzyme to negative potentials at bare HMDE resulted
in disturbances of the structure of the surface-attached enzyme, as
detected by CPS (Figure 12). The extent of
the urease unfolding was time- and temperature-dependent, when the
proteins were exposed to negative potentials (Figure 12B,C). Dependence of peak H
on the negative polarizing current
(−I
str, Figure 12A) showed secondary denaturation of the surface-attached urease
during the CPS recording at I
str less
negative than ∼ –30 μA. Urease enzymatic
activity was observed also at a thiol-modified amalgam surface,
238
and at this surface the protein was less vulnerable
to the effects of the electric field. It was concluded that earlier
observed loss of enzymatic activity, resulting from a 10 min exposure
of the protein to −0.58 V vs Ag/AgCl/3 M KCl,
275
was not due to reduction of the disulfide bonds as suggested
by Santhanam et al.,
275
because the enzyme
showed the best activity in its reduced form, while its oxidation
caused a decrease of its activity.
238
Moreover,
no disulfide bonds in native urease molecule were found.
285
The loss of the enzyme activity at negative
potentials probably resulted from the protein reorientation, after
reduction of the Hg–S bonds (formed by accessible Cys residues),
followed by prolonged electric field effect on the surface-attached
protein.
Figure 12
CPS responses of the native and denatured proteins at bare and
DTT-modified mercury electrodes. (A) Dependence of CPS peak H area
of 20 nM native (−●–, − –○– −)
and denatured (−■–, − –□– −)
urease on stripping current, I
str, at
bare (solid line) and DTT-modified (dashed) HMDEs. Urease was adsorbed
at accumulation potential of −0.1 V for accumulation time, t
A, of 60 s from McIlvaine buffer, pH 7, with
26 mM GdmCl in a thermostated electrolytic cell at 25 °C, and
CPS analysis proceeded at the given I
str. (B,C) Chronopotentiograms of 20 nM native (black) and denatured
urease (red) on a bare HMDE at (B) different striping currents at
25 °C and (C) different temperatures using I
str −25 μA. Adapted with permission from
ref (238). Copyright
2013 Elsevier.
5.6
Concluding
Remarks
Section 5 represents the first
comprehensive review of EC
analysis of practically all proteins based on combination of CHER
with CPS at mercury-containing electrodes. To obtain CPS protein CHER
signal (peak H) under conditions close to physiological, the protein
has to contain at least one type of catalytically active aa residue,
that is, Arg, Lys, Cys, or His. Most probably, proteins not containing
any of these residues, if any, are extremely rare. In other words,
this CPS method can be applied for analysis of practically any protein
in proteomics, biomedicine, and elsewhere. Another condition for obtaining
protein peak H is the accessibility of the catalytically active aa
residues for the electrode process. In a native folded protein, aa
residues buried in the interior of the molecule and/or located far
from the electrode surface may remain catalytically silent. However,
they can become accessible after the protein denaturation. Using chemically
modified electrodes (e.g., with different thiols) can help to change
the orientation of protein molecules at the surface and the accessibility
of some aa residues. Mechanistic aspects of the catalytic hydrogen
evolution were elucidated >50 years ago,
286
and even recent papers
265,287
are behind the progress
in understanding of redox processes in nonprotein components of conjugated
proteins, including electron transfer
80
and hydrogen tunneling in enzymes.
83
Application of CPS to proteins <10 years ago opened the door not
only for sensitive EC determination of nonconjugated proteins but
also for a new type of protein structure-sensitive analysis based
on the ability of the electric field forces to denature proteins attached
to the negatively charged electrode surface. By adjusting the time
intervals of the protein exposure to the electric field effects to
milliseconds (by choosing the appropriate current density in CPS),
as well as other experimental conditions, such as temperature and
ionic strength, the surface-attached protein denaturation can be minimized
and the stability of proteins as well as of DNA–protein complexes
(section 7.5) at the surfaces can be investigated.
We believe that CPS studies of nonconjugated proteins at Hg electrodes
are a challenging field that deserves further attention of electrochemists
and biochemists from both theoretical and experimental points of view.
6
Label-Free Protein Analysis in Biomedicine
Progress in understanding of changes in the protein structure at
the mercury electrode surface made it possible to apply electrochemistry
in studies of proteins important in biomedicine. For example, studies
of α-synuclein (AS) protein, which is involved in Parkinson’s
disease,
288,289
showed that peak H at HMDE as
well as oxidation signals at carbon electrodes could be used to study
aggregation of this protein.
156
HMDE was
particularly sensitive to preaggregation changes,
254
detected at short incubation time periods preceding AS
oligomerization observable by a dynamic light scattering. EC studies
of amyloid peptides involved in Alzheimer’s disease suggested
that EC techniques have a good chance to become of great value also
in better understanding of an aggregation process in Alzheimer’s
disease.
156,221
Application of electrochemistry
in cancer research and especially studies of CPS responses of the
tumor suppressor protein p53 and its mutants appear very interesting.
105,240
The CPS responses of wild type and mutant proteins agreed well with
changes of the X-ray crystal structures resulting from a single aa
exchange in these proteins.
105
As compared
to X-ray crystal analysis, nuclear magnetic resonance, and other methods,
the CPS analysis worked with picomole protein amounts and yielded
instant results, suggesting that EC methods may complement classical
methods of protein structure analysis in the future.
6.1
Neurodegenerative
Diseases
Among
a number of human diseases that are associated with protein misfolding,
290,291
particular attention has been paid to a group of diseases in which
protein conversions into insoluble fibrils play a critical role.
292−295
The final forms of aggregated proteins have frequently a well-defined
fibrillar nature denominated as amyloid. About 20 neurodegenerative
diseases include Alzheimer’s, Parkinson’s, and Creutzfeldt–Jakob’s
diseases. Alzheimer’s and Parkinson’s disease are the
most common neurodegenerative disorders among the elderly. Aggregation
processes of amyloid peptides and proteins can be induced in vitro
and in vivo by a variety of agents and conditions. Numerous point
mutations responsible for the disease were identified in the AS gene.
296−298
Aggregation of amyloid peptides and proteins in vitro is commonly
studied by several methods such as circular dichroism spectroscopy,
light scattering methods, thioflavin T or Congo red fluorescence,
electron microscopy, and atomic force microscopy.
299−304
Aggregates of amyloid peptides and proteins involved in Parkinson’s
and Alzheimer’s disease are highly polymorphic, with mature
amyloid fibrils constituting the predominant structure in fully aggregated
amyloids.
299,300,302,305−310
Increasing evidence from studies in different organisms and in vitro
systems indicated that intermediate aggregation products, such as
soluble oligomers of amyloidogenic peptides and proteins, are responsible
for amyloidosis
311,312
and are the toxic agents.
313,314
Parkinson’s disease is associated with the formation
of amyloid fibrils of the AS. This 14 kDa protein (first described
in 1988)
315
is natively unfolded and comprises
140 aa residues contained in three regions: (i) aa’s 1–60
constitute the N-terminal region including the hexamer motif KTKEGV;
(ii) the central region with highly amyloidogenic NAC sequence (aa’s
61–95), containing two additional KTKEGV sequences; and (iii)
the C-terminal region (aa residues 96–140) rich in acidic residues
and prolines, including three highly conserved Tyr residues;
316
this region is presumably disordered under
most conditions.
At first sight, EC might seem of little use
in AS analysis, as
this protein does not contain any redox center for reversible EC.
Tryptophan is absent, but there are four Tyr residues that can undergo
oxidation at carbon electrodes.
154,271,317
To our knowledge, EC was applied in studies of AS
for the first time in 2004.
226
Among the
EC methods tested, the best results were obtained with SWV oxidation
peaks of Tyr at carbon paste electrodes and CPS electrocatalytic (reduction)
peak H using HMDE. Both methods reflected fibrilization of AS in vitro
by the decrease of their signals, but the changes in peak H were much
larger than the decrease observed in the AS oxidation signals. EC
studies were combined with atomic force microscopy and circular dichroism
spectroscopy. Recently, it has been proposed that oligomeric intermediates
rather than the fibrils themselves can be the pathogenic agents of
Parkinson’s disease.
301,318−321
Minor changes in the adsorption state of AS at solid spectroscopic
graphite were followed through the shift of the Tyr oxidation potential,
consistent with the compact and less-compact/unfolded AS.
157
HMDE was particularly sensitive to preaggregation
changes in AS. In an early stage of a standard aggregation assay,
peak H increased and shifted to less negative potentials. In the following
interval (in which a dynamic light scattering indicated AS oligomerization),
peak H diminished, its potential shifted in the opposite direction,
and AS adsorbability decreased. These early changes in the interfacial
behavior of the protein were attributed to disruption of long-range
interactions and to destabilization of AS, while the subsequent changes
were related to the onset of oligomerization. EC methods (CPS analysis
and alternating current voltammetry) together with SDS-PAGE, optical
spectroscopy (UV/vis absorption, steady-state and dynamic fluorescence,
and dynamic light scattering), MS, and atomic force microscopy were
used for monitoring and characterization of stable covalent oligomeric
species (dimers, trimers, and higher oligomers) produced in vitro
by the photoinduced oxidative formation of side-chain tyrosyl radicals
of AS. It was concluded that EC methods represent new and simple tools
for the investigation of amyloid formation.
322
It has been shown that various factors affect the progression
of
the AS and amyloid β (Aβ)-peptide fibrillation.
294,300,311−314,323−326
Metals,
327−330
organic solvents and acidic pH’s,
305
pesticides and herbicides,
331
and oxidative
stress stimulate AS aggregation in vivo and in vitro.
319,332
Inhibition of the AS aggregation can be observed in the presence
of other factors,
333,334
including certain flavonoids
with therapeutic potential in Parkinson’s disease treatment.
335−341
Among them, baicalein has shown a significant inhibitory effect
on the AS aggregation.
334−341
This flavonoid possesses antioxidant properties, and upon oxidation,
it can form quinones interacting with AS and inhibiting its fibrillation.
224
Chan et al. followed the AS aggregation in
the presence of Cu(II) ions and baicalein (inhibiting the AS aggregation)
by measuring changes in SWV Tyr oxidation peak Y at screen printed
carbon electrodes.
342
In an agreement with
previous studies,
226,254
this peak decreased in the course
of AS aggregation. In the presence of baicalein, the AS oxidation
peak greatly increased, probably due to the presence of overlapping
oxidation peak of baicalein itself. Surprisingly, after 24 h of AS
aggregation, peak Y heights observed in the absence or presence of
0.1 mM baicalein or 5 mM Cu(II) ions were about the same. A rather
high concentration of AS (50 μM) was used for the analysis as
compared to those used for EC aggregation studies of AS at Hg electrodes
(2 μM or lower to measure CPS peak H).
In Alzheimer’s
disease, the Aβ-peptides involved represent
a paradigm for studies of amyloid formation and conformation.
300
These peptides result from cleavage of the
transmembrane amyloid precursor protein.
300,343,344
Diagnosis of Alzheimer’s
disease in patients is still difficult, and suitable biomarkers are
sought. Protein tau and its phosphorylation have recently appeared
as attractive candidates. This 50–65 kDa heat stable protein
has many functions, including maintaince of structural integrity of
microtubules.
223,345−347
Tau binding and stabilization of microtubules is disturbed in cells
affected by Alzheimer’s disease where tau hyper-phosphorylation
and aggregation are evident. The role of Cu(II) ions in tau protein
dysfunction has recently attracted attention. This protein was studied
by some techniques such as surface plasmon resonance-based immunochip
223
and also CV and X-ray photoelectron spectroscopy.
348
Tau protein was immobilized at a gold electrode,
and EC methods allowed the detection of Cu(II) ion binding and differentiation
between normal tau and phosphorylated tau films and revealed ion displacement
by Zn(II) ion for phosphorylated tau but not for normal tau films.
The majority of information about the peptide assemblies was obtained
from full-length Aβ-peptides Aβ1–42 and
Aβ1–40.
319,349−360
The aa sequence of the human Aβ1–42 is 1-DAEFRHDSGYEVHHQKLVFFAEDVGSNKGAIIGLMVGGVVIA-42.
In addition to the
full-length Aβ-peptides, a number of Aβ-fragments
generated in vitro were found to form amyloid fibrils. Among them,
the Aβ16–22, included in the hydrophobic C-terminal
region and containing the KLVFF core, represented one of the most
studied fragments.
361−369
It was suggested that an Aβ1–42 conformation
with the C-terminus forming inside the wall of a hollow core and the
N-terminus plays an important role in the peptide fibrilization.
370
EC methods have been applied also in
studies of Aβ-peptide
aggregation,
156
in addition to a variety
of methods mentioned above. Interaction of amyloid peptides and proteins
with lipid bilayers at Au and Hg
371,372
electrodes
was also studied. In contrast to the EC analysis of AS employing both
Hg and carbon electrodes, EC analysis of Aβ-peptides relied
predominantly on voltammetric oxidation signals of Tyr residues (peak
Y) at carbon electrodes. Vestergaard et al.
224
observed a decrease in Tyr voltammetric oxidation peak Y at GCE
in the course of Aβ1–42 and Aβ1–40 aggregation and found a difference in the rate
of aggregation of
these two peptides. EC analysis was complemented with fluorescence
of thioflavin T and atomic force microscopy analysis. Later, Aβ1–40 aggregation in
the presence and absence of Zn(II),
Cu(II), and Mg(II) ions was studied using SWV peak Y at GCE and electron
microscopy.
373
While Mg(II) ions were almost
without effect, Zn(II) and Cu(II) stimulated the peptide aggregation.
In the absence of metal ions, the time dependence of peak Y was sigmoidal,
but in the presence of metal ions, the lag period almost disappeared
as a result of rapid metal-induced aggregation. However, after prolonged
aggregation (incubation of 50 μM Aβ1–40 for almost 150 h at 37 °C), peak Y heights
of metal-treated
peptides were significantly higher than this peak produced by Aβ1–40 peptide aggregated
in the absence of metals. Electron
microscopy displayed different morphologies of the peptide aggregates
obtained in the presence and absence of Zn(II) or Cu(II). No precautions
preventing bacterial growth (resulting from very long incubation time)
were mentioned in this paper.
373
Interactions of benzothiazole dyes (such as thioflavin T and [2-(4′-methylamino)phenyl)
benzothiazole], BTA-1) with Aβ-peptides were well documented
by fluorescence methods.
300,374−379
Recently, interactions of these dyes with Aβ1–42 and Aβ1–40 peptides not containing
oxidizable
Tyr residues (aa sequences of the peptides in rats) were studied.
380
Using DPV and CV, it was shown that thioflavin
T and BTA-1 produce oxidation peaks at screen-printed carbon electrodes.
Dramatically different behavior of oxidation peaks of positively charged
thioflavin T and its neutral analog BTA-1 in the course of the Aβ-peptide
aggregation was observed. Aggregation of these peptides in the presence
of thioflavin T resulted in an unexpected increase of the oxidation
thioflavin T peak after 24 h of incubation. The authors speculated
that changes in the peak heights after longer incubation times resulted
from changes in thioflavin T binding sites along the peptide β-sheets,
while very early changes of the dye oxidation signals were due to
oligomerization of the peptides.
Carbon nanotube (CNT)-modified
electrodes are widely recognized
for their electrical conductivity and catalytic effects.
381,382
These electrodes were used to study Aβ1–42 peptide aggregation and disaggregation
processes induced by the
β-sheet breaker LPFFD.
383
Tyr oxidation
peaks were measured by DPV or CV at relatively low Aβ1–42 peptide concentrations (down
to 10 μM) during the aggregation
process in vitro. Antiaggregation effects of the peptide inhibitor
LPFFD were observed in combination with conventional methods. In fibrils,
Tyr residues were unexposed, while in earlier peptide forms exposition
of Tyr was observed. The authors believe that this method opens a
new avenue in EC studies of Aβ-peptides aggregation and better
understanding of the mechanism that causes Alzheimer’s disease.
In addition to typical Aβ-peptides, Tyr homopolymer (poly(Tyr))
was used as a model for studies of protein aggregation, which was
accompanied by changes in Tyr oxidation signals at carbon electrodes.
384
These changes were explained in terms of varying
accessibility of Tyr residues for the EC oxidation.
In principle,
two classes of in vitro measurements can be distinguished:
(i) usual measurements in a bulk solution, for example, monitoring
the fluorescence emission by amyloidophilic dyes,
385−387
and (ii) recently applied techniques such as quartz crystal microbalance,
388−393
surface plasmon resonance,
394,395
or microcantilever
measurements,
388,391
allowing measurements of the
growth of amyloid fibrils bound to a surface. In these techniques,
reliable attachment of the peptide or protein to the surface plays
a critical role.
The attachment of amyloid fibrils or other
protein structures to
matrixes can be achieved either via an amide coupling reaction
394,395
or through antibody–antigen interactions.
393
Alternatively, Cys-containing proteins can be attached
to some metal surfaces such as gold or mercury via the Au–S
or Hg–S bonds. This type of binding can be applied neither
to Alzheimer’s Aβ-peptides nor to Parkinson’s
AS protein, which do not contain Cys residues. Recently, a novel strategy
was described, based on the attachment of small molecule linkers to
protein fibrils, to immobilize the protein nanostructures to Au surfaces.
396
This strategy involved the reaction of 2-iminothiolane
and cystamine with the amyloid fibrils, which enabled their covalent
linkage to gold surfaces via Au–S bond. Such attachment was
irreversible, allowing quantitative analysis by biosensors that can
revolutionize the way we look at a process of an amyloid growth. Moreover,
this study contributed to a better understanding of the nature and
relative importance of covalent versus noncovalent forces acting on
protein superstructures at metal surfaces. This strategy appears attractive
showing that the chemical reactions do not disturb the amyloid structures.
In recent years, de novo designed peptides were synthesized, and their
fibrilization and other properties were systematically studied.
299
Cys residue can be easily added to Aβ-peptides
in the course
of their synthesis. Peptide-based octamer YYKLVFFC containing the
KLVFF (Aβ16–20) core domain with additional
terminal Tyr and Cys residues was designed in Hamley’s laboratory,
397
to enable the peptide functionalization. Cys
residue allowed covalent attachment to mercury and other metal surfaces
as well as its chemical modification using its −SH group. The
two Tyr residues served as fluorescence tags, potentially enabling
bioconjugation, or responsiveness to enzyme (e.g., by phosphorylation/dephosphorylation
reactions). The self-assembly of YYKLVFFC in solution has been investigated
by various methods such as a variety of spectroscopic, scattering,
and microscopic ones.
398
The tendency
of fibrils to spontaneous orientation was investigated
in a detailed study with a focus on the orientation of the peptide
alignment (in bulk solution and dried films) by X-ray diffraction
and near-edge X-ray absorption fine structure polarized Raman spectroscopy,
linear UV dichroism, and Fourier transform infrared spectroscopy.
399
These approaches provided detailed information
about the orientation of fibrils and of the constituent Tyr fluorophores.
A similar approach was used to modify the Aβ12–28 peptide with a Cys residue at the
C-terminal end with its subsequent
immobilization onto gold electrodes.
397
Using this platform, it was possible to electrochemically monitor
the interaction of the peptide with Congo Red as well as with a β-sheet
breaker peptide.
EC and interfacial properties of the above
YYKLVFFC peptide and
two other Aβ peptides YEVHHQKLVFF and KKLVFFA were investigated
at carbon and predominantly at mercury electrodes. The interfacial
behavior of these model amyloid peptides at the metal|aqueous solution
interface was studied by voltammetric and CPS analysis. All three
peptides adsorbed in a wide potential range exhibited different interfacial
organizations depending on the electrode potential. At the least negative
potentials, chemisorption of YYKLVFFC peptide occurred through the
formation of a metal–sulfur bond. This bond was broken at about
−0.6 V. The peptide then underwent a self-association at more
negative potentials, leading to the formation of a “pit”
characteristic for a 2D condensed film (Figure 13).
221
Under the same conditions, the other
peptides did not produce such a pit. Formation of the 2D condensed
layer in YYKLVFFC peptide was supported by the time, potential, and
temperature dependences of the interfacial capacity, and the presence
of the 2D layer was reflected also by the peptide CPS signals due
to the CHER. The ability of YYKLVFFC peptide to form the potential-dependent
2D condensed layer has not been reported for any other peptide or
protein molecules. At this stage, it is not clear to which extent
is the YYKLVFFC peptide 2D condensation related to the known oligomerization
and aggregation of Alzheimer amyloid peptides.
Figure 13
(A) Capacity–potential
curves of 1 μM peptide YYKLVFFC
(black), YEVHHQKLVFF (red), and KKLVFFA (blue) in 35 mM Na-phosphate,
pH 7 (dashed line). Peptides were adsorbed at −0.1 V, for t
A of 120 s at HMDE, followed by recording of
ac voltammogram with a frequency of 150 Hz, amplitude of 5.0 mV, and
a scan rate of 8.0 mV/s. (B) Capacity–potential curves of 1
μM peptide YYKLVFFC recorded in 35 mM phosphate buffer, pH 7.0
at different temperatures, as indicated on the graph. Accumulation
potential E
A of −0.5 V; accumulation
time t
A of 120 s. Adapted with permission
from ref (221). Copyright
2013 Elsevier.
6.2
Tumor
Suppressor Protein p53
Shortly
after the invention of CPS protein analysis with DTT-modified Hg electrodes,
239
the new method was applied for investigation
of the tumor suppressor protein p53.
105
This protein plays a critical role in the cellular responses to
DNA damage by regulating the gene expression of factors involved in
DNA repair, cell cycle, and apoptosis.
400,401
p53 protein
is inactivated by mutation in about one-half of human cancers. Most
oncogenic mutations are located in the DNA-binding core domain of
the protein.
102,402
Better understanding of the
molecular basis of p53 inactivation in cancer is essential for the
development of novel anticancer strategies.
403
The structural effects of many oncogenic p53 mutants were intensively
studied by X-ray crystallography and complementary techniques (reviewed
in ref (404)). The
most frequent highly destabilized cancer-associated mutant, R175H,
has, however, eluded a detailed structural characterization, indicating
the need for complementary techniques to study this and other unstable
mutants. R175H is located in the L2 loop of the DNA-binding domain
in a vicinity of the zinc coordination sphere (Figure 14A). The mutation induced substantial
structural perturbation
in the zinc-binding region (Figure 14B), causing
increased conformational flexibility and loss of the zinc ion and
sequence-specific DNA-binding activity.
404,405
Similar effects were found in the wild-type (wt) protein upon removal
of the zinc ion.
406
Figure 14
Structure of DNA-binding
domain of p53. (A) Overall structure of
T-p53C (PDB entry 1UOL).
101
Sites of cancer mutations investigated
in this study (V143A, R175H, F270L, and R273H) are highlighted as
green stick models. (B) Close-up view of the zinc coordination sphere,
with the four zinc ligands shown in magenta. (C–F) CPS peak
H of wild type T-p53C (black) and mutant R175H (red) at DTT-HMDE in
50 mM phosphate, pH 7 at (C) 11.1 °C, (D) 13.9 °C, (E) 15.9
°C, and (F) 20.3 °C. (G–J) CPS peaks H of (G) wt,
(H) R175H, (I) R273H, and (J) V143A treated by 0 mM (red), 5 mM (blue),
10 mM (green), and 20 mM (cyan) EDTA at 0 °C for 10 min. CPS
measurements were performed at 18 °C. Adapted with permission
from ref (105). Copyright
2011 American Chemical Society.
Superstable variants of wt p53 and mutant core domains (T-p53C),
for which high-resolution structures are available, and which are
better suited for physicochemical studies than the native p53 proteins,
101,407
were used for the CPS analysis. Large differences between the CPS
responses of wt and R175H proteins were found in a wide temperature
range (Figure 14C–F). Below 13 °C,
wt p53 did not produce any peak as compared to the mutant, which yielded
a well-developed peak H (Figure 14C), suggesting
better accessibility of aa residues involved in the electrocatalysis
of the mutant protein. Between ∼16 and 20 °C, wt T-p53C
yielded a single peak H, while the mutant displayed two peaks (Figure 14E, F). Wt
and mutant R175H were treated with EDTA
to remove zinc ion from their molecules. Treatment of wt T-p53C with
5 mM EDTA for 10 min resulted only in shifts of peak H1wt to positive potentials,
while 20 mM EDTA produced in addition to
peak H1wt a broad peak H2wt at −1.94
V, resembling thus the CPS profile of untreated R175H (Figure 14G). The same EDTA
treatment of R175H was almost
without effect on its CPS peaks (Figure 14H),
in agreement with the expected absence of Zn2+ in this
mutant protein. EDTA treatment of other mutants produced similar effects
in their CPS profiles as in that of wt T-p53C (Figure 14I,J), albeit at lower EDTA
concentrations. CPS profiles of
all untreated mutants differed from that of the wt protein.
It was suggested that the sharp peak H1 is related to accessible
electrocatalytically active groups in an ordered T-p53C layer. The
broad peak H2 might involve distorted or partially unfolded protein
regions, strongly influencing the structure of the layer at the electrode
surface. The electrode process responsible for this peak required
higher temperatures than peak H1, which was produced already at 8.6
°C in R175H and at 13.9 °C in wt T-p53C (Figure 14).
105
Considering the
relations between the appearance of peak H2 and the effect of the
zinc removal from T-p53C by EDTA (Figure 14G,H), it was tentatively suggested that
in R175H the Cys residues
in the perturbed zinc-binding site (Cys176, Cys238, and Cys242) significantly
change the structure of the adsorbed protein layer and contribute
to CHER. Most of the results were obtained with DTT-modified HMDE,
but similar differences between wt and mutant protein CPS profiles
were achieved using DTT-AgSAE.
105
However,
the square wave voltammetric oxidation signals of wt and R175H mutant
at carbon electrodes and cyclic voltammetric catalytic peaks close
to −1.9 V at DTT-HMDE differed only a little.
105
6.3
Analysis of Poorly Soluble
Membrane Proteins
Among proteins occurring in mammals, about
30% are bound to membranes.
408,409
Contrary to the rest
of the proteins, which are mostly water-soluble
and can easily be analyzed using EC techniques, membrane proteins
are poorly soluble or insoluble in aqueous solutions and/or unstable
outside the lipid membrane. Because of these problems, information
about the EC behavior of membrane proteins was rather limited. Only
recently has it been shown that by using the adsorptive transfer voltammetry
and CPS, membrane proteins solubilized by a detergent can be attached
to mercury or carbon electrodes and their reduction and oxidation
responses studied with a protein-modified electrode immersed in a
blank background electrolyte.
410,411
In this way, a transmembrane
protein Na+/K+-ATPase (NKA, a sodium–potassium
pump) was attached to the electrodes and investigated in detail.
410
Nonionic surfactant octaethylene glycol monododecyl
ether (C12E8) was used for solubilization of
NKA. Using C12E8, well-developed peak H, but
not peak S, was obtained at HMDE (Figure 15A). In contrast, water-soluble cytoplasmic
loop C45, which was analyzed
in the absence of C12E8, yielded both peaks
H and S (Figure 15B). It was concluded that
C12E8 interfered with reduction of the Hg–S
bond but not with CHER, responsible for peak H, under the given experimental
conditions. NKA and its C45 loop were detectable at a femtomole level.
At pyrolytic graphite electrodes, oxidation peaks of Tyr and Trp residues
were obtained (see section 4).
Changes
in NKA structure in dependence on ATP binding were studied using adsorptive
transfer and usual CPS at bare HMDE
411
in
the arrangement mentioned above. Peak H shifted to less negative potentials,
and peak area increased with increasing ATP concentration, indicating
better accessibility of proton-donating aa residues to the electrode
surface, most probably resulting from opening of the cytoplasmic part
of the NKA molecule. This behavior agreed well with changes in peak
H of aqueous-soluble riboflavin-binding protein after riboflavin binding.
256
Figure 15
Chronopotentiograms of (A) Na+/K+-ATPase
(NKA) and (B) its C45 loop. CPS experiment at HMDE, concentration
of proteins: (A) 10 μM and (B) 500 nM, t
A of 30 s; supporting electrolyte, Britton–Robinson
buffer, pH 6.5; stripping current I
str, −10 μA. (C) CPS records of 5 μM C45 loop before
(black) and after (red) incubation with cisplatin. (D) Dependence
of peak heights (C45 peak S, C45 peak H2, and cis-Pt) on the concentration
of cisplatin. Concentration of cisplatin (for C) was 40 μM,
activated complex was used for all experiments. EC parameters: supporting
electrolyte 0.2 M phosphate buffer, pH 7.4; t
A of 30 s, I
str of −20 μA.
“*” in inset of panel A: NKA transmembrane part. Adapted
with permission from refs (410) and (412). Copyright 2012 Wiley-VCH Verlag GmbH&Co
and 2012 Elsevier.
The above techniques were utilized
in the investigation of the
effects of cisplatin binding to NKA.
412
Earlier studies indicated that cisplatin could bind to proteins
in blood or in cytosol.
413−417
It was hypothesized that acute kidney failure accompanying cisplatin
administration in cancer therapy might be related to inhibition of
NKA.
418−422
Testing of platinum drugs effects on NKA revealed almost no influence
of carboplatin and oxaliplatin (known to be much less nephrotoxic
than cisplatin) on NKA activity. In contrast, the cisplatin binding
reduced NKA activity by 50%, and this inhibitory effect was substantially
reduced in the presence of 0.5 mM DTT. EC studies of C45 loop showed
that peak S decreased with increasing cisplatin concentration and
its complete disappearance at 2-fold excess of cisplatin over C45,
indicating cisplatin binding to Cys residues localized on the surface
of C45. Disappearance of peak S was accompanied by a formation of
peak H2 (not produced by intact C45), which grew with increasing cisplatin
concentration. However, a decrease of peak H1 with increasing cisplatin
concentration was observed (Figure 15C,D).
EC studies were complemented by matrix-assisted laser desorption/ionization
time-of-flight (MALDI-TOF) MS experiments and determination of free
−SH groups by Ellman’s reagent. It was concluded that
(i) inhibition of NKA activity could be involved in nephrotoxicity
in cisplatin-treated patients, and (ii) cisplatin binds to Cys residues
in C45 loop and particularly to Cys367 close to the phosphorylation
site. The results of the NKA analysis
411
show that CPS analysis can be used not only for studies of structural
changes and intermolecular interactions in water-soluble proteins
at bare
155,230,232,233,238,254,256,322
and thiol-modified electrodes,
105,228,238,239
but also for the investigation
of poorly soluble proteins.
410,411,423
7
DNA–Protein Interactions
Interactions
of proteins with DNA in cells control many aspects
of DNA metabolism ranging from gene transcription, DNA replication,
DNA-damage repair, or chromosome segregation to higher order structure
organization of chromatin. Particularly, transcription factors constitute
typically about 5% of the total number of genes in most species, highlighting
the importance of proper gene expression regulation.
11
Basal transcription factors (e.g., TATA-box binding protein,
TBP) constitute general transcription complexes, which are regulated
by intricate networks of transcription activators, repressors, and
cofactors (e.g., immune responses converge on NF-κB and AP-1
transcription factors). Furthermore, the transcription regulation
can be regulated through master transcription factors (e.g., factors
initiating cell differentiation programs or “genome guardian”
p53).
102
A great number of protein
complexes (e.g., a single-strand binding
(SSB) complex) are involved in DNA replication, DNA repair, and homologous
recombination.
424
Given the high frequency
of endogenous DNA damage, it is of great importance for every cell
to have adequate repair mechanisms to keep them healthy. To deal with
different kinds of base damage, cells are equipped with proteins recognizing
chemically modified DNA and/or structurally altered DNA strands (e.g.,
MutY, MutS, HMG proteins). DNA-binding enzymes like helicases (e.g.,
XPD) and endonucleases (e.g., EndoIII) then assist in damage removal
(e.g., base excision repair (BER) processes). Eventually, DNA polymerases
and ligases finish the job by filling gaps and recovering DNA strands.
425
On the basis of the nature of the damage, timing
(through the cell cycle), and chromatin localization of the damage,
the correct mechanisms must be employed again highlighting the importance
of regulation at multiple levels. Deregulation of the above processes
is deleterious for cells and organisms. Not surprisingly, many of
the above protein–DNA complexes are involved in cell protection
against tumorigenesis, underlining their importance for human health.
It is therefore extremely important to investigate the mechanisms
of DNA–protein binding and the nature of complexes formed between
DNA and proteins. In the past two decades, we witnessed a great expansion
from 100 to about 2500 of high-quality structures of DNA–protein
complexes (including DNA-based aptamers; January 2014 in PDB, http://www.rcsb.org/pdb/;
and in NPIDB, http://npidb.belozersky.msu.ru/).
426
These structures have provided valuable
insights into the principles of DNA–protein binding.
The phosphate backbone of DNA provides a negatively charged surface
for the binding of basic side chains (Lys, Arg) of proteins.
11,427
Such DNA–protein contacts are utilized by structure-specific
(i.e., sequence nonspecific) proteins for DNA structure recognition.
For the sequence-specific binding, these contacts stabilize the protein–DNA
interaction mostly mediated by hydrogen bonds between bases and proteins.
Base pairs inside the double-stranded (ds) DNA are accessible through
either major or minor groove. Most sequence-specific binding proteins
(e.g., transcription factors) approach the major groove (Figure 16A), which can easily
accommodate an α-helix
element of particular protein domain (e.g., helix-turn-helix). However,
crystal structures provided insights also into special DNA–protein
binding modes. For example, some proteins use β-ribbon or loop
elements for major groove binding, and a few proteins bind into the
minor groove. Particularly, TBP binding to minor groove partially
unwinds and kinks DNA (Figure 16B), facilitating
the initial step of transcription.
428
Figure 16
Different
DNA–protein binding modes. (A) Most proteins insert
helix element into the major groove of DNA molecule. For example,
helical bZIP motif of the AP-1 transcription factor binds within the
major groove of the specific DNA sequence (PDB: 1FOS).
881
(B) Some proteins employ, however, β-sheets for their
binding into the minor groove of DNA. For example, TBP protein binds
to minor groove and partially unwinds and kinks DNA (PDB: 1YTB).
428
In addition to the high-resolution
X-ray crystal analysis, a number
of methods have been used in studies of DNA–protein interactions.
57
However, applications of EC analysis to studies
of DNA–protein interactions were until recent years rather
scarce. Considering that both DNA and proteins are electroactive and
can be analyzed with high sensitivity, application of electrochemistry
in DNA–protein analysis appears natural and very promising.
In difference to a large number of reviews on EC analysis of nucleic
acids
15−36
and proteins,
44,155,210,229,263,429−432
comprehensive reviews on electrochemistry of nucleic acid–protein
interactions are missing. Numerous reviews on DNA or ribonucleic acid
(RNA) aptamers
432−442
include interactions of such aptamers with proteins, but usually
changes in EC signals of nucleic acids are measured. In this Review,
nucleic acid and peptide aptamers will be mentioned only in connection
with biomarkers (section 9). This section will
focus on new ways of label-free EC analysis of DNA–protein
interactions, which play significant roles in nature (such as DNA
sequence-specific binding), and in which changes in the protein EC
signals are monitored.
7.1
Early Work
To
our knowledge, the
first EC papers on DNA– or RNA–protein interactions
were published more than 25 years ago. In 1986, the course of RNase
cleavage of (a) RNA immobilized at the electrode and (b) in solution
was investigated by voltammetry.
276
More
than 10 years later, DNA was immobilized at a gold electrode, and
its enzymatic cleavage by deoxyribonuclease I (DNaseI) as well as
formation of RNA duplexes at the electrode surface were monitored
by quartz crystal microbalance.
443
This
work showed that surface-confined DNAs and RNAs could be manipulated
at the gold electrode surface. Almost at the same time, cleavage of
supercoiled DNA by DNaseI in solution and at the mercury electrode
surface was investigated by ac voltammetry.
444
Nicking of circular DNA resulted in a specific peak, which enabled
investigation of the DNA enzymatic cleavage. Kinetics of the DNA cleavage
at the electrode surface showed restricted accessibility of DNA at
the surface as compared to solution cleavage. Cleavage of DNA adsorbed
at positively charged electrode was inhibited, while at negatively
charged electrode stimulation of the cleavage was observed. In all
of these experiments, changes in DNA signals were measured, but in
principle, EC analysis of DNA–protein interactions could rely
also on changes in protein signals. Surprisingly, no attempt was made
to use changes in Brdička’s catalytic response of Cys-containing
proteins for this purpose (section 5.5). This
might be due to alkaline pH used for Brdička’s catalytic
response, which is not favorable for studies of DNA–protein
interactions. In 2005, it was shown that thiol end-labeled DNAs produced
catalytic peaks at potentials less negative than those of Cys-containing
peptide 9-mer [Lys8]-vasopressin in cobalt-containing solutions.
445
Using oligodeoxynucleotide HS-(TCC)7 and adsorptive transfer stripping DPV, this system
was used at pH
7.5 to study H2A histone interaction with DNA,
271
showing a decrease in the DNA signal after the histone
binding. No decrease was observed when DNA and histone were incubated
in 1 M NaCl, preventing DNA histone binding.
Among the first
papers on EC detection of DNA–protein interactions, there was
the method developed by Barton et al.
446
This method was based on dsDNA-mediated charge transport.
18,34,446
Binding of a base flipping enzyme,
MHhaI, to dsDNA greatly decreased the EC signal of DNA labeled with
daunomycin, indicating that this binding disturbed the DNA base stacking.
447
Using a DNA repair enzyme MutY, which binds
to 8-oxoG:A and G:A mismatches, single-base mismatches in DNA were
detected.
446
Shortly afterward, it was
shown that the [4Fe–4S] cluster, contained in MutY, can be
utilized in the detection of MutY–DNA interactions.
448
As compared to free MutY, which shows no EC
signal, the DNA-bound MutY displayed a reversible couple on cyclic
voltammograms at gold electrodes. This method required relatively
high concentrations of MutY (0.8 mM).
448
These experiments marked the beginning of interesting research on
DNA–protein interactions in Barton’s laboratory, which
still continues (see section 7.2).
Almost
at the same time, another technique was proposed by Palecek’s
group,
449
showing that binding of practically
any protein to DNA can be detected using the so-called double-surface
technique (DST) (reviewed in ref (34)) combined with EC detection of the protein electrocatalytic
peak H (section 5.2). In DST, the DNA–protein
interaction or DNA hybridization was performed at one surface (usually
magnetic beads, reviewed in ref (450)) and the EC detection of these events at another
surface (usually metal or carbon electrodes). This technique was proposed
to overcome difficulties in DNA hybridization experiments using complex
biological matrixes. Similarly to the work of Barton et al.,
448
DNA repair protein was used to detect mismatched
bases in DNA
449
but instead of MutY, the
MutS protein was used. MutS plays an important role in the DNA repair
system in prokaryotic and eukaryotic cells,
451−454
recognizing unpaired and mispaired bases in duplex DNA, and this
protein was shown as a useful tool for the detection of point mutations.
449,455−459
In DST experiments, biotinylated DNA was first attached to the magnetic
beads, followed by their incubation with the MutS protein solution
and magnetic separation of the DNA–MutS complexes (including
extensive washing). Next, the protein was dissociated from its DNA
complex and detected electrochemically at mercury electrodes using
peak H (section 5.2). In this way, tens of
attomoles of this protein were detected. The sensitivity of the determination
at carbon electrodes was by 3 orders of magnitude lower.
449
This highly sensitive label-free detection
of MutS protein opened the door to the development of DNA chips for
a high-throughput EC determination of proteins, as well as to the
detection of point mutations and insertions/deletions in DNAs. These
methods represented a new approach in the analysis of nucleic acid–protein
interactions, which could be applied to both DNA and RNA, and to a
large number of nucleic acid-binding proteins. DST methods were also
applied using proteins (e.g., p53) attached to the beads and interacting
with DNA in solution, followed by EC detection of DNA.
460
In 2005, Kerman et al.
461
used single-walled
carbon nanotube (SWCNT)-modified screen-printed carbon electrodes
to investigate interaction of the SSB protein with DNA. Bacterial
SSB protein is a homotetrameric protein playing an important role
in DNA replication, recombination, and repair.
462
It binds selectively to ssDNA and facilitates DNA unwinding
by helicases.
463
Each monomer of the protein
contains 3 Trp and 4 Tyr residues (section 4). DNA was bound to the SWCNT-carbon electrode
via its end amino
link, and the DNA-modified electrode was immersed into the SSB protein
solution (10–50 μg/mL). Binding of the protein to ssDNA
resulted in an appearance of the protein oxidation peak and in a decrease
of the DNA guanine signal. No protein peak resulted from interaction
of double-stranded DNA with the SSB protein solution.
7.2
DNA Charge Transport for DNA–Protein
Sensing
The work of Barton’s group, which started
with MHhaI–DNA interaction studies,
446
has continued up to the present and was reviewed in 2012.
464
Here, only a brief summary will be given, and
recent results will be discussed. From the beginning, this group was
primarily oriented to the merits of the DNA charge transport (CT)
34,465
and only secondarily to the development of a method for detection
and investigation of the DNA–protein interactions. They devoted
their attention to the electronic properties of the DNA double helix
and showed that DNA is an effective conductor of charge and that stacked
bases function as the path of this conductivity.
34,464,465
Electronic coupling of the donor
and acceptor to the π-stack is required, and DNA CT is highly
sensitive to the structural integrity of the stacked base pairs and
can report the integrity changes on a single base pair level. Bases
in the DNA double helix are stacked next to each other with an interplanar
spacing similar to that of graphite, forming thus a conductive path
of overlapping π-orbitals that extends along the DNA helical
axis. Although the merits of the DNA CT were recently reviewed,
34,465
a debate about various aspects of this CT has continued,
466−469
suggesting that DNA CT and experimental arrangement of its EC studies
are far from simple. Intercalation of the redox label, its charge,
and the way of the label tethering to DNA as well as the structure
of the DNA monolayer at the gold electrode surface are among the factors
affecting the DNA CT.
After thorough studies of the DNA CT,
464
Barton asked a question whether repair proteins
could utilize DNA CT in signaling of the DNA damage throughout the
genome. This question became even more attractive considering that
a subset of DNA repair proteins contains redox [4Fe–4S] clusters.
Examining metalloproteins from the BER proteins showed that [4Fe–4S]2+ clusters were
not necessary in the protein folding process.
106
Crystal structures of DNA complexes with two
BER proteins (MutY and EndoIII) revealed the proximity of these clusters
to the DNA backbone.
120,134,470
Studies of these proteins at DNA-modified gold electrodes showed
their midpoint potentials in the region potentially required for a
biological redox switch and suggested much higher affinity for DNA
in their oxidized state.
18,467
These results helped
to answer the above Barton’s question positively and to propose
a model on how the BER proteins could cooperate in searching the genome
and binding DNA to accomplish its eventual repair (Figure 17).
We shall not explain here this model (which
agrees well with the
experimental data of Barton’s group) in greater detail, because
its biological implications are out of the scope of this Review. Of
course, it is an electrochemist’s wish to have a chance to
make conclusions about biological processes in cells based on signals
reflecting the DNA–protein interactions at electrode surfaces.
Yet mostly real cell biology is much more complex than even the highly
complicated electrode surfaces. Nevertheless, Barton’s model
offers the interesting possibility of the DNA repair mechanism, which
deserves further attention.
Figure 17
Model for a DNA-mediated search by repair proteins.
(1) When the
cell undergoes oxidative stress, guanine radicals are formed, triggering
a repair protein to bind DNA. (2) DNA-binding protein is oxidized,
releasing an electron that repairs the guanine radical. (3) Another
repair protein binds to a distant site. As it binds to DNA, there
is a shift in the redox potential of the protein, making it more easily
oxidized. (4) The protein could then send an electron through the
DNA base pair stack that travels to a distally bound protein, scanning
the intervening region for damage. (5) If the base pair stack is intact,
charge transport occurs between proteins. The repair protein that
receives the electron is reduced and dissociates. (6) If a lesion
is present (red), charge transport is attenuated, and the repair proteins
will remain bound in the oxidized form and slowly proceed to the site
of damage. Adapted with permission from ref (464). Copyright 2012 American
Chemical Society.
It is interesting that
searching for damage in the genome is neither
limited to BER glycosylases nor to the presence of [4Fe–4S]
clusters in the proteins. Various [4Fe–4S] cluster-containing
DNA binding proteins not related to DNA repair were identified.
471
Testing of DNA-bound XPD protein (ATP-dependent
helicase from Sulfolobus acidosaldarious) showed the same voltammetric peak potentials
as those obtained
with BER enzymes.
472
Further work showed
that the XPD redox signal responded to addition of ATP, suggesting
that the helicase function can be investigated electrochemically.
The [4Fe–4S] clusters in XPDs from archae appeared as relatively
labile and not necessary to maintain the protein structure. A number
of DNA-binding proteins involved in genome maintenance, such as FancJ,
Dna2, primase, and DNA polymerases, were also shown to contain [4Fe–4S]
clusters.
473,474
The question of whether these
proteins are involved in genome damage signaling has not been answered
yet. The mechanism of bacterial ferritin Dps involvement in DNA protection
was examined.
475
It was shown that protective
effects of Dps vary with iron content of the protein and that the
DNA charge transport may be involved in the mechanism by which Dps
protects the genome of pathogenic bacteria from a distance.
In the studies of DNA–protein interactions based on DNA
CT, relatively high protein concentrations were used, making the methods
poorly competitive, when compared to other methods not relying on
electrochemistry,
57
such as gel electrophoresis.
Typically, a DNA-modified Au surface was prepared by self-assembly
of thiolated DNAs to form a monolayer. Such DNA monolayer should offer
good accessibility of relatively large protein molecules targeting
specific DNA sequences. While certain control over the DNA surface
density was possible, it was difficult to control the dispersion of
DNA molecules within the film, where high density clusters of DNA
molecules could be formed,
476,477
in which the DNA-binding
protein could hardly find its specific sequence or recognize the DNA
structure. Only recently did Furst et al.
478
propose an alternative approach to form a functionalized homogeneous
DNA-free mixed SAM followed by conjugation of DNA to this SAM. They
labeled DNA with a cyclooctyne moiety tethered to the DNA 5′-end,
followed by modification of Au electrodes with an alcohol-terminated
SAM doped with an azide-capped alkyl thiol. This process was followed
by a copper-free click reaction in which cyclooctyne-labeled dsDNAs
were coupled to the film via azide–alkyne cycloaddition. In
this way, the tendency of DNA helices to cluster in the large high
density domains was significantly decreased, allowing the formation
of very low-density DNA monolayers containing as little as 5% of DNA
in the surface layer. The low-density monolayers offered a better
accessibility to interacting protein than conventional high-density
films as demonstrated with detection of TBP binding at the nanomolar
level.
478
Using the two-electrode
platform, this system was recently applied
for the detection of mammalian DNA methyltransferase1 (DNMT1) in human
tumors by Furst et al.
479
Several methods
of EC detection of DNA methylation are available,
480
and some of them are based on differences of restriction
cleavage of nonmethylated and methylated DNA.
481
Because methylation does not affect CT, DNA methylation-sensitive
restriction enzyme was used by Furst et al.
479
to transform the DNA methylation state into an EC signal. The principle
of this method is shown in Figure 18, showing
clearly that methylation prevents DNA restriction cleavage (blue arrow)
and does not thus interfere with the CT, while in the absence of methylation,
restriction cleavage of DNA (red arrow) results in a significant decrease
of DNA at the surface and in diminishing EC signal. It is important
to note that in this method, biopsied tumor tissues and crude colorectal
cancer cell lysates were used, thus making this new approach potentially
applicable in cancer research and clinical testing.
Figure 18
Electrochemical platform
(right) and scheme (left) for the detection
of human methyltransferase activity from crude cell lysates. (right)
The electrochemical detection platform contains two electrode arrays,
each with 15 electrodes (1 mm diameter each) in a 5 × 3 array.
Multiple DNAs are patterned covalently to the substrate electrode
by an electrochemically activated click reaction initiated with the
patterning electrode array. Once a DNA array is established on the
substrate electrode platform, electrocatalytic detection is then performed
from the top patterning/detection electrode. (left) Overview of electrochemical
detection scheme at each electrode of the 5 × 3 array. DNA, patterned
onto the bottom electrode using the copper-activated click chemistry,
is electrocatalytically detected from the top electrode using methylene
blue (MB+) as the electrocatalyst and ferricyanide for amplification.
Crude cell lysate is then added to the surface containing the patterned
DNA. If methyltransferase (green) is present (blue arrows), the hemimethylated
DNA on the electrode is methylated (green dot) by the methyltransferase
to a fully methylated duplex; if methyltransferase is not present
(red arrows), the hemimethylated DNA is not further methylated. A
methylation-specific restriction enzyme, BssHII (purple), is then
added. If the DNA is fully methylated (blue arrows), the electrochemical
signal remains protected, and the DNA is not cleaved. However, if
the DNA remains hemimethylated (red arrows), it is cut by the restriction
enzyme, and the electrocatalytic signal associated with MB+ binding
to DNA is diminished significantly. Adapted with permission from ref (479). Copyright
2014 Proceedings
of the National Academy of Sciences of the United States of America.
7.3
Electrochemical
Impedance Spectroscopy (EIS)
In the field of biosensing,
EIS is particularly well suited for
the detection of binding events on the electrode/transducer surface
(reviewed in refs (482) and (483) and in
section 8.5.1). This method was recently applied
for a label-free detection of DNA–protein binding using thiolated
DNA bound to gold electrodes, backfilled either with 6-mercapto-1-hexanol
484
or with 4-mercapto-1-butanol.
485
EIS was measured in the presence of K4Fe(CN)6/K3Fe(CN)6. Chang and Li
484
investigated the TATA-box specific binding
of TBP, known to use its β-sheet domain to bind the DNA minor
groove (Figure 16B).
486
They observed a significant impedance increase and obtained high
sensitivity (down to 0.8 nM TBP) and a good specificity. They showed
that formation of a DNA triplex (instead of duplex DNA) destabilized
the DNA–protein complex, while binding of daunomycin to DNA
duplex completely abolished the TBP binding to DNA. The presence of
5 μM BSA did not interfere with TBP binding.
Tersch and
Lisdat
485
studied the binding of a transcription
factor NF-κB to the DNA recognition sequence GGGRNNYYCC (where
R stands for purine and Y for a pyrimidine base). They found that
incubation of the 25-mer DNA-modified electrode with 33 μg/mL
NF-κB resulted in a decreased charge transfer resistance, while
capacitance of the DNA–protein layer decreased only slightly.
In the same experiment, in which DNA without the recognition sequence
was used, no change in the impedance of the electrode was observed.
In addition, cleavage of dsDNA by the restriction enzyme BamHI at the electrode was
studied using EIS and CV with methylene blue
end-labeled DNA. It was necessary to remove DTT (usually present in BamHI solutions)
because it strongly affected the surface
impedance. After treatment of dsDNA containing the restriction sequence
(GGATCC), the charge transfer resistance strongly decreased in agreement
with removal of the enzyme cleaved DNA fragments from the electrode.
Similarly, CV reflected the DNA restriction cleavage by a decrease
of the methylene blue peaks.
Tersch and Lisdat
485
did not consider
earlier results of Chang and Li,
484
and
we can only speculate about reasons of differences in their data.
Figure 19A shows a relatively small decrease
of the charge transfer resistance after binding of the NF-κB
transcription factor to dsDNA, contrasting a large increase in the
charge transfer resistance after TBP binding to DNA (Figure 19B), observed by Chang
and Li.
484
Experimental arrangements in Figure 19A and B somewhat differed: (a) screening the
electrode with a different
thiol and interference of DTT reported only in Tersch and Lisdat work;
485
and (b) different potentials under which the
EIS measurements were performed: Chang and Li measured at 0.2 V, while
Tersch and Lisdat at open circuit potential. However, these differences
need not represent the only reason for the very large difference between
the EIS published by these authors (Figure 19). It is not excluded that the DNA binding
modes and different properties
of the TBP and NF-κB proteins contributed to the different results
of their EIS measurements. The NF-κB p50 transcription factor
recognizes specific DNA sequence through the major groove binding.
In addition, a number of phosphate contacts stabilize the complex
and embrace the DNA molecule inside the protein wrap. Tersch and Lisdat
considered neutralization of this phosphate wrapping being behind
the signal decrease.
485
In contrast, the
TBP binds DNA in an unusual way through the minor groove using a β-sheet
motif (Figure 16B). The minor groove binding
induces bending of the DNA by 80° and facilitates base pair melting.
The increased impedance signal might be thus attributed to the exposition
of bases. Such a conclusion would imply additional benefit of the
EIS analysis, that is, the ability to reflect the protein–DNA
binding mode. More work will be however necessary, including additional
control experiments, to arrive to a more definite conclusion and to
show what are the advantages and drawbacks of application of EIS to
DNA–protein interaction studies.
Figure 19
(A) Impedance spectra
of a gold electrode with NF-κB-specific
dsDNA before (a) and after (b) incubation with 33 μg/mL NF-κB
p50, as measured by Tersch and Lisdat.
485
(B) Nyquist plots of gold electrode modified with DNA duplexes before
and after interaction with BSA and TBP, as reported by Chang and Li.
484
Electrolyte: 10 mM PBS (pH 7.0) with 10 mM
[Fe(CN)6]3–/4– and 10 mM NaCl.
Frequency: from 0.1 Hz to 100 k Hz. Amplitude: 10 mV. Bias potential:
0.20 V. Adapted with permission from refs (485) and (484). Copyright 2011 and 2009
Elsevier.
7.4
E-DNA Sensors
E-DNA sensor was developed
about 10 years ago in the laboratory of A. Heeger
487
(reviewed in refs (25) and (34)) as a tool for the DNA nucleotide sequence analysis.
Later, it was
shown that this sensor could be used also as an aptamer-based, sensor
detecting low molecular weight compounds and some proteins.
488
Recently, it was shown that principles of these
sensors could be used also in studies of naturally occurring DNA-binding
proteins. The sensor is composed of a ds or single-stranded (ss) DNA
probe, which is redox-labeled and covalently attached to an electrode.
Upon protein binding, the signal produced by the redox label was attenuated,
which was explained by reduction of the collision efficiency between
the DNA probe label and the electrode by the bulky structure of the
DNA-bound protein. Binding of TBP and MhaI methyltransferase to their
specific recognition sequences in dsDNA, as well as bindings of SSB
protein from E. coli and eukaryotic
replication protein A to ssDNAs, were studied.
496
The relative decrease of the label signal was rather small
with short DNA probes (e.g., 20 bases) and increased significantly
with the probe length (up to 70 bases). However, the absolute value
of the label signal decreased with increasing probe length. It is
generally accepted that DNA probe density at the electrode surface
is important for the probe accessibility in different DNA interactions,
34,489−494
and it may be particularly critical in DNA binding to bulky protein
molecules.
495,496
Accessibility of surface-attached
DNA probes for protein interactions was tested using DNase I. As a
result of the DNA enzymatic cleavage, the label was removed from the
DNA probe and the signal decreased. It was shown that the accessibility
of the DNA probes was (as expected) improved at lower probe densities.
495
Yet even at the lowest densities investigated,
a significant fraction of dsDNA probes remained inaccessible to the
enzymatic cleavage. This might be due to formation of high-density
clusters of dsDNA (see section 7.2). By linking
an antigenic peptide epitope to the DNA probe, the E-DNA sensor supported
detection of antibodies.
497−499
In this way, proteins which
do not bind DNA, can be detected by E-DNA sensor. This principle
was recently applied for detection of protein IP-10, a biomarker (Interferon-γ
inducible Protein-10 kDa or CXCL 10) for the diagnosis of inflammation.
500
To detect this protein, methylene blue-labeled
ssDNA was attached to Au electrode via a thiol group. This DNA was
hybridized with peptide nucleic acid to which a 21-aa peptide binding
element was grafted to recognize and bind IP-10 protein. The binding
event was indicated by a decrease of a signal from the DNA label.
The sensor worked in undiluted blood serum and did not require repetitive
washing, showing properties that might be utilized in point-of-care
diagnostics. The probability that, in E-sensor, redox impurities from
the biological materials can give false positive signal is rather
low. To produce such a signal, the impurity should remain firmly bound
to the electrode surface resisting washing, and its response should
appear at the same potentials as that of the DNA label.
7.5
CPS Peak H in Sensing of DNA–p53 Interaction
In the above methods, either changes in signals of the DNA label
were measured or DNA itself served for the charge transfer through
its duplex structure. Here, we will show that changes in CPS peak
H can also indicate DNA–protein binding and provide information
about the stability of the DNA–protein complex.
The tumor
suppressor protein p53 binds DNA by several modes,
501,502
and the sequence-specific binding of the p53 core domain (p53CD)
to the dsDNA consensus (DNACON) site (involved in apoptosis
and cell cycle arrest) was most intensively studied.
503,504
This sequence contains two copies of the half-site 10-base pair
motif RRRC(A/T)|(T/A)GYYY separated by up to 13 bp (vertical bar indicates
the center of symmetry within the half site). The crystal structure
showed binding of four p53 molecules to the DNACON sequence.
503,504
Such binding caused bending and twisting of the DNA.
505
It was found that wt and mutant p53CD produced
CPS peak H at DTT-modified mercury electrodes, sensitively reflecting
changes in the protein structure (section 6.2).
105
The CPS peak H was utilized for
investigation of the p53CD–DNA sequence specific binding,
240
using the same arrangement as in free p53CD
studies
105
(section 6.2). The p53–DNA complex was prepared in the test tube
using a usual DNA binding buffer
459
and
adsorbed at HMDE or SAE followed by the electrode washing and CPS
recording with I
str −35 μA.
240
The sequence-specific binding resulted in a
disappearance or decrease of the p53CD peak H (Figure 20A). In contrast, the peak
H was almost not influenced by p53CD
interaction with a nonspecific DNA, not containing the consensus sequence
(DNANON). Similarly, the complex of mutant protein R273H
406
(not binding to DNA) displayed almost the same
peak H as the free protein (Figure 20B). Removal
of zinc from the wt p53CD (known to inhibit the DNA binding)
105,406
resulted in the same effect.
The decrease in the peak H height,
resulting from the p53 binding
to DNA, was related to changes in the accessibility of electrocatalytically
active aa residues (particularly Lys, Arg, His, and Cys) at the electrode
surface.
155,170,264
Such changes may cause more difficult electrocatalysis, which can
result in shifting of the peak H to more negative potentials and eventually
can lead to almost complete disappearance of this peak.
Figure 20
(A) Sequence-specific
binding of wild-type p53 core domain (p53CD)
to dsDNACON as detected by CPS at a DTT-modified hanging
mercury drop electrode (DTT-HMDE) using I
str −35 μA at 21 °C. Free p53CD (black), sequence-specific
p53CD–DNACON complex (red), and a mixture of p53CD
with 40-mer dsDNA not containing the consensus sequence (blue, p53CD
+ dsDNANON). (B) Interaction of mutant p53CD R273H with
dsDNACON (showing no DNA binding). Free p53CD R273H (black),
p53CD R273H + dsDNACON (red), p53CD R273H + dsDNANON (blue). (C) Peak H of p53CD (black)
and p53CD complexes with spacer-containing
DNAs: DNACON–GC (green), DNACON–AT (blue), and DNACON–ATAT (magenta); I
str −35 μA at 21 °C. (D–F) Peak
H of p53CD (black), p53CD–DNACON complex (red),
and p53CD–DNANON (blue) at I
str of (D) −25 μA, (E) −40 μA, and
(F) −50 μA. (G) Dependence of peak H1 height of free
p53CD (black), p53CD–DNACON complex (red), and p53CD–DNANON (blue) on stripping current
(−I
str). Adapted with permission from ref (240). Copyright 2014 Elsevier.
High sensitivity of peak H for
the p53CD sequence-specific DNA
binding was observed only at a sufficiently negative I
str intensity (Figure 20A,E,F).
For example, I
str
–35 μA was sufficient for discrimination between a sequence-specific
and a much weaker nonspecific p53CD binding (Figure 20A), while at I
str
–25 μA, such discrimination was not possible (Figure 20D). Under these conditions,
even the stable sequence-specific
complex was disintegrated at the electrode surface, producing almost
the same CPS response as free p53 protein. Nonspecific binding of
p53 to DNANON resulted in a similar response even at a
much higher I
str intensities (−40 μA; Figure 20E),
but further increase of the –I
str intensities displayed a significant difference between the CPS responses
of free p53 and p53–DNANON (Figure 20F), suggesting that even the weakly bound nonspecific
complex
was not completely disintegrated at the electrode surface, because
of very short exposure to negative potentials. Earlier, it was shown
that the DNA sequence and structure could modulate the affinity of
p53 for its binding sites
449,506
and that DNA sequences
involved in a cell cycle arrest do not have spacers interspersed between
the two half-sites and bind usually p53 with a higher affinity
507,508
than DNA sequences involved in apoptosis and containing interspersed
spacers between the two half-sites. Figure 20C displays CPS responses of free p53CD
and compares them to the three
different p53 sequence-specific complexes with spacer-containing DNAs.
These data suggest that the DNA containing the longest ATAT spacer
produced the CPS response closest to that of free p53 protein.
It appears that by controlling the temperature and/or I
str intensities in CPS, it will be possible to make a
conclusion about the stabilities of different surface-attached p53CD–DNA
complexes on the ground of their susceptibility to the electric field
effects acting at the interface. Recently, DNACON bearing
acrylamide or vinylsulfonamide groups covalently cross-linked with
Cys residue of p53CD was prepared.
509
A
comparison of EC and other properties of p53CD–DNA complexes
with covalently bound versus noncovalently bound DNACON appears of utmost interest.
7.6
p53–DNA
Sequence-Specific Binding As
Detected by Signals of Labeled DNA
The double-surface technique
(DST) mentioned in section 7.1 was used for
the detection of p53 protein sequence-specific binding to DNA. To
increase the sensitivity of the detection, labeled DNAs were used.
510−513
Tail-labeling of DNA with many nitrophenyl tags
510
resulted in a significant increase in sensitivity as compared
to previous label-free DNA analysis.
460,514
However,
this approach required an organic chemistry laboratory with highly
qualified personnel. Recently, a much simpler DNA labeling technique,
based on covalent binding of Os(VIII)bipy complex to thymine residues
in a 20-mer tail, was used.
460
Using the
electrocatalytic adsorptive transfer stripping DPV, nanogram and subnanogram
amounts of the DNA–Os(VIII)bipy adduct were determined. In
competition experiments, relative affinities of different DNACON to the immobilized
p53 protein were tested. DNA labeling
consisted of mixing DNA with Os(VIII)bipy and dialysis after 2 h of
reaction time at room temperature. DST with immobilized protein or
DNA
449,460
can be used practically to investigate any
DNA (or RNA)–protein interactions. In its present form, this
technique is laborious and not easy to use for highly parallel analysis.
The combination of DST with microfluidic technique would help to overcome
these problems.
7.7
Concluding Remarks
In the recent
decade, electrochemistry significantly extended the field of DNA–protein
interactions. Not only were new EC methods for DNA–protein
binding detection developed,
112,464,466−469,484,485
but also new approaches were offered including interesting views
of DNA-mediated search by repair proteins
464
and testing of DNA–protein complex stability at electrified
interfaces.
240
It can be expected that
at least some of the EC methods and approaches will be further developed
and new ones will appear. Great potentiality of EC analysis as a new
tool for DNA–protein interaction studies will be utilized,
and this analysis will become a useful technique among a number of
well-established traditional methods. Investigation of multiprotein–DNA
complexes playing important roles in transcription mechanisms
515
and chromatin structure dynamics
516
may represent a challenge for bioelectrochemists
in the coming decades.
8
Electrochemical Analysis
of Glycoproteins
8.1
Glycomics
Analysis
of precisely controlled
post-translational modifications of proteins,
517
often with addition of only a small functionality (e.g.,
single phosphorylation), which can change activity of a kinase up
to 108 times,
518
is a big challenge
for analytical chemistry at present. Enzymatic addition of a glycan
(i.e., oligosaccharide chain covalently bound to proteins or lipids)
in a process of glycosylation is a highly abundant form of co- and
post-translational modification of proteins. Approximately 70% of
cytosolic proteins and about 80% of membrane-bound human proteins
are estimated to be glycosylated.
519
Glycans
take active part in many physiological processes,
37,520−525
but they are also involved in pathological processes as well, triggered
by adhesion of different pathogens to host tissues, in neurological
disorders and in cancer.
526−534
Better understanding of glycan involvement in pathological processes
may be thus essential to cure diseases.
535−542
Glycan changes can be used in early stage diagnostics of numerous
diseases, including various types of cancer with already approved
biomarkers.
534,543−549
Moreover, many approaches previously applied for treatment of diseases
are currently being revisited, taking into account glycan biorecognition
to achieve high efficiency, low side effects, high serum half-life,
or low cellular toxicity of drugs/therapeutics.
526,550−553
The first antibody with controlled glycan composition was already
launched to the pharmaceutical market, an achievement glorified by
the authors as “a triumph for glyco-engineering”.
554
Glycomics is aimed to reveal finely tuned
biorecognition of glycans in the cells based on graded affinity, avidity,
and multivalency of glycans.
555,556
Glycans are preferential
molecules for coding/storing biological information, because the number
of possible unique sequences as compared to nucleic acids or proteins
is staggering (Figure 21).
557,558
The theoretical number of glycan hexamers is 8 orders larger in
comparison to peptide hexamers and 11 orders of magnitude larger than
nucleic acid sequences.
559
The eukaryotic
cellular glycome can contain up to 500 000 glycan containing
biomolecules built from a pool of more than 5000 unique glycans.
556,560
This glycan variation might be behind complexity of human phenotype,
even though there is a relatively small human genome. Complexicity
of glycans together with their similar physicochemical properties
are the main reasons why progress in glycomics has lagged behind advances
in analysis of DNA and proteins.
561
Glycans
can be formed from monosaccharide units with a different bond between
them (branching). For example, sialic acids can be attached to galactose
in three different ways (2–3, 2–6, or 2–8 bond).
Figure 21
Graphical
representation of complexity of glycans, showing variability
of sugar building blocks, multiple branching, and attachment points.
Adapted with permission from ref (37). Copyright 2013 Springer Science and Business
Media.
The most important current analytical
method for structural glycoprofiling
of proteins is MS relying on different ionization techniques, mass
analyzers, and detection platforms.
12,562−565
The combination of MS with separation techniques including a diverse
range of chromatographic approaches (reversed-phase, hydrophilic,
graphitized carbon-based, and lectin affinity modes), gel electrophoresis
and capillary electrophoresis, and integration of various glycan modifications/labeling
steps has been successfully applied in the identification of glycan
isomers, glycosylation sites, and in revealing glycan microheterogeneity.
566,567
Moreover, such instrumental machinery can elucidate not only the
structure of a glycan, but it can also characterize the corresponding
protein. Instrumental techniques have been successfully applied in
better understanding of progression of various forms of cancer or
for the identification of new prospective glycan-containing biomarkers.
566,568−571
Lectins are a heterogeneous group of proteins recognizing
free
or bound mono- and oligosaccharides or whole cells (Table 1).
572,573
Lectins are not catalytically
active nor involved in the immune system of higher organisms and have
been found in viruses, bacteria, fungi, plants, and animals exhibiting
similar sequence motifs. The lectin binding site is very often a shallow
groove or a pocket displayed at the protein surface.
519
Mostly four main aa residues are part of a biorecognition
site in lectins such as asparagine, Asp, glycine (Arg in Con A), and
an aromatic residue responsible for interaction with glycan via hydrogen
and hydrophobic bonds.
574
Ionic interactions
are involved in the biorecognition of negatively charged glycans containing
sialic acids. Lectins as natural glycocode readers
559
are popularly applied in a wide range of traditional
575−577
and advanced sensing protocols.
37,38,40,41,527,529,578
Lectins are traditionally isolated from natural sources, but nowadays
a recombinant DNA technology helps to produce lectins with high purity.
527,578
Table 1
Specificity, Source, and Other Properties
of the Most Commonly Used Lectinsa
lectin
specificity
source
source type
M
w (kDa)
pI
SUc
glycoprotein
metal ions
AAL
α-l-Fuc
Aleuria aurantia
F
72
9
2
Con
A
α-d-Man, α-d-Glc
Canavalia
ensiformis
P
104
6.3–7
4
Ca2+, Mn2+
MAA-I
Gal-β-(1–4)-
GlcNAc
Maackia amurensis
P
130
4.7
2
yes
MAA-II
Neu5Ac-α-(2–3)β-Gal
Maackia amurensis
P
130
4.7
2
yes
PHA-L
branched β-(1–6)GlcNAc
Phaseolus
vulgaris
P
126
4.2–4.8
4
yes
Ca2+, Mn2+
PHA-E
oligo Man
Phaseolus
vulgaris
P
126
6–8
4
yes
Ca2+, Mn2+
SNA-Ib
Neu5Ac-α-(2–6)Gal
Sambucus
nigra
P
140
5.4–5.8
4
yes
SNA-II*
Gal, GalNAc
Sambucus
nigra
P
RCA-I/RCA120
β-d-Gal
Ricinus communis
P
120
7.8
2
yes
RCA-II/RCA60
Gal-β-(1–4)-
GalNAc
Ricinus communis
P
60
7.1
1
yes
UEA-I
α-l-Fuc
Ulex europaeus
P
63
4.5–5.1
2
yes
Ca2+, Mn2+, Zn2+
WGA
β-d-GlcNAc
Triticum
vulgaris
P
36
>9
2
no
Ca2+
a
Table based on ref (888).
b
SNA (EBL-elderberry bark lectin)
is often available only as SNA without any specification, despite
the different binding abilities of the two isolectins.
c
F, fungi; P, plant; SU, subunits;
Gal, galactose; GalNAc, N-acetylgalactosamine; Glc,
glucose; GlcNAc, N-acetylglucosamine; Fuc, fucose;
Man, mannose; Neu5Ac, N-acetylneuraminic acid.
Lectins as natural glycan-recognizing
proteins
559,579,580
can be very helpful to fractionate
or preconcentrate glycoproteins from other nonglycosylated proteins,
566
which is essential for robust glycan measurements.
Lectins of course can be applied in glycoprofiling in a direct way,
that is, to be integrated into a microarray format of analysis with
a fluorescent reading.
527
An emerging lectin
microarray-based technology offers highly parallel analysis with a
minute sample consumption desperately needed for cost-effective monitoring
of glycan changes.
581,582
Therefore, direct analysis of
intact glycoproteins/glycolipids and glycans on the surface of cells
is possible.
530,583−585
Moreover, this technology is routinely used in the analysis of glycan
changes as a result of numerous diseases or for the discovery of new
biomarkers. This technology can be thus complementary to the approach
based on MS-based glycan analysis.
529,584
Furthermore,
the glycomic data set obtained from lectin microarrays together with
microRNA data confirmed the involvement of microRNA in regulation
of expression of glycan processing enzymes.
586
Lectin microarrays are suitable for obtaining preliminary information
about changed glycosylation rather than providing detailed information
about the exact glycan structure/composition that can be acquired
only by sophisticated instrumental analysis. Another drawback of lectin
microarrays with fluorescent detection is a requirement to have lectins
or a sample fluorescently labeled, which is an additional step resulting
in analysis variability due to an uneven labeling efficiency of the
molecules.
527,529
Moreover, nonviable cells were
found to exhibit stronger fluorescence as compared to viable ones,
when assayed on lectin microarrays and fluorescent labeling of cells
affected a biorecognition process on microarrays.
527,529
Besides a requirement to use labels with lectin microarrays, this
technique can not detect a low level of glycoproteins with a typical
LOD in the sub nanomolar or low nanomolar level and offers only a
narrow working concentration window.
Thus, alternative and more
sensitive routes for glycoprofiling
with application of lectins in combination with various transducing
schemes are being sought.
528,576,577
Application of nanotechnology, advanced surface modification protocols,
and novel transducing schemes will help to overcome limitations of
lectin microarrays. In particular, utilization of nanomaterials has
a beneficial effect on sensitivity, detection limit, working concentration
window of the device, and, in certain cases, also label-free mode
of operation, and a real-time monitoring of a biorecognition process
may be feasible.
38−41,520,587−590
Advanced and highly sensitive detection schemes are therefore needed
to detect very low concentrations of glycoproteins without a requirement
to release glycans from the protein prior to analysis. Moreover, label-free
methods for glycoprofiling are of particular interest, because the
binding event is not compromised by the label. All of these requirements
can be addressed by utilization of EC detection platforms offering
low LOD, simple miniaturization, and integration into numerous multiplexed
formats of analysis. In this section, a short historical overview
of analysis of glycoproteins by various EC techniques, fulfilling
“must-have attributes” of glycoprofiling, is provided
together with application of glycoprofiling as a tool for diagnosis
of various diseases.
8.2
EC Analysis of Glycoproteins
EC detection
of glycoproteins either in the isolated state or present directly
on the surface of various types of cells is the field developing at
a tremendous pace with a focus to address limitations of the well-established
technique of lectin microarrays for glycoprofiling.
37,527,529
Section 8 provides a summary of glycan detection by EC lectin biosensors in
various perspective label-free formats of analysis (EIS and capacitance-based
detection) together with numerous reports relying on application of
various labels. Moreover, approaches in which “artificial lectins”
based on boronate derivatives were used are provided as well. A novel
method of carbohydrate covalent modification by Os complexes and application
of catalytic hydrogen evolution reactions in carbohydrate analysis
are discussed. Additionally, analysis of a wide range of glycoprotein
biomarkers by EC methods is extensively covered (section 9.5). This section provides
the first comprehensive
summary of the diverse range of possibilities that the EC detection
platform can offer in glycan/carbohydrate analysis. The subject was
previously partially covered with emphasis given on pulsed amperometric
detection of carbohydrates released from glycoproteins.
591
Short reviews were published with a focus on
EIS-based glycan biosensing
37−39
or pioneering work (until the
end of the year 2010) in EC glycobiosensing.
41
The most recent review only partly covered the subject by referring
to only four relevant papers.
42
8.3
Early Studies of Glycoproteins
In
this section, only studies based on a (bio)recognition of a carbohydrate
moiety on the glycoprotein surface by lectins and boronate derivatives
integrated with EC detection will be discussed. A historical perspective
gives an overview about the application of EC detection in the field
of glycomics from the early 1980s. Analysis of mono-/oligosaccharides
and detection of glycopeptides with some pretreatment steps are provided.
Moreover, analysis of intact glycoproteins and determination of glycoproteins
directly on the surface of various cells are described.
8.3.1
Analysis of Mono-/Oligosaccharides Released
from Glycoproteins
Glycoproteins were detected in the past
by a combination of various techniques. The first step was a cleavage
of a glycan moiety from a protein or a peptide backbone either enzymatically
(Figure 22) or chemically.
592
The second step was separation of carbohydrates released
from the glycan by chromatographic, electrophoretic, and other techniques.
592
The final step was detection of separated carbohydrates
by fluorescent or EC means.
593
The main
advantage of using EC detection of saccharides was a direct detection
without any labeling needed, and thus a simpler, faster, and more
efficient carbohydrate analysis was possible as compared to fluorescent/optical
reading.
49
In addition, EC analysis is
among the most cost-effective and flexible analytical tools with instrumentation,
which can be miniaturized and integrated with various separation techniques
(i.e., capillary zone electrophoresis).
49,593
There are
intrinsic limitations of this approach including possible epimerization
and degradation of carbohydrates at elevated pH needed for EC analysis
and a need to know molar response for each carbohydrate to be assayed.
Moreover, the method requires quite pure carbohydrates free from aa
residues, peptides, and organic acids, to avoid interference with
EC detection.
594
Thus, the EC response
to a particular carbohydrate had to be known in advance by quantification
of carbohydrate standards, complicating acquisition of reliable data.
594
Figure 22
Enzymatic release of whole glycans or carbohydrates
from glycoproteins.
Abbreviation of enzymes: endo-β-GlcNAc-ase, endo-β-N-acetylglucosaminidase (endo H);
PNGase F, peptide:N-glycosidase F (PNGase F); α-Man-ase, α-mannosidase;
β-Man-ase, β-mannosidase; NANA-ase, N-acetylneuraminic acid hydrolase (sialidase);
β-Gal-ase, β-galactosidase;
and β-HexNac-ase, β-N-acetylhexosaaminidase.
EC analysis of carbohydrates in
a reliable way was challenging
for some time due to unwanted adsorption of products of carbohydrate
oxidation on the electrode surface. More specifically, carbohydrates
undergo electrocatalytic oxidation triggered by surface gold oxides
generated electrochemically. In the O-transfer reaction, multiple
electrons are exchanged between the electrode and carbohydrate with
production of formate and smaller aldaric acids. Radical intermediates,
formed through interfacial electrode reaction with carbohydrates,
can rapidly foul the electrode.
49
This
problem was effectively solved in 1981, when Hughes and Johnson first
introduced a pulsed amperometric detection (PAD) of carbohydrates
at Pt electrodes using a triple-pulse potential waveform allowing
detection of carbohydrates with frequency of at least 1 Hz.
595
The technique further developed by Johnson’s
group was soon
applied in carbohydrate analysis using a commercially available instrumentation.
596
The combination of a PAD detector with high-performance
liquid chromatography offered a LOD for mono- and disaccharides at
picomole levels, and its integration with capillary zone electrophoresis
further reduced LOD down to femtomole range
49,597
with a sample volume of 5–100 μL.
591,595,598
Implementation of EC detection
with capillary zone electrophoresis, however, required decoupling
of the capillary zone electrophoresis and EC electronics to avoid
electronic interference.
599
Currently,
EC analysis of carbohydrates is performed on electrodes made from
transition metals (Cu, Ni, Co, and Ru) at a constant potential, lowering
a risk for surface poisoning by carbohydrate oxidation byproducts.
49
To better understand the structure of
a glycan present on the surface
of a glycoprotein, it was very important to identify glycosylation
sites on the protein surface, to elucidate primary structure of the
glycan, to determine the position of glycoside bonds, etc.
49
This is why carbohydrate analysis by PAD had
to be complemented with other instrumentation including nuclear magnetic
resonance and an array of various techniques of MS and chromatography.
12,565,592,600
A quadruple-potential waveform was introduced improving a long-term
reproducibility of glycan analysis,
601
when
PAD was coupled with chromatographic separation.
591
Despite continual effort in using PAD with separation techniques,
the current instrumentation of choice is MS combined with liquid chromatography
based on fluorescent detection or capillary electrophoresis-based
laser-induced fluorescent detection (section 8.1).
570
Biochemical characterization
of glycans by enzymatic hydrolysis
played a key role in revealing carbohydrate structure. For example,
carbohydrates can be cleaved from the glycan structure one by one,
that is, through the removal of terminal sialic acid by sialidase
602
followed by removal of newly exposed galactose
by β-galactosidase,
603
and so on
598
(Figure 22). The other
way is to release an intact glycan from the protein en-block using
glycopeptidases, that is, endo-β-N-acetylglucosaminidase
(endo H) or peptide:N-glycosidase F (PNGase F).
594,604,605
Both of these enzymes have the
ability to provide information about the glycan structure, in which
endo H hydrolyzes the linkage between the two core N-acetylglucosamine
moieties in a high-mannose or hybrid glycan, while PNGase F cleaves
the β-aspartylglucosaminyl bond of all known types of N-linked
oligosaccharides (see Figure 22). Moreover,
deglycosylation with PNGase F converts Asn residue to Asp, leading
to a change in the mass detectable by MS.
605
Usefulness of enzymes in structural characterization of released
complex carbohydrates from the protein/peptide was proved by application
of an enzyme array in digestion of a glycan with released monosaccharides
detected by PAD.
598,604
This approach has some limitations,
such as a limited number of enzymes requiring high purity, problem
distinguishing between similar mono-/oligosaccharides, and a need
to work with pure carbohydrates rather than with complex mixtures.
599,606
The O-linked carbohydrates are enzymatically released from the protein
using Pronase digestion, that is, by a mixture of several proteases
leaving glycan attached only to a single serine or threonine residue.
It is worth mentioning that application of enzymes in glycan cleavage
can be expensive, when extensive characterization of glycans has to
be performed,
570
and thus cost-effective
methods are continuously evolving.
12,566,570
8.3.2
Detection of Glycopeptides
Cleaved from
Glycoproteins
The first study that focused on the analysis
of glycopeptides by PAD was published in 1988 by Hardy and Townsend,
when a glycoprotein fetuin was cleaved into glycan-containing peptides.
607
Quantitative analysis of glycopeptides by PAD
was successfully validated by nuclear magnetic resonance.
607
In the next study, PAD analysis of intact carbohydrates
linked to asparagine was performed, and further information about
a carbohydrate composition was revealed using enzymes.
608
An alternative to PAD analysis of carbohydrates
attached to biomolecules is a sinusoidal voltammetry,
609,610
which can be compared to oscillographic polarography.
611
EC analysis of carbohydrate still attached
to a short peptide was successfully applied for glycoprofiling of
pure glycoprotein (a coagulation factor VII) produced by a recombinant
technology.
612
8.3.3
Analysis
of Intact Glycoproteins
Lectins (see section 8.4.1) are well suited
to get preliminary information about spatial organization of a glycan
moiety on the glycoproteins without glycan liberation.
573,613,614
Thus, it is logical and practical
to rely on their function to recognize glycans on glycoproteins in
their natural, intact form. For example, 26 different lectins recognizing N-acetylgalactosamine
were used in glycoprofiling of samples
from patients suffering from breast cancer and showed subtle differences
in the structure of complex glycans.
600
Although antibodies raised against glycan moieties can detect glycans
on glycoproteins with higher affinity and specificity as compared
to lectins,
529,535,537
their application in direct glycoprofiling of proteins is questionable
due to their limited commercial availability.
600
However, when available, they can be effective in early
cancer detection.
599,615,616
Most importantly, carbohydrate arrays can assist in finding novel
glycan specificities of already utilized antibodies and lectins.
599,617−621
Because lectins are almost exclusively applied in direct glycoprofiling
of glycoproteins, details about their integration into EC formats
of detection will be discussed in the forthcoming sections.
8.3.4
Glycoprofiling of Intact Cells
The first application
of glycoprofiling of intact cells was published
in 2001 with two completely different approaches. In the first one,
Luong’s group applied EIS to study the attachment and growth
of insect Spodoptera frugiperda Sf9
cells on eight modified gold electrodes present at the bottom of tissue
culture wells.
622
Because the cells were
cultivated in the wells for up to 20 h, a change in impedance was
not attributed only to the cell density on the electrode, but to the
products of a metabolic activity as well. The experimental setup allowed
one to monitor the growth inhibitory effect of key explosives with
inhibitory constants calculated.
622
In the second approach, a very interesting strategy was introduced,
623
which involved a selective binding of six types
of microbial cells to surfaces modified by 10 different lectins via
glycans with subsequent EC monitoring of their respiratory activity
by chronocoulometry. In the first step, the lectins determine the
amount and cell types attached to the membrane. In the second step,
the amount of bound viable cells was chronocoulometrically detected
in the presence of two redox probes, menadione (reaching intracellular
redox enzymes) and ferricyanide (a shuttle between menadione and the
electrode). During the measurement, formate and succinate, substrates
of enzymes involved in a respiratory chain, were present (Figure 23). This EC investigation
is universal and can be
applied to a diverse range of cells. Principal component analysis
of the results showed that it is possible to distinguish between all
microbial species tested in the study on an array of electrodes.
623
Figure 23
Schematic representation of EC detection of
cells with lectins
by monitoring of respiratory activity of the cells with a two-mediator
system.
Later, the authors extended this
approach to distinguish between
four different E. coli subspecies on
an array of 32 screen-printed carbon electrodes.
624
Again, a principal component analysis helped to distinguish
between all four E. coli subspecies
based on differences in a respiratory activity after being bound to
lectins.
624
Moreover, mainly label-based
approaches on how to detect intact cells of a different origin and
status are discussed in section 9.5.5.
8.4
Biorecognition Molecules
8.4.1
Lectins
– Useful Components for Glycoprofiling
of Proteins
Lectins are proteins binding to free or bound
mono- and oligosaccharides or whole cells. A binding site of lectin
is a shallow groove or a pocket displayed at the protein surface with
involvement of four main aa residues such as asparagine, Asp, glycine
(Arg in Con A), and an aromatic residue responsible for interaction
with glycan via hydrogen and hydrophobic bond, while ionic interactions
are involved in biorecognition of negatively charged glycans containing
sialic acids (see section 8.1). Thus, a binding
preference of commonly used lectins is known quite well.
37
However, unknown lectin binding specificity
was revealed, when some glycan immobilization was controlled at a
nanoscale.
625−629
EC characterization of glycan surfaces can provide information about
the quality of one- or two-component SAM deposited on gold surfaces,
when probed by CV or EIS.
630,631
An interesting approach
on how to probe the specificity of lectins on carbohydrate modified
interfaces was described recently. The glycan receptive interface
was developed from a compound containing a thiol group on one end,
glucose or galactose on the other end, and a middle part of the molecule
containing a quinone redox moiety. Such a molecule after incubation
on a gold surface formed a gold–thiol bond with glucose or
galactose exposed to the solution. Quinone moiety of the molecule
served for signal generation via CV or DPV. Upon incubation of the
electrode with lectins, a decrease of EC signal was observed, enabling
detection of two lectins, Con A and peanut agglutinin, down to micromolar
concentration.
632
A nanoscale-controlled
interface formed on gold nanoparticle-modified electrodes by sequential
deposition of thiolated saccharides and various blocking thiols (cysteamine,
11-mercaptoundecanoic acid, 6-mercaptohexanol, and 3-mercapto-1-propanesulfonate)
was applied for investigation of Con A binding. The study showed that
a blocking/diluting thiol had a detrimental effect on Con A binding
to the surface-immobilized saccharide with the best blocking/diluting
thiol (3-mercapto-1-propanesulfonate) allowing detection of Con A
down to nanomolar level.
633
Binding specificity
of six different lectins on glucose- and galactose-modified graphene
interfaces revealed that lectins can be detected down to low nanomolar
level on “electrified” anthraquinonyl glycoside surfaces
(Figure 24).
634
Moreover,
such a system was applied for analysis of a hepatoma cell line down
to 1 × 104 cells/mL.
634
When discussing graphene as a prospective transducing surface in
glycan measurements, it is important to take into account that such
material was proved to induce a strong interaction with Con A lectin,
leading to lectin denaturation.
635
Thus,
a linker layer was suggested to be inserted between graphene and lectin
ensuring lectin activity after binding.
635
Applications of lectins in EC assays will be discussed in the forthcoming
sections.
Figure 24
Probing the specific sugar–lectin interactions on surface
modified by (a,c) glucose and (b,d) galactose. The interaction between
lectin and immobilized saccharide was detected by either (a,b) DPV
or (c,d) EIS. Bare screen printed electrode was modified by graphene
oxide (GO), then by antraquinone containing glucose (GO-GA1) or galactose
(GO-GA2), and interaction between saccharide-containing surfaces was
probed by two lectins: Con A, recognizing glucose; and peanut agglutinin,
recognizing galactose. All experiments were performed in Tris-HCl
(pH 7.0). Adapted with permission from ref (634). Copyright 2013 Macmillan Publishers
Ltd.:
Scientific Reports.
8.4.2
Lectin
Engineering
A novel trend
is to apply recombinant DNA technology for producing lectins with
improved characteristics for construction of various lectin-based
biodevices. Production of lectins by recombinant technology has distinct
advantages as compared to lectin isolation and purification from natural
sources such as time- and cost-effectiveness, high yields, low batch-to-batch
variation, and high purity.
527,636
Moreover, recombinant
expression of lectins in prokaryotic hosts produces lectins without
glycosylation. The presence of glycans on lectins can complicate glycoprofiling,
when a sandwich configuration (primary lectin/glycoprotein/secondary
lectin) is applied in sensing. Recombinant lectins can be produced
with various tags applied for simple purification process, but applicable
for an oriented lectin attachment on different surfaces, as well.
580
Other attractive concepts of how to prepare
lectins with advanced binding properties include multimerization of
lectins. By incubation of biotinylated lectins with streptavidin,
a 4–40-fold increase in sensitivity of analysis
637
was observed (Figure 25A). Further, a redox switchable preparation of a lectin dimer
from
its monomers with an agglutination activity increased 16-fold as compared
to a lectin monomer
638
(Figure 25B). Introduction of a boronate moiety into lectin
with affinity increasing 2–60-fold for its glycan analyte
639
(Figure 25C) is another
good example of improved lectin forms. Because such recombinant or
modified lectins have been applied in bioanalysis relying on a fluorescent
reading,
640−642
they are not discussed here in detail, but
can be combined with EC detection in the future for robust and advanced
glycoprofiling.
Figure 25
Various ways to prepare modified lectins with improved
binding
properties. (A) Modification of a lectin by a biotin derivative, which
upon incubation with a dye-labeled streptavidin forms a lectin multimer.
Adapted with permission from ref (637). Copyright 2013 American Chemical Society.
(B) A redox switchable formation of a lectin dimer involving a thiolated
form of a lectin. Adapted with permission from ref (638). Copyright 2004 John
Wiley & Sons. (C) A scheme of increased strength of interaction
between BAD (boronic acid-decorated) lectin and a glycan with involvement
of lectin binding site and boronate derivative in the biorecognition.
Adapted with permission from ref (639). Copyright 2013 American Chemical Society.
8.5
Label-Free
EC Detection
The usual
EC techniques employed for a label-free analysis of intact glycoproteins
include EIS (Figure 26) and capacitance sensing.
A transducing mechanism of these techniques is described shortly below.
At the end of this section, it will be shown that glucosamine-containing
poly- and oligosaccharides catalyze hydrogen evolution at Hg electrodes.
Figure 26
A typical
interfacial layer of the EIS-based biosensor after SAM
formation (1), immobilization of lectins (2), and biorecognition of
a glycoprotein (3) with corresponding Nyquist plots showing shifts
in the R
CT value with an increased loading
of the surface.
8.5.1
Electrochemical
Impedance Spectrometry (EIS)
of Glycoproteins
EIS as the most frequent label-free technique
utilized in glycoprofiling is based on application of a small alternating
potential amplitude to the electrode. This technique can provide interfacial
characteristics utilizable in (bio)sensing by fitting Nyquist and
Bode vector plots.
37
The most often charge
transfer resistance R
CT extracted from
a Nyquist plot (a diameter of a semicircle in Figure 26) is applied for biosensing
purposes. EIS has been extensively
used as an efficient tool for reliable analysis of surface conditions
such as biorecognition binding events and desorption.
434,483,643
In its simple form, EIS
lacks selectivity. To apply EIS for analysis of biomacromolecules,
sophisticated selectivity improvements based on biological recognition
systems were elaborated. For example, in the case of detection of
glycans in glycoproteins, the selectivity of the glycoprotein binding
event relies on the specificity of the glycan binding to lectins,
which can be immobilized on the electrode surface (see below). Nonspecific
binding of other proteins and various compounds to the surface is
prevented usually by screening the electrode with binary or ternary
thiol layers including thiols with terminal betaine moiety.
644
Electrode surfaces after immobilization of
the biorecognition element can be blocked to prevent nonspecific interactions
by filling the surface with some proteins such as bovine serum albumin,
casein, or other molecules. In these arrangements, the specificity
of the EIS analysis was greatly increased, allowing analysis of body
fluids and cell lysates.
The first paper dealing with analysis
of an intact glycoprotein
by a label-free EC method based on EIS was published in 2006.
645
Authors employed boronic acid and its derivatives,
which have a high affinity toward carbohydrates and can be effectively
applied for sensing purposes. Thin films of poly(aniline boronic acid)
and poly(aminobenzoic acid boronate) electropolymerized on the surface
of GCE were applied for biorecognition of glycoproteins. Horseradish
peroxidase (HRP) and glucose oxidase were used as model glycoproteins,
and the study showed that both proteins were effectively recognized
by the boronic acid-containing films, but the sensor exhibited a nonspecific
binding in the presence of BSA. The study showed that 5 μM glucose
oxidase exhibited a shift in R
CT from
5.1 to 6.3 kΩ,
645
indicating that
such a sensor is not very sensitive and cannot be applied in early
detection of glycoprotein-based biomarkers. Application of lectins
can significantly increase specificity of glycoprotein EIS-based analysis
as compared to analysis based on utilization of boronic-acid functionalized
film because lectins can specifically recognize different glycan structures
(section 8.1).
Lectins started to be
applied in combination with EIS measurement
of glycoproteins in 2007.
646
Con A lectin
covalently attached to a one-component SAM layer of 11-mercaptoundecanoid
acid (MUA) was tested for a specific detection of HRP. Interestingly,
after covalent binding of Con A, R
CT decreased
from 15.6 kΩ (MUA SAM on gold) to 10.1 kΩ, and R
CT further decreased to a value of 8.0 kΩ
upon incubation with a micromolar concentration of HRP.
646
This unexpected decrease of R
CT after formation of protein layers is most likely a
result of screening of a negative charge of pure MUA SAM by both proteins
Con A and HRP.
Analysis of sialic acids is important because
it is involved in
numerous physiological and pathological processes.
647
Thus, there is no doubt that lectin biosensors able to
detect these carbohydrates can be useful for diagnostic purposes.
The first paper dealing with detection of physiologically relevant
glycoproteins (fetuin and asialofetuin) by EIS was published by the
Joshi’s group.
648
The lectin biosensors
with either peanut agglutinin (galactose-specific) or Sambucus nigra agglutinin (SNA,
sialic acid-specific)
immobilized on printed circuit board electrodes were able to detect
corresponding asialofetuin (predominantly without terminal sialic
acid) or fetuin (with terminal sialic acid) in a short time (∼80
s). Both glycoproteins could be detected down to 150 fM level with
a possibility to see microheterogeneity of glycan composition on fetuin
by the biosensor relying on determination of changes in the R
CT. Moreover, the authors noticed a difference
in glycoprofiling of proteins depending on the provider of SNA lectin,
suggesting that the lectin from different sources can contain different
levels of SNA isoforms and the source of lectins has to be chosen
with special care.
648
This label-free concept
of analysis was recommended for point-of-care ultrasensitive detection
of early cancer stages because cancer biomarkers including carcinoembryonic
antigen (CEA), carbohydrate antigen 125 (CA125), prostate specific
antigen (PSA), and mucin 1 (MUC1) or carbohydrate antigen 15-3 (CA15-3)
are glycosylated.
648
Oliveira’s
group was intensively involved in the application
of EIS method for the detection of glycoproteins. In their first study,
Con A and a lectin from Cratylia mollis CramoLL were immobilized on gold nanoparticles
with polyvinyl butyral,
and these complexes were subsequently adsorbed on the surface of gold
electrodes with BSA (preventing nonspecific protein adsorption).
649
EIS allowed detection of sub micromolar concentration
of a glycoprotein ovalbumin with both lectins by monitoring changes
in the R
CT. The Con A biosensor offered
a linear response toward ovalbumin up to 200 μg/mL, while the
CramoLL biosensor only offered up to 100 μg/mL. A comparative
investigation showed that CV can be applied for analysis of glycoprotein
ovalbumin as well.
649
CramoLL lectin in
combination with EIS was successfully applied in glycoprofiling of
various types of microbes, including Escherichia coli, Serratia marcescens, Salmonella
enteric, and Klebsiella
pneumonia, and CV investigation provided similar results.
650
Two lectins, Con A and Ricinus
communis agglutinin, integrated with EIS were employed
for selective discrimination of prokaryotic strains (Escherichia coli DH5a, Enterobacter
cloacae, and Bacillus subtilis) and eukaryotic cells (yeast Saccharomyces cerevisiae
and human HeLa cell line) down to 1 × 103 cfu/mL
(cfu = colony forming units).
651
From EIS not only R
CT but also capacitance
of the surface can be read, which can be applied for evaluation of
a specific binding.
652
The surface of the
silicon chip with an array of electrodes present in nanowells was
applied for biosensing. Two lectins were covalently immobilized on
such an interface, and the biosensor performance was tested with standard
glycoproteins and a protein isolated from a cultured human pancreatic
cancer cell line BXPC-3. The results obtained by EC investigation
with both glycoproteins were in good agreement with glycan composition
and affinity of lectins for a particular glycan. The biosensor exhibited
high reliability of assays and a good agreement with enzyme-linked
lectin assays (ELLA, enzyme-linked immuno sorbent assay, an (ELISA)-like
method using lectins instead of antibodies). LOD of the biosensor
for its analyte was 5 orders of magnitude lower, and the assay time
of the biosensor was much shorter as compared to ELLA.
652
In a series of recent publications from Tkáč’s
group, a systematic effort was devoted to prepare highly sensitive
lectin biosensors based on EIS for analysis of glycoproteins containing
sialic acid.
644,653−655
In the first report, density of a lectin covalently immobilized
on a modified gold surface was controlled by adjusting the ratio of
a functional thiol in a mixture with a diluting thiol.
653
Surface patterning process was monitored by
atomic force microscopy, showing differences in surface roughness
caused by different molecules being attached to the surface (Figure 27). The lectin
biosensor for analysis of sialic
acid was able to detect two glycoproteins, fetuin (8.7% sialic acid)
and asialofetuin (≤0.5% sialic acid), down to femtomolar concentration.
The biosensor response to these two glycoprotein analytes was proportional
to the content of sialic acid.
653
Figure 27
AFM images
of the gold surfaces during a patterning procedure starting
with the bare gold (upper left), the gold surface modified by a mixed
SAM (upper right), the surface with covalently attached SNA I lectin
(lower left), and the surface after being treated with a blocking
agent (lower right). Scale of z-axis was adjusted
in a way to clearly see topological features on the surface after
each modification step. Adapted with permission from ref (653). Copyright 2013 Springer
Science and Business Media.
Recently, a 3D lectin biosensor for analysis of glycoproteins
containing
sialic acid based on a gold nanoparticle-modified interface was developed.
654
After a careful optimization of the 3D interface,
it was possible to detect two glycoproteins containing sialic acid
down to attomolar concentration, which is still the lowest LOD for
analysis of glycoproteins. Again, the lectin biosensor was able to
detect the level of sialic acid residues on fetuin and asialofetuin
quantitatively, but the level of nonspecific interaction was quite
high, reaching 23% of a specific signal.
654
Finally, a direct comparison between sensitivity of the 2D biosensor
versus 3D biosensor, taking into account the absolute amount of immobilized
lectin, showed that a 3D configuration was by 61% more sensitive as
compared to the 2D biosensor (Figure 28).
655
Glycoproteins can be detected down to the attomolar
level even on a 2D interface, when after binding of a glycoprotein
to a lectin-modified gold surface, an additional lectin layer in a
sandwich configuration is formed.
644
Figure 28
A graphical
representation drawn to scale of interfaces applied
to build (A) the 3D biosensor based on integrated 20 nm gold nanoparticles
or (B) the 2D biosensor (upper image). The 3D biosensor was built
on a planar gold surface by chemisorptions of 11-aminoundecanethiol
for attachment of 20 nm gold nanoparticles (spheres). (A) On every
gold nanoparticle, a mixed SAM composed of 11-mercaptoundecanoic acid
(MUA) and 6-mercaptohexanol was formed for covalent immobilization
of lectin. (B) The 2D biosensor was formed by incubation of a planar
gold with MUA and mercaptohexanol for covalent attachment of a lectin.
In the lower part of the figure, comparison of the response of the
2D and the 3D biosensor to its analyte fetuin (Fet) with concentration
close to the LOD, represented in a Nyquist plot (left), and calibration
graphs for detection of fetuin (Fet) by both biosensors (right) are
shown. Adapted with permission from ref (655). Copyright 2014 Enterprise Strategy
Group.
Besides all of the above-mentioned
examples, when a Faradaic EIS
was measured using a redox probe (i.e., a mixture of ferri- and ferrocyanide
was applied for a biorecognition), it was also possible to utilize
EIS in a non-Faradaic mode of operation without any redox probe present
during measurements.
656
In such a case,
however, it is important to optimize ionic strength of the electrolyte
employed for measurements. For example, an increase of a double layer
thickness and thus sensing distance from the electrode surface, from
∼0.1 to ∼10 nm, can be achieved by changing the buffer
concentration from 100 to 1 mM. This is why diluted phosphate buffer
was used for analysis of ovarian cancer SKOV3 cells with a LOD of
only 4 captured cells.
656
EIS-based
analysis of glycoproteins is the only exception, where
LOD observed for analysis of glycoproteins is on the same level or
better (fM to aM level) as compared to the LOD of protein analysis
based on immunoassays.
657−659
Even though there is so far
only one study, when EIS-based lectin biosensor was applied in the
analysis of complex samples such as human serum,
644
recently developed DNA- and immuno-sensors working in saliva,
660
bovine serum,
661
and human serum
659,662−669
indicate that sensitive and selective EIS-based lectin glycoprofiling
in complex samples will be widespread in future diagnostics.
8.5.2
Capacitance Measurements
A novel
capacitive EC measurement of biorecognition was introduced by Berggren
in 1997.
670
This method samples the current
response triggered by a potentiostatic step (typically 50 mV or so
in an amplitude) at a frequency of 50 kHz with a possibility to extract
a value of capacitance of the layer.
671
There is, however, a requirement to work with this step potential
measurement in diluted electrolytes (i.e., 10 mM) to avoid fast current
decay to get enough data points before current output decays to zero,
and this is the main reason for high current sampling.
671
This EC technique was successfully applied
in the analysis of a glycoprotein, human immunoglobulin G (IgG), on
a modified SAM layer with immobilized Con A.
672
The lectin biosensor was able to detect its analyte down to 10 nM
within 15 min in a flow injection mode, and the biorecognition could
be followed in real time. Moreover, the native and aggregated form
of the IgG could be distinguished by the biosensor, and aggregated
IgG was detected with a concentration as low as 0.01% of the total
IgG.
672
Thus, the lectin biosensor can
be effectively applied in a control of quality during heterologous
proteins’ production.
A LOD obtained here for a step-potential
capacitance method-based
analysis of a glycoprotein is much higher as compared to detection
of proteins with antibodies, when LOD in the sub femtomolar level
670
or even sub attomolar level on a gold nanoparticle-modified
interface
673
was observed.
8.5.3
Catalytic Hydrogen Evolution in Polysaccharides
Very
recently, a new possibility of EC analysis of polysaccharides
(PS) and oligosaccharides (OLS) based on CHER appeared.
246,263,674,675
This new method has been developed using free PS and OLS, but it
has not been yet tested on glycans cleaved from glycoproteins. Until
recently, label-free direct EC reduction or oxidation of PS and longer
OLS under conditions close to physiological was not possible. In 2009,
it was found that some sulfated PS are able to produce CHER at Hg
electrodes.
675
Using CPS, carrageen HPS peaks were obtained, similar to CPS peak H produced by proteins
(see section 5.2). However, as compared to
unfolded proteins, much larger PS concentrations were necessary to
obtain peak HPS. Later, it was shown that using adsorptive
transfer (ex situ) stripping, it was possible to adsorb sulfated PS
directly from seawater and analyze them in buffered solutions.
674
Very recent data suggest that the other
types of OLS and PS, such as chitosan, have the ability to catalyze
hydrogen evolution reaction as well.
246,676−678
In the past decades, chitosan as a biodegradable material with many
interesting properties (antimicrobial, anti-inflammatory, and anticholesterolemic
activities) has attracted great attention, making it potentially useful
in biomedicine, but also in other fields of everyday life.
679
Chitosan occurs as a major structural component
of the cell wall of some fungi, and it can be easily prepared by chemical
deacetylation of more abundant chitin.
677
Chitosan mostly exists as a random linear copolymer of d-glucosamine and N-acetyl-d-glucosamine
units. Commonly, if the number of acetoamide groups is less than 30%,
the PS is termed chitosan.
676
Chitosan
has been used for modification of various surfaces, including electrodes,
680
but possibilities of its direct EC detection
were rather limited.
678−680
It has been shown that chitosan produces
voltammetric and chronopotentiometric reduction peaks (in a wide pH
range) at mercury and solid amalgam electrodes (Figure 29A–C) and can be determined
down to concentrations of
1 μg/mL and below. Chitosan peaks occurring at highly negative
potentials (E
p ≈ −1.8 to
−1.9 V, Figure 29) are well separated
from the background discharge. These peaks were assigned to the CHER
and were much larger than those of carrageenans, suggesting that chitosan
is a much better catalyst than carrageenans.
Figure 29
(A,B) CPS and (C) SWV
curves of chitosan at mercury electrodes.
(A,C) 10 μg/mL of chitosan at HMDE and (B) 15 μg/mL of
chitosan at solid amalgam electrode. Accumulation time, t
A, 60 s; stripping current intensity, (A) I
str, −70 μA; (B) I
str, −40 μA; (C) frequency 20 Hz; (D) CPS curves of 12
μM chitohexaose (red) and N,N′,N″,N‴,N‴′,N‴″-hexaacetylchitohexaose
(blue); t
A, 60 s; I
str, −40 μA. Background: 0.1 M sodium acetate,
pH 5.2 (dashed). Adapted with permission from ref (246). Copyright 2014 Elsevier.
At a concentration of 10 μg/mL,
chitosan produced a high
SWV peak in 0.1 M sodium acetate, pH 5.2, while carrageenan yielded
no signal under the same conditions. Chitosan oligomers yielded CHER
peaks similar to those of chitosan, while acetylated (chitin) oligomers
were inactive (Figure 29D). These results suggested
that free −NH2 groups in glucosamine residues are
responsible for the chitosan CHER. In agreement with this suggestion,
chemical deacetylation of the above chitin oligosaccharides resulted
in voltammetric and chronopotentiometric peaks similar to those of
chitosan. In weakly acid media, chitosan hexamer was detectable at
nanomolar concentrations, but it can be expected that more detailed
studies will offer LODs at the picomolar level.
In contrast
to chitosan, PS-containing acetylated glucosamine residues,
such as hyaluronic acid, heparin, and chondroitin sulfate, did not
produce any significant reduction signal (not shown) even at much
higher concentrations than that of chitosan (Figure 29A). Finding the very strong
ability of glucosamine-containing
PS and OLS to catalyze hydrogen evolution opens the door for a simple
label-free EC analysis of PS and OLS, including glycans of glycoproteins.
Such glycans frequently contain N-acetylated glucosamine
or galactosamine residues (not producing CHER signals), which may
become electroactive as a result of chemical or enzymatic deacetylation
(Figure 29D). More work will be, however, necessary
to find out how useful will be the CHER signals in glycoprotein analysis.
8.6
Label-Based EC Detection
Label-free
EC methods are simple, sensitive, and convenient for glycoprotein
measurements. However, covalent and noncovalent labeling of the protein
moieties in glycoproteins, as well as specific labeling of glycans
either in their free state or directly in the glycoprotein, may have
some advantages. Interestingly, DNA analysis started with label-free
methods, and later label-based approaches prevailed.
34
Different types of labels are available for EC detection
of protein moieties in glycoproteins with enhanced sensitivity of
detection. Besides traditional amplification agents such as HRP and
alkaline phosphatase,
681−685
nanomaterials such as metallic nanoparticles,
686−688
carbon nanotubes,
382,689
and quantum dots (QD)
690,691
are applied for amplification of a binding event due to their attractive
EC properties and electrocatalytic activities. Here, we introduce
carbohydrate modification with Os(VI)L complexes (where L is a nitrogenous
ligand), which can be used for labeling of only a glycan part of biomolecules
such as glycoproteins and glycolipids, and which is less known in
the glycomic community. An advantage of this labeling protocol is
that a complex with a redox label can be formed after biorecognition,
not disturbing the binding event.
8.6.1
Covalent
Labeling with Os(VI) Complexes
Osmium tetroxide complexes,
Os(VIII)L, binding covalently to pyrimidine
residues in single-stranded DNA and RNA (Figure 30Aa) have shown their usefulness
as probes of DNA structure
in vitro (reviewed in ref (692)) and in cells (reviewed in ref (693)). In contrast,
six-valent Os(VI)L complexes
were shown to bind ribose (but not deoxyribose) in nucleosides
692
(Figure 30Ab). It was
found that products of this reaction were electroactive, displaying
redox couples on CV of Os(VI)L-modified ribosides (Figure 30C) in a wide pH range
(Figure 30D) at mercury and carbon electrodes, similar but not identical
to those of the base-Os(VIII)-modified ribosides.
694
In addition to redox couples seen on CV, reaction products
of some Os(VI)L complexes yielded electrocatalytic peaks (see peak
Vc in Figure 30C,D). Os(VI)L riboside reactions
turned out to be useful for end-labeling of RNA and were utilized
in sensing of microRNA.
242,695,696
Figure 30
(A) Reaction of Os(VIII)L and Os(VI)L complexes with different
parts of a nucleoside showing Os(VI)L complex specifically modifying
ribose moiety. (B) Fragment of Os(VI)L-modified dextran. (C) Adsorptive
stripping cyclic voltammograms of 10 mM base-modified (blue) and sugar-modified
thymine riboside (red), and sugar-modified adenine riboside (black).
(D) Dependence on pH, 10 mM sugar-modified thymine riboside, HMDE,
with stirring; Britton–Robinson buffer, pH 7.0; scan rate (C)
2 V/s, (D) 1 V/s; t
A of 60 s; E
A of 0 V, step potential 5 mV. Adapted with
permission from ref (694). Copyright 2007 John Wiley & Sons, Inc.
It was also shown that the above reactions yield a redox
active
product, which could be useful in EC analysis of PS and OLS.
229,244,697−699
Thus, PS lacking any redox moiety can be transformed into electroactive
species by their reaction with six-valent Os(VI)L complexes (Figure 30B).
694
Os(VI)L did not
produce any electroactive adducts with DNA and proteins, suggesting
high specificity in the glycoprotein measurements. The reaction is
very simple and does not require any special equipment. The complex
can be only mixed with the carbohydrate at room temperature, and the
adduct is formed within hours. Using a ligand exchange process
699
or elevated temperature
245
(e.g., 75 °C), the reaction can proceed in about 15
min. Excess of the reagent may interfere with the EC determination,
and mostly a purification step such as dialysis or separation on a
chromatographic column or membrane is necessary. This step can be
omitted when the adsorptive transfer stripping (ex situ) method
276,700
is used with carbon electrodes.
697
In
adsorptive transfer stripping experiments, the electrode with strongly
adsorbed PS adduct is washed to remove the weakly adsorbed Os(VI)L
complex. The PS-modified electrode is then transferred to the electrolytic
cell containing blank background electrolyte followed by voltammetric
measurement. In this way, the purification step is avoided, and the
PS adsorption can be performed from a small analyte drop (e.g., 3–10
μL, depending on the electrode size). Using adsorptive transfer
stripping, an abundance of monomeric carbohydrates (e.g., glucose)
does not interfere with PS determination as it can be easily washed
away from the electrode.
700
Properties
and EC behavior of PS–Os(VI)L adducts can be
significantly influenced by the nature of the chosen ligand. Table 2 shows some ligands
that are useful in EC analysis
of PS–Os(VI)L adducts. For example, by using Os(VI)bpds (bpds
= bathophenanthrolinedisulfonic acid), negative charges can be introduced
in the PS or OLS adducts. Using Os(VI)bipy, electrocatalytic peaks
can be obtained allowing the determination of OLS at the picomolar
level.
244
The reaction of Os(VI)temed (Table 2) producing PS and OLS adducts appears particularly
interesting. These adducts can be determined using adsorptive stripping
method (in situ) at mercury electrodes directly in the reaction mixture,
without any purification step because free Os(VI)temed adsorbs very
weakly on Hg electrodes.
700
Moreover, it
has been shown that some glycan isomers can be distinguished on the
ground of differences in the voltammetric responses of their Os(VI)L
adducts.
701
Table 2
Nitrogenous
Ligands (L) Applied in
Os(VI)L Complexes for Carbohydrate Modification
a
Ligands shown in
bold were mostly
applied in PS modification.
8.6.1.1
Glycans Can Be Modified with Os(VI)L Directly
in Glycoproteins and Recognized by Voltammetry from Nonglycosylated
Proteins
Recently, it has been shown
702
that by using Os(VI)L complexes, glycans can be modified
directly in glycoproteins and detected voltammetrically at carbon
electrodes (Figure 31A). Using square wave
voltammetry and pyrolytic graphite electrode, linear dependence of
peak heights on Os(VI)temed-modified avidin concentration was obtained
between 100 nM and 1 μM (Figure 31B).
In AdTS experiments (using 8 μL drops of Os(VI)temed-modified
avidin), hundreds of femtomoles of avidin were sufficient for the
analysis. The analysis could be performed directly in the reaction
mixture when Os(VI)temed was used for the avidin modification. However,
experiments with Os(VI)bipy required separation of the adduct from
the reaction mixture. The combination of Os(VI)bipy with mercury electrodes
resulted, however, in electrocatatalytic signals allowing sensitivity
down to the picomolar level.
Figure 31
(A) AdTS SWV of unpurified 5 μM Os(VI)temed-treated
avidin,
complex avidin–biotin, and streptavidin (STV); (B) concentration
dependence of peak α (E
P ≈
−0.85 V) of avidin-Os(VI)temed, four replicative measurements.
Inset: Detail of the concentration range 100–1000 nM. Adapted
with permission from ref (702). Copyright 2014 Elsevier.
8.6.2
Covalent Labeling with
Other Redox Labels
In 2006, Joseph Wang’s group introduced
a concept of the
lectin biosensor based on application of QD as a label in glycan measurements.
703
Even though the biosensor was not exposed to
glycoproteins as an analyte, the authors in the study selected important
glycan determinants, which were conjugated to CdS QD. The lectin biosensor
developed on a modified gold electrode was first incubated with a
QD-conjugated glycan, and then this complex was displaced by an untagged
sugar being a “sample”. Analysis of the amount of QD-conjugated
glycan not displaced by the “sample” solution was performed
via stripping voltammetric detection on a mercury coated GCE, with
a LOD of 0.1 μM.
703
Other forms of
QDs composed of CdTe, ZnO, and CdSe were applied in glycoprofiling,
as well.
704−707
Ferrocene redox moiety has been successfully applied in labeling
of Con A lectin
708
or a gold nanoparticle
coloaded with Con A
709
for further glycoprofiling.
Moreover, Con A was labeled with daunomycin for EC analysis of ovalbumin
down to the sub nanomolar level
710,711
or with Ru
complex for detection of E. coli cells
down to 127 cells/mL by electrochemiluminescence.
712
Another way to use the redox labels is to attach them directly
to the analyte, as in the case of electrostatic attraction between
negatively charged ovomucoid and positively charged ZnO quantum dots.
The labeled analyte was then detected by the biosensor with immobilized
Con A down to the picomolar level.
707
Label-based
approaches have not been that often applied in the detection of isolated/purified
glycoproteins, but rather for the analysis of glycoproteins attached
to the surface of various types of cells (section 9.5.5).
It can be concluded that there is still plenty of
room for improvement
of label-based glycoprotein measurements with applications of lectins,
when compared to the most sensitive protein assay schemes performed
by antibodies, using various labels offering LODs in the femtomolar
to attomolar levels.
713−716
8.7
Concluding Remarks and Future Prospects
It is very important that a palette of currently used lectins mainly
of a plant origin
717
will be extended by
lectins isolated from other higher organisms such as galectins and
selectins.
718,719
It will expand binding specificities
for subtle changes in the glycan composition and will be essential
for highly reliable diagnosis of pathological processes. Alternatively,
artificial lectins such as lectin multimers, boronate-decorated lectins,
or redox switchable lectins can be beneficial for the development
of novel lectin-based EC biosensors. Recombinant technology will in
the future undoubtedly allow us to expand the range of lectins produced
with various tags for oriented and controlled immobilization of lectins
as an important prerequisite for sensitive and selective glycan measurements.
Additional options of how to increase the library of biorecognition
elements are to produce highly stable DNA aptamers or lectin-like
peptide aptamers with a high affinity for their glycan targets (section 9).
From a transducer point of view, it can
be anticipated that many different ways of how nanomaterials can be
integrated into the EC detection platform of detection will be developed.
This can be done by direct modification of electroactive surfaces
by nanomaterials or by advanced patterning protocol and by using nanomaterials
as amplification tags, helping to produce lectin biosensors/biochips
working in an ultrasensitive and selective way. Further, it can be
anticipated that EC-based biosensors will compete in a future with
instrumental techniques (MS, liquid chromatography, capillary electrophoresis)
or lectin microarrays only if such devices are integrated into a biochip
format offering multiplexed glycan measurements. At the same time,
it will be desirable to develop redox switchable immobilization protocols
for selective and controlled immobilization of lectins on electrodes
within an array of electrodes.
Recent advances in synthetic
chemistry applied for the synthesis
of asymmetrically branched glycans,
720
which
resemble naturally occurring glycans, or better understanding of the
enzymatic action of glycan processing enzymes,
721
can assist in the development of bioreceptive surfaces
applicable in finding novel glycan-binding biorecognition elements,
including antibodies.
9
Detection of Biomarkers
Generally speaking, biomarkers for diseases may consist of any
measurable or observable factors that indicate normal or disease-related
processes in vivo or patient responses to therapy. Such biomarkers
encompass, for example, physical symptoms, expressed proteins, mutated
DNAs and RNAs, processes such as cell death or proliferation, and
serum concentration of small molecules. Significant progress has been
made in the identification of biomarkers by means of such technologies
as DNA microarrays and proteomic approaches, including mass spectrometry,
resulting in an increasing number of potential biomarker candidates.
Proteomic and genomic analyses generate vast amounts of data, but
these approaches do not appear to be the best suited for a routine
cancer clinical testing due to their complexity. The laborious protocols
have to be performed by highly skilled technicians to achieve acceptable
reproducibility. If a complex pattern of gene expression is reduced
to a few genes, biosensor-based detection may become advantageous
for practical testing, because it is more user-friendly, faster, less
expensive, and less technically demanding than microarray or proteomic
analyses.
722
The topic of EC detection
of protein biomarkers has been the subject
of several review articles,
50−53
and here we wish to focus mostly on glycoprotein
biomarkers and show that detection strategies were developed to recognize
the protein part itself as well as a glycan part of the glycoproteins.
We shall deal mostly with the detection of cancer-related protein
biomarkers found at elevated levels in body fluids already during
the early stages of cancer development, acting as indicators of the
onset, progression, or recurrence of the cancer. Moreover, biomarkers
for other diseases, such as diabetes, neurodegenerative diseases,
AIDS, etc., will be also mentioned.
It is now well accepted
that measurement of a single biomarker
is often insufficient because of variations in the given protein expression
among human population and also due to the fluctuation of biomarker
levels within the single individual. Let us take prostate specific
antigen (PSA) as an example. PSA is a glycoprotein secreted by a prostate
gland, which is present in small amounts in serum of healthy men but
becomes elevated in prostate cancer, the most common form of cancer
in men in the U.S. and Europe.
546
The majority
of PSA in the blood is bound to serum proteins, but there are also
tiny unbound amounts, referred to as free PSA; both of them together
form total PSA. Testing of serum PSA was routinely applied for diagnosing
prostate cancer and for monitoring the disease progress, with total
PSA values below 4 ng/mL considered safe while levels above 10 ng/mL
indicated the presence of a disease. However, besides prostate cancer,
there are some benign conditions that may cause PSA levels to rise
above the threshold level, including inflammation and enlargement
of the prostate, or even prostate biopsies and surgeries (which give
rise to false positives). Biopsy-detected prostate cancer was found
among 15% of men that had otherwise normal PSA levels of 4 ng/mL or
below (yielding false negatives).
723
For
these reasons, the U.S. Preventive Services Task Force and UK National
Screening Committee do not recommend PSA screening anymore as it may
lead to “overdiagnosis” or “overtreatment”.
When the patient has borderline or moderately increased total PSA
values, it may be useful to monitor the ratio of free PSA to total
PSA and thus help to eliminate unnecessary biopsies in men with total
PSA values above 4 ng/mL. This ratio decreases with increasing risk
of prostate cancer. In addition, glycoprofiling of PSA with serum
concentration in a gray zone (4–10 ng/mL) could be applied
as a supplementary test in the diagnosis of this type of cancer.
It should be also noted that many protein biomarkers indicate more
than one disease, and thus a single cancer biomarker is frequently
not unique to a specific type of cancer. For these reasons, a combination
of multiple biomarkers is preferred, which could result not only in
improved accuracy, but also in the increase of a sample throughput
and reduction of cost per test. Detection of panel of biomarkers is,
however, complicated due to large variations in concentrations for
different markers in serum, ranging from low picograms to hundreds
of nanograms per mL. For example, the level of interleukin 6 (IL-6),
a cytokine associated with different types of tumors, is about 1000-fold
lower in healthy individuals than the level of PSA.
724
This requires a development of a reliable detection method,
spanning all of the concentration ranges for the chosen panel of biomarkers.
Contrary to nucleic acids, whose molecules can be recognized (and
separated) on the ground of their complementarity, proteins do not
possess such a unique system for specific recognition of individual
protein types. Therefore, different strategies need to be employed
for capturing specific proteins. The most widely used ones are based
on application of (i) antibodies (in so-called immunoassays), (ii)
nucleic acid aptamers, (iii) peptide aptamers, and (iv) glycan-binding
lectins for detection of glycoproteins. All of these approaches will
be described after a brief summary of protein labeling often used
for biomarkers detection and for probing protein structures.
9.1
Labeling of Proteins
Usually, the
label is introduced into the EC biomarker assay to greatly amplify
the signal and thus to achieve substantially lower LODs. The vast
majority of authors use enzymes or nanomaterials (especially nanoparticles)
as labels in immunosensor or aptasensor format. Enzymes, such as horseradish
peroxidase (HRP) and alkaline phosphatase (AP), provide high, steady,
and reproducible signal amplification.
725
If higher sensitivity is required, single enzyme approach may be
combined with an additional amplification process (i.e., redox cycling),
or using multienzyme labels per detection probe. As compared to enzymes,
nanomaterials show better long-term stability and are more easily
prepared. Nanomaterials used in EC assays mostly comprise various
types of nanoparticles, carbon nanotubes (CNT), nanowires, or graphene
sheets, which usually serve as immobilization substrates for accumulation
of increased amounts of electroactive molecules, as well as catalysts
or nanoelectrodes. It is common to combine both enzymes and nanomaterials,
such as by loading multiple enzymes at the CNT surface, in a single
assay to achieve even lower LODs. More details on the construction
and application of biosensors using enzymes or nanomaterials as labels
are given below in this section, where specific examples are described,
and also in recent reviews.
725−732
9.1.1
Chemical Modification of Proteins for Probing
Their Structure and Activity
Chemical agent can serve not
only as a signal amplifier, but also as a probe of protein structure
(similarly to chemical probes of the DNA structure
34
). Although there are many reagents available for modification
of aa residues in proteins, only few exhibit reasonable selectivity
for a particular residue under conditions close to physiological.
In most cases, the reagents are electrophilic molecules that target
nucleophilic functional groups.
733
These
chemicals were utilized for probing accessibility of the reactive
moieties upon changes in the protein structures due to unfolding,
or intermolecular interactions. The probing of aa residues in proteins
was used in connection with various detection techniques, including
optical methods,
734−739
chromatography combined with site-specific proteolysis,
740−742
and mass spectrometric techniques.
733,743−745
Literature devoted to EC analysis of chemically modified peptides
or proteins for probing their structure is rather scarce. Several
papers reported on the synthesis and applications of peptides using
electrochemically active ferrocene derivatives.
746−754
For instance, the approach chosen in Kraatz’s laboratory
is based on the ability of kinases to transfer a redox-labeled phosphoryl
group to surface-bound peptides that are highly specific substrates
for the particular protein kinase. For this purpose, they mostly applied
5′-γ-ferrocenoyl-ATP (Fc-ATP) as a cosubstrate for peptide
phosphorylation. After the Fc-phosphoryl group was transferred to
the peptide, the presence of the redox active Fc group was detected
electrochemically (Figure 32). The EC response
enabled monitoring the kinase activity and its substrate, as well
as the effect of small molecule inhibitors on protein phosphorylation.
The authors developed peptide biosensors, for example, for papain,
755
protein kinase C,
754
serine/threonine kinase,
756
sarcoma-related
kinase,
752,757
HIV enzymes,
753,758
STAT3 dimerization,
759
or even amyloid β peptides.
760
More recently, this strategy was extended to
an immunoarray format, in which authors utilized anti-Fc antibodies
for visualization of the protein kinase-driven transfer of the phosphate
group from Fc-ATP to the hydroxyl group of peptides or proteins.
761
Figure 32
Protein kinase C-catalyzed phosphorylation
of SIYRRGSRRWRKL peptide (with phosphorylated
serine underlined)
using ferrocene-labeled ATP (ATP-Fc) as a substrate. After a transfer
of γ-phosphate-Fc group to the serine residue of the peptide,
the surface-attached Fc groups are detected via EC techniques at thiol-modified
gold electrodes. Adapted with permission from ref (754). Copyright 2008 Royal
Society of Chemistry.
Another option is to probe the accessibility of specific
Trp residues
in proteins with a chemical agent combined with EC detection. Trp
has a special status in several aspects (ref (762) and references therein)
and is relatively rare in natural proteins. It often plays critical
roles in the arrangement of the protein tertiary and quaternary structures,
763−765
and is frequently found in active sites of enzymes and proteins
containing specific binding sites for other molecules.
735,740,741,766,767
The indole side group of Trp
has a characteristic fluorescence spectrum, and can be utilized as
intrinsic fluorophore in protein analysis.
406,768,769
In addition, the Trp yields
an electro-oxidation signal at carbon electrodes (see section 4).
54,55,271,317,410
Several chemical agents are available, which react with the Trp
indole group, including 2-hydroxy-5-nitrobenzyl bromide, sulfenyl
halides, or 2-nitrobenzenesulfenyl chloride and N-bromosuccinimide.
736,737,743,767
However, these agents exhibit
reactivity also toward other aa residues and particularly cysteine.
Increased specificity toward Trp can be achieved by optimization of
reaction conditions. Chemical reactivity of individual Trp residues
within a protein reflects their accessibility toward solvent, making
it thus possible to distinguish between buried and exposed residues
and/or monitor shielding of residues within the protein binding sites
(e.g., in avidin–biotin).
735,736,740,741,766,767
As compared to the above-mentioned
chemical agents, a complex of
eight-valent osmium tetroxide and 2,2′-bipyridine (Os(VIII)bipy)
was shown to be a more specific electroactive label for Trp in peptides
and proteins.
770−772
Os(VIII)bipy forms a stable adduct with
the Trp indole moiety,
770,771,773
similar to adducts formed by pyrimidine residues in Os(VIII)bipy-treated
nucleic acids.
34,692,772
Analogically to Os(VIII)bipy-modified DNA, the Os(VIII)bipy-modified
peptides (containing Trp) produce an electrocatalytic signal at the
Hg electrode enabling their highly sensitive determination.
770
Modification of several peptides with Os(VIII)bipy
was recently studied using capillary electrophoresis and MALDI-TOF
MS.
771
Although several other aa residues
in proteins also exhibit reactivity toward the osmium reagents,
773
no other stable aa adducts (except Trp) bearing
the Os moiety were identified. Oxidation products of cysteine or methionine
(cysteic acid or methionine sulfone, respectively) were detected by
MALDI-TOF MS,
773
but they did not display
EC signals characteristic for peptide Os(VIII)bipy adducts. Modification
of protein Trp’s with Os(VIII)bipy can be analyzed by adsorptive
transfer stripping voltammetry directly in the reaction mixture at
carbon electrodes.
774
Electrocatalysis
of peptide/protein Os(VIII)bipy adducts at Hg electrodes is more sensitive
but requires separation of the adduct from the reaction mixture. The
technique was applied to monitor changes of accessibility of Trp residues
during the formation of specific molecular complexes such as streptavidin–biotin
under conditions close to physiological.
9.2
Immunoassays
Because principles,
applications, as well as recent advances in immunoassays were thoroughly
reviewed,
728,732,775−777
we will limit ourselves only to a brief
summary here. To our knowledge, first EC analysis of the antibody–antigen
interaction was performed already in the 1950s by Breyer and Radcliff,
778
who used polarography to detect interaction
of azo-protein with rabbit antiserum. They observed a decrease of
the azo-protein polarographic wave as a result of interaction of this
protein with the specific antiserum. Today, most EC immunoassays are
performed in an ELISA-like format, in which primary antibody (Ab1) immobilized at
the immunosensor (mostly electrode) surface
captures the protein analyte present in the sample, followed by the
addition of an enzyme-labeled secondary antibody (Ab2)
and EC detection of the enzymatic product. In this way, large amplification
of the signal is obtained, because one molecule of the enzyme (per
one molecule of the biomarker) leads to a generation of many electroactive
molecules from an enzymatic reaction. It was Heineman and Halsall
who in the middle of the 1980s developed sensitive enzyme-linked EC
immunoassays for proteins and other molecules.
779
They used AP enzyme yielding electroactive products, and
obtained low LOD in the range of pg to ng/mL. Heineman’s team
continued this research and was later among the first who used microfluidics
in EC immunoassays of proteins. Limoges et al.
780
elaborated theoretical analyses of amperometric and voltammetric
AP-labeled immunosensor responses, and identified key factors behind
a high sensitivity of detection. Besides AP, also HRP and glucose
oxidase were shown to be suitable labels.
776
The original Ab2/enzyme format has been increasingly
replaced with more sophisticated amplification strategies in which
the Ab2 is conjugated, for example, with nanoparticles
(metal, iron oxide, semiconducting quantum dots, etc.), CNTs, multienzyme
clusters, or even with their combinations, greatly amplifying the
response (Figure 33).
Figure 33
Scheme of amplification
strategies for detection of protein cancer
biomarkers in EC immunoassays. Surface-immobilized primary antibody
(Ab1) captures an antigen, which is then detected using,
for example, (a) a simple secondary antibody/enzyme (Ab2/enzyme) or (b) antibody/nanoparticle
(Ab2/NP) bioconjugate,
which are now often replaced with more sophisticated systems, in which
the secondary antibody is coupled with, for example, (c) biotin/streptavidin/enzyme,
882
(d) different quantum dots (Ab2/QD),
883
(e) enzyme-modified carbon nanotubes (Ab2/CNT/multienzyme),
689
(f) magnetic
beads bearing cluster of enzymes (Ab2/MB/multienzyme),
882
or (g) quantum dot-dendrimer nanocomposites
(Ab2/dendrimer/QD).
884
Typical materials for the preparation
of the immunosensor surface
are metals (e.g., gold, platinum), semiconductors (e.g., indium tin
oxide), and carbon. One of the key factors determining the quality
of the immunosensor is immobilization of the antibody. Depending on
the surface, as well as on protein properties, different immobilization
techniques were developed, including a simple physical adsorption,
entrapment into a polymer matrix, covalent immobilization, or bioaffinity-based
interactions (Figure 34). While physical adsorption
is simple and quick, its weakness is nonspecificity and a random orientation
of the immobilized antibody, greatly decreasing antigen binding. In
the study by Salam and Tothill, covalent attachment led to a 250-fold
improvement of a LOD as compared to a simple physical adsorption.
781
However, covalent attachment may yield heterogeneous
population of differently oriented proteins.
782
For oriented antibody immobilization, site-specific noncovalent
bioaffinity interactions are better suited, improving further efficiency
of the antigen binding. An ideal orientation of the antibody is when
the Fc region is in contact with the surface, and the Fab region (which
binds the antigen) protrudes to the solution (Figure 34i). This can be achieved in
several ways,
732
such as via proteins A or G, His-tag system, DNA-directed
antibody conjugation, avidin–biotin binding, etc. (Figure 34). Nanomaterials greatly
enhance the sensitivity
of the immunoassays, although the antibody orientation is usually
random. Their main advantages lie in their large surface-to-volume
ratio, faster reaction kinetics, and good compatibility with biomolecules.
Figure 34
Examples
of antibody immobilization to the surface. The binding
can be achieved via, for example, (a) physical adsorption, (b) entrapment
into a polymer matrix, (c) thiol groups, (d) protein A or G, (e) DNA-directed
immobilization (by site-specific coupling of protein G to DNA oligonucleotide),
(f) avidin–biotin system, (g) nanoparticle, or (h) carbon nanotubes
(linked via carboxyl groups at CNT). (i) Structure of the antibody
with Fab and Fc region.
Special care should be taken to minimize noise (usually caused
by nonspecific adsorption of proteins or other molecules on the electrode
or other surfaces involved in the assay), especially in label-free
techniques such as EIS (section 8.5.1), because
any adsorbed interfering molecule causes a change in the resulting
signal (giving rise to false positives or negatives). Introduction
of blocking reagents usually saturates remaining binding sites at
the surface, leading to a formation of a dense gap-free layer. Such
a layer not only improves binding capacity of antibodies to the antigens,
but also enhances a long-term stability of the sensing layer. However,
the blocking reagents, which are usually proteins (BSA, casein), detergents
(Tween-20), or polymers (such as polyethylene glycol), require stringent
washing following their addition, making the assay more time-consuming
and complex. Recent progress in tailoring the gold electrodes with
ternary thiol layers greatly improved the signal-to-noise ratio (S/N)
in DNA hybridization sensors,
783,784
and it can be expected
that application of such layers will also improve the S/N in immunoassays.
9.2.1
Cancer Biomarkers
Several examples
of EC immunoassays for cancer protein biomarkers detection are listed
in Table 3. It should be noted that not all
papers demonstrate detection of multiple biomarkers using samples
from body fluids, but rather focus on a single protein, sometimes
present in a plain buffer. Such studies may be useful from a methodological
point of view, when demonstrating proof of concept, but they are not
very relevant in clinical settings. New strategies should be therefore
developed for measuring both high and low concentrations of different
protein biomarkers in the same sample in the presence of thousands
of other serum proteins, usually occurring at much higher concentrations.
Despite these difficulties, some encouraging results were already
obtained. For instance, EC immunosensor arrays (ECIAs), based on miniaturization
and low cost of the entire assay, were constructed for multiplexed
EC immunoassay.
785−790
In 2003, Kojima et al. designed ECIA for detection of two tumor
markers: α-fetoprotein (AFP, a plasma protein elevated in several
types of cancer) and β2 microglobulin.
788
Shortly afterward, Wilson reported ECIAs for
the simultaneous quantitative detection of seven tumor markers.
790
Such ECIAs were prepared by immobilizing immunoreagents
on photolithographic and sputter-deposited layers adhering to a glass
substrate. In a different study, a multiplexed analysis of a tenascin
C, glycoprotein present in the extracellular matrix expressed in adults
in cancerous tissues (solid tumors like glioma or breast carcinoma),
was performed on a chip having nine independent electrodes with immobilized
primary antibodies.
791
When probed with
a HRP-labeled secondary antibody (against primary antibody), unoccupied
binding sites produced an insoluble film, leading to high R
CT observed with EIS. Conversely, the higher
was the concentration of the analyte, the lower was the R
CT of the electrode surface observed with EIS. The biosensor
was able to detect 14 ng (48 fmol) of tenascin C, sufficient for clinical
diagnostics.
791
Immunosensing strategies
are being increasingly coupled with various
nanomaterials. For instance, in 2010, a graphene sheet sensor platform
and functionalized carbon nanospheres labeled with HRP-secondary antibodies
(Ab2) for detection of α-fetoprotein (AFP), a plasma
protein elevated in several types of cancer, were proposed.
792
Enhanced sensitivity was achieved by (i) using
the multiconjugates of HRP-Ab2-carbon nanospheres onto
the electrode surface through “sandwich” immunoreactions
and (ii) functionalized graphene sheets-chitosan, which increased
the surface area, capturing a large amount of primary antibodies and
amplifying the detection response 7-fold, as compared to that without
graphene modification and labeling. Rusling’s group has successfully
designed several immunosensors for detection of various cancer biomarkers.
793
By using conductive nanostructured electrode
platform based on films of SWCNT forests,
689
they achieved LOD for PSA of 4 pg/mL. About an 8-fold increase in
sensitivity as compared to the above SWCNT forests was achieved by
using 5 nm glutathione-decorated Au nanoparticles, containing carboxylate
groups for attachment of large amounts of capture antibodies combined
with multienzyme magnetic beads bioconjugate.
794
Magnetic beads (1 μm in a diameter) contained 7500
HRP labels along with Ab2. Using this platform, LOD of
0.5 pg of PSA per mL in 10 μL of undiluted serum (corresponding
to 5 fg of PSA in the analyzed sample) was achieved, what is near
or below the normal serum levels of most cancer biomarkers. An excellent
correlation with standard ELISA assays in cell lysates and sera of
cancer patients was obtained. An ultrasensitive EC immunoassay protocol
with signal amplification via formation of a sandwich configuration
with Ab2 complexed to dendrimer-containing silver nanoparticles
could detect PSA down to 10 fM.
795
More
recently, Rusling’s group has simultaneously detected four
oral cancer biomarkers, IL-6, IL-8, vascular endothelial growth factor
(VEGF), and VEGF-C, directly in diluted sera obtained from 78 oral
cancer patients and 49 negative controls, with different serum levels
for each biomarker (Figure 35).
796
Good clinical sensitivity (aM levels) and specificity
for early stage tumor detection was reached, and, again, results were
confirmed by a good correlation with ELISA. Wan et al. have proposed
an immunoarray using so-called universal nanoprobe consisting of multi-HRP
labels and antirabbit antibodies loaded onto multiwalled carbon nanotubes
(MWCNT) for simultaneous detection of PSA and IL-8.
797
This strategy required three different antibodies, monoclonal
antibody against the biomarker, which was immobilized at the sensor
surface, polyclonal rabbit antibody against the biomarker recognizing
a different epitope, and antirabbit antibodies attached to the universal
nanoprobe, binding to polyclonal rabbit antibodies. In this way, multiple
antigens could be detected with only a single labeled antibody, leading
to a reasonable LOD of 5 pg/mL for PSA and 8 pg/mL for IL-8. Other
nanomaterial-based strategies for EC detection of tumor biomarkers
involved, for example, gold nanoparticle-modified screen-printed carbon
electrodes,
798,799
bilayer nano-Au and nickel hexacyanoferrates
nanoparticles,
800
nanosilver-doped DNA
polyion complex membrane,
784
bimetallic
AuPt nanochains,
637
or ferrocene liposomes
combined with MWCNT.
801
Figure 35
(A) Gold nanoparticle
(AuNP)-based immunosensor. The immunosensor
involves attached Ab1, which captures antigen from a sample,
followed by incubation with Ab2-magnetic bead-HRP (Ab2-MB-HRP), providing multiple
enzyme labels for each PSA bound.
The detection step involves immersing the immunosensor into a buffer
containing a mediator, applying voltage, and injecting H2O2. (B) Results for AuNP
immunosensor incubated with PSA
present in 10 μL of a calf serum (ng/mL labeled on curves, dashed
lines), cell lysates (HeLa and LNCap cells), and human patient serum
samples (1–3) (solid lines) for 1.25 h, followed by an injection
of 10 μL of 4 pmol/mL of Ab2-MB-HRP. (C) Validation
of AuNP sensor results for cell lysate and human serum samples by
comparing against results from an ELISA determination (relative standard
deviation ∼10%) for the same samples. Adapted with permission
from ref (794). Copyright
2009 American Chemical Society.
Table 3
List of Immunoassay-Based EC Studies
of Protein Biomarkers
biomarker
amplification
strategy
LODa
glycoprotein
ref
PSA
Ab2/AP
1.4 ng/mL
yes
(889)
PSA
Ab2/CNT/multi-HRP
4 pg/mL
yes
(689)
PSA
MB/HRP
0.5 pg/mL
yes
(794)
PSA
graphene/HRP/Ab2/Au NPs
2 pg/mL
yes
(890)
PSA
MWCNT/AuNP/HRP
0.4
pg/mL
yes
(891)
PSA
label-free (EIS)
1 ng/mL
yes
(892)
PSA
Ab2/HRP/MWCNT
5 pg/mL
yes
(797)
IL-8
8 pg/mL
no
PSA
multi-HRP/MB/Ab2
0.23 pg/mL
yes
(50)
IL-6
0.3 pg/mL
no
PSA
Ab2/HRP
2 ng/mL
yes
(893)
CEA
0.2 ng/mL
yes
CA 15-3
5.2 U/mL
yes
CEA
AuNP/Ag deposition
0.5 pg/mL
yes
(894)
CEA
graphene/Au
1 pg/mL
yes
(895)
AFP
microspheres/HRP
yes
CA
125
Ab2/dendrimer/CdS QD
0.005 U/mL
yes
(884)
CA 15-3
Ab2/dendrimer/ZnS QD
0.003 U/mL
yes
CA 19-9
Ab2/dendrimer/PbS QD
0.002 U/mL
yes
PSMA
virus matrix/PEDOT
100 pM
yes
(896)
AFP
graphene/Au–Pd nanocrystals
5 pg/mL
yes
(897)
IL-6
multi-HRP
10 fg/mL
no
(898)
IL-6
Ab2/MB/multi-HRP
10 fg/mL
no
(796)
IL-8
15 fg/mL
no
VEGF
8 fg/mL
yes
VEGF-C
60 fg/mL
yes
CA 125
SPCE/sol–gel/Ab2/HRP
0.5 U/mL
yes
(787)
CA 15-3
0.2 U/mL
yes
CA 19-9
0.3 U/mL
yes
CEA
0.1 μg/L
yes
a
PSA, prostate specific
antigen;
IL-6, interleukin 6; IL-8, interleukin 8; CEA, carcinoembryonic antigen;
CA, cancer antigen; AFP, α-fetoprotein; CRP, C-reactive protein;
LOD, limit of detection; PSMA, prostate specific membrane antigen;
VEGF, vascular endothelial growth factor; Ab2, secondary
antibody; AP, alkaline phosphatase; CNT, carbon nanotube; HRP, horseradish
peroxidase; MB, magnetic bead; AuNP, gold nanoparticle; QD, quantum
dot; PEDOT, poly(3,4-ethylenedioxythiophene).
9.2.1.1
Electric Field Effects
It has
been shown that application of positive potentials to the electrode
with immobilized DNA stimulates DNA hybridization, while a negative
electric field destabilizes DNA duplex.
34
Similarly, it has been observed that electric field can be utilized
in EC immunosensors. Application of positive potentials greatly increased
the rate of protein binding, while negative potentials were used to
prevent a weak nonspecific binding, improving thus the S/N ratio.
787
Originally, Wu et al.
785
used the sandwich assay in combination with a screen-printed carbon
electrode array and electron-transfer mediators to develop disposable
ECIAs for a low-cost multiplexed immunoassay of proteins. The multiplexed
immunoassay needed, however, a long incubation time due to the slow
diffusion of an antigen in an unstirred layer necessary for the immunocomplex
formation. Earlier, various technologies were explored to accelerate
the immunoreaction on the surface, including low-power microwave radiation,
802
magnetic stirring,
803
and magnetic field combined with superparamagnetic labels.
804
An electrophoresis-assisted optical immunoassay
was developed to accelerate the protein transport.
805−807
These technologies required additional equipment, resulting in the
increased cost and inconvenience of assays. To speed the incubation,
Wu et al.
787
constructed integrated electric
field-driven ECIAs for multiplexed immunoassay, in which antibodies
against carbohydrate antigens CA 125 (used especially for monitoring
of a therapy progress or recurrence of ovarian cancer), carcinoembryonic
antigen (CEA, a surface glycoprotein elevated in colorectal carcinoma),
CA 15-3 (a serum biomarker in breast cancer), and CA 19-9 were labeled
with HRP and immobilized on biopolymer/sol–gel modified electrodes.
In the presence of Au nanoparticles, the HRP showed enhanced EC responses.
Formation of immunocomplexes resulted in a decrease in the EC signals,
due to the impedance changes caused by the nonconductive immunocomplexes
and blocking of the electron transfer between the electrode and HRP-labeled
antibodies. The ECIA benefited from the electric field-driven incubation
strategy, and the EC end-point detection of four protein biomarkers
could be accomplished in less than 5 min. Literature on the electric
field effects on proteins is rather scarce, as compared to a large
number of papers on DNA, resulting from decades of studies of the
electric field effects on DNA.
34
Consequently,
these effects on proteins are much less understood. The incubation
time depended on the strength of an electric field/driving potential
and on a solution pH.
787
Shortening of
the incubation time was explained by increased immunoreaction rates
due to the faster transport of the antigens to the immunosensor interfaces.
More work will be necessary to understand fully all aspects of the
electric field effects on the surface protein–protein interactions.
9.2.2
Neurodegenerative Diseases
It was
mentioned in section 6.1 that both oxidation
signals at carbon electrodes and peak H at HMDE could be used to study
aggregation of α-synuclein (AS), a neuronal protein involved
in Parkinson’s disease. Its importance naturally led also to
the development of other EC strategies for its analysis, such as a
dual amplification strategy in which GCE was modified by gold nanoparticle-based
dendrimer loaded with thionine.
683
AS was
nonspecifically bound to this surface, then incubated with a primary
antibody, followed by an addition of secondary antibody labeled with
HRP adsorbed on a gold nanoparticle surface. CV was used to monitor
EC signal generated by the HRP in the presence of H2O2 with adsorbed thionine. LOD
in a high femtomolar range for
the analyte was 2–3 orders of magnitude lower as compared to
the approach without amplification by a gold nanoparticle-loaded dendrimer
or a secondary HRP-labeled antibody.
9.2.3
gp160
as a Marker of AIDS
An early
detection of HIV infection is desirable, and thus highly effective
bioanalytical methods have to be developed. One way to detect this
virus is to look for antibodies against HIV in patients serum. However,
these antibodies are usually not present in the serum in an initial
highly infectious phase. Therefore, an alternative way is to look
for glycoproteins on the surface of HIV virus such as gp160 and gp120.
808
An immunoassay using antibodies raised against
this antigen offered LOD down to the low picomolar level.
809
The method relied on a nanostructured layer
deposited on GCE working as a catalyst to reduce H2O2, followed by an amperometric
monitoring of a current at −300
mV, which decreased with an increased concentration of gp160. Spiking
of serum samples by this antigen revealed a recovery of 96–102%
with results being in excellent agreement when compared to ELISA.
809
9.3
Nucleic Acid Aptamers
The term aptamer
was first used in 1990 by Ellington and Szostak to describe artificial
RNA molecules binding to a small analyte.
810
The name aptamer comes from the Latin expression aptus (to fit) and from the Greek
word meros (part).
Aptamers are single-stranded oligonucleotides with a size of 15–60
nucleotides selectively binding a wide range of biomolecules, including
whole cells.
811
DNA/RNA aptamers have several
attractive properties such as a relative simplicity of chemical modification,
simple regeneration/reusability, and thermal and chemical stability.
812
The small size of DNA/RNA aptamers is a key
in achieving high interfacial densities, resulting in the construction
of selective and sensitive bioanalytical interfaces. As compared to
DNA, RNA is more flexible from a structural point of view, and thus
RNA aptamers can be theoretically raised against a wider range of
analytes.
813
However, a major limitation
of using RNA is their susceptibility to chemical and/or enzymatic
degradation and the time-consuming process of its preparation. Aptamers
resistant to chemical/enzymatic degradation can be prepared by modifications
of the DNA or RNA backbone or by introduction of modified bases.
814
As compared to the immunoassays, literature
on aptamer-based EC detection of protein biomarkers is rather scarce.
Nevertheless, some recent papers appeared showing that aptamers might
be also useful as biorecognition elements when constructing EC platforms
suitable for clinical diagnostics (Figure 36).
Liu et al.
815
developed an EC
DNA aptamer-based
biosensor for detection of interferon gamma (IFN-γ), the aberrant
expression of which is associated with a number of autoinflammatory
and autoimmune diseases. Thiolated DNA aptamer conjugated with methylene
blue as a redox tag was immobilized on a gold electrode, followed
by a binding of IFN-γ, which caused the aptamer hairpin to unfold,
moving methylene blue away from the electrode and decreasing electron
transfer rate. Using SWV, LOD was estimated as 0.06 nM.
Aptamers
were employed also in a study aimed at the detection of
human cellular prions, PrPC, playing a role in prion diseases.
816
The biosensor comprised MWCNTs modified with
polyamidoamine dendrimers, which in turn were coupled to DNA aptamers
used as bioreceptors. The signal originated from a ferrocenyl redox
marker incorporated between the dendrimer and the aptamer layer. Interaction
of aptamers with prion proteins led to a change of the EC signal,
with prion proteins detectable down to 1 pM concentration.
Besides
protein molecules, aptasensors were utilized also in the
detection of whole cancer cells with biomarkers exposed at the cell
surface. Feng et al. reported an EC sensor for multiple cancer types
using functionalized graphene and the aptamer AS1411, the first clinical
trial II aptamer, with a high binding affinity and specificity to
the overexpressed nucleolin on the cancer cell surface.
817
The sensor having LOD as low as 1000 HeLa cells
(using EIS) could be regenerated and reused. More recent work employing
aptamer-aided recognition was described, in which a cancer cell-aptamer
binding event mediated by an AP-catalyzed silver deposition reaction
was followed by an EC detection.
818
In
this work, Ramos cells detected down to 10 cells were used as a model
for possible analysis of blood cell cancer or Burkitt’s lymphoma.
Zhang et al. studied the HL-60 leukemic cell line using a DNA aptamer
against this cell line.
819
DNA aptamer
was first hybridized with capture DNA conjugated to gold nanoparticles.
Upon cells binding to DNA aptamer, capture DNA attached to the gold
nanoparticle was released from DNA aptamer and detected on GCE modified
by CdS QDs and DNA complementary to capture DNA. After hybridization
on a modified GCE, an energy transfer between gold nanoparticles and
QDs was registered in the form of electrochemiluminescence at an applied
potential of −1200 mV vs SCE. The signal increased with increasing
cell number with LOD of 20 cells/mL.
819
So far, DNA aptamers have not been used for EC analysis of
glycoproteins
via glycan recognition, but quite a few reports refer to effective
production of DNA aptamers against glycan moieties of glycoproteins,
for example, from Binghe Wang’s laboratory.
820
An extended library consisting of modified nucleotides
was used for the generation of DNA aptamers against glycoproteins
such as a necrosis factor receptor superfamily member 9,
821
vascular endothelial cell growth factor-165,
or IFN-γ.
822
Boronate affinity capillary
proved to be an efficient tool for selection of novel DNA aptamers
against glycoproteins with a quite high affinity (∼10 nM),
and a process was completed within 2 days with a small consumption
of glycoproteins or oligonucleotides (10–20 μL).
823
The proof of the concept approach was applied
for the selection of DNA aptamers against HRP, but can be applied
to other glycoproteins with a diagnostic potential.
823
Although the aptamer-based approach for sensing purposes
is promising,
further studies using real human samples are necessary. In such studies,
modification of the aptasensor surface will be very important to avoid
nonspecific adsorptions. In real biological samples, probably worse
LODs will be obtained.
Figure 36
Different approaches for detection of proteins
using nucleic acid
aptamers. (A) Redox-labeled aptamer alters its conformation after
aptamer–protein complex formation, positioning the label closer
to the electrode.
885
(B) Strategy employing
two aptamers, electrode-immobilized aptamer for capturing the protein
and a second aptamer labeled with enzyme for EC monitoring of enzymatic
reaction.
886
(C) Approach similar to that
in (B), with the second aptamer being labeled with gold nanoparticle–ferrocene
conjugate.
887
9.4
Peptide Aptamers
The term peptide
aptamer was first applied by Colas in 1996
824
to define a protein having a variable peptide sequence displayed
on an inert scaffold protein. Peptide aptamers exhibit affinity constant
toward protein analytes comparable to antibodies.
811
Immobilization of peptide aptamers in a biochip format
would generate an “army of terracotta soldiers” with
unique “facial” features (binding sites). Such proteins
exhibit a high solubility and chemical/thermal stability, a small
uniform size of a scaffold protein, and peptide aptamers can be produced
in a cost-effective way using heterologous techniques.
813
Such proteins could be immobilized with high
density on surfaces proving high sensitivity and selectivity of bioanalytical
devices.
811
Peptide aptamers are isolated
from combinatorial libraries during a selection procedure, when the
whole library is incubated with an analyte of interest, and thus there
is no requirement to know the structure of the analyte and/or the
mechanism of the binding in advance.
825
The scaffold protein has to be pretreated to “silence”
its original biological activities with a possibility to tolerate
peptide sequences without changing its overall structure.
826
Ferrigno’s group developed peptide aptamers
based on a scaffold constructed by mutations of Stefin A protein (a
cysteine protease inhibitor). This scaffold allows inserting more
than one peptide sequence, and peptide aptamers are working well after
being immobilized on various interfaces.
827
The most systematic studies with the application of EC methods
in combination with peptide aptamers were realized with peptide aptamers
based on a Stefin A triple mutant scaffold. This scaffold is made
from a Stefin A protein by introducing three point mutations to block
its binding with its interaction partners. Various peptide inserts
against CDK2 and CDK4 proteins (kinases playing an important role
in the cell cycle),
828−832
biomarkers of systemic sclerosis,
833
or
C-reactive protein
834
were introduced into
this scaffold protein. Moreover, a library of peptide aptamers based
on Stefin A triple mutant scaffold can be applied for the identification
of novel disease biomarkers and for the development of novel clinical
assays.
835
Estrela et al. developed
an EC method of protein detection by utilizing
peptide aptamers to sense a change in the interfacial charge as a
result of a biorecognition.
828−830
In the first study, such changes
were monitored in a real-time by an open circuit potential measurement
using an ultralow input bias current instrumentation amplifier.
828
Three different peptide aptamers (pep2, pep6,
and pep9) differing in peptide inserts and two analytes CDK2 and CDK4
(both present in complex cell lysates) were applied in the study.
These label-free measurements were complemented by EIS assays with
a soluble redox probe. Results indicated that a negative shift of
an open circuit potential was consistent with an increase in R
ct and a decrease in capacitance of the interfacial
layer after analyte binding.
828
The
Davis group compared a bioanalytical performance of two different
immobilized peptide aptamers with the performance of the immobilized
antibody to detect C-reactive protein.
834
Three detection techniques were applied in such a comparison, including
fluorescent protein microarray, surface plasmon resonance, and EIS.
In surface plasmon resonance and microarray configurations, antibody
outperformed peptide aptamers from an analytical perspective. If EIS
was applied for protein detection, increased assay sensitivity was
observed for detection of C-reactive protein with immobilized peptide
aptamers as compared to the device based on the immobilized antibody,
but both biorecognition molecules were able to detect the analyte
down to the 300 pM level.
834
Three
important challenges ahead in the field of multiplexed protein
analysis have recently been postulated by Colas, including generation
of highly stable, specific biorecognition molecules, production of
high-density arrays, and a very sensitive detection platform.
836
All of these challenges were addressed in a
pioneering work from Walti’s group.
832
The first challenge was addressed by utilization of peptide aptamers
pep2 and pep9 based on a Stefin A triple mutant scaffold raised against
proteins CDK2 and CDK4. Second, high density of immobilized peptide
aptamers was achieved using an original masking/demasking procedure
(Figure 37A).
837
An array of 10 gold 20 μm wide microelectrodes 15 μm
apart was first masked by a methyl-terminated polyethylene glycol-terminated
thiol. On-demand immobilization of a peptide aptamer on electrode
#1 within an array was performed in two steps. The first step involved
application of a negative voltage of −1.4 V vs Ag/AgCl for
120 s where a reductive desorption of a thiol mask was applied on
the electrode #1, while the rest of microelectrodes within an array
were held at −0.2 V (Figure 37B). In
the second step, a bare gold microelectrode #1 was covered by a peptide
aptamer via a cysteine–gold bond (Figure 37C). The procedure could be repeated on
the rest of gold microelectrodes
to selectively pattern a whole array by different peptide aptamers
(Figure 37D). The third challenge was addressed
by an application of a label-free EIS, when a phase difference between
an applied working potential and measured current, Φ(ω),
was utilized for sensing purposes. A typical dependence of Φ(ω)
as a function of applied frequency ω is shown in Figure 37E, indicating a binding
of CDK2 to a pep9 modified
surface, while a binding of CDK2 on pep2 and on a reference surface
was negligible. The device, which was able to detect CDK2 in a clinically
relevant concentration range from 25 pM to 100 nM, worked reliably
well in a cell lysate. The authors concluded that biochip fabrication
is scalable, enabling the generation of high-density, submicrometer
electrode arrays, unlike the conventional printing pin-based technologies
with resolutions of the order of 0.1 mm or more, for massively parallel,
ultrasensitive, and label-free analysis of proteins present only in
a single cell.
832
Figure 37
Electrochemically triggered
immobilization of peptide aptamers
within a biochip. (a) All microelectrodes are initially protected
by a mask from mPEG, resisting protein adsorption. (b) Release of
a mask from the electrode #1 by a highly negative voltage. (c) Funcionalization
of the electrode #1 with a peptide aptamer. (d) Independent functionalization
of the whole array by various peptide aptamers by repeating steps
(a)–(c). (e) Analysis of CDK2 bound to pep2, pep9, or to a
reference surface covered by mPEG as change in a phase shift, ϕ(ω),
which is a phase difference between an applied working potential and
measured current. A typical dependence of ϕ(ω) as a function
of applied frequency ω used in the analysis is shown. mPEG is
a methyl-terminated polyethylene glycol containing thiol. Reprinted
with permission from ref (832). Copyright 2008 BioMed Central.
Even though peptide lectin aptamers have not been raised
yet, it
is possible that a restricted range of aa’s enriched in four
aa types involved in glycan recognition by lectins will help to design
such proteins with high selectivity and affinity toward glycoproteins
in the future.
9.5
Analysis of Glycoprotein
Biomarkers
In sections 9.2, 9.3, 9.4, we described strategies
involving recognition
of protein biomarkers by antibodies or aptamers, that is, strategies
aimed at protein molecules themselves. In this section, we would like
to pay attention to the application of EC methods in the analysis
of glycoprotein biomarkers or in glycoprofiling of intact, mainly
cancerous cells, utilizing recognition of glycan (nonprotein) part
of the biomarker, as described more generally in section 8. Protein glycosylation,
which is the most common
post-translational modification in higher organisms including human,
means that a wide range of different glycoproteins can be used as
disease biomarkers for early detection of pathological processes.
This is a relatively new, yet very interesting approach with a potential
to develop assays for the detection of diseases such as diabetes,
rheumatoid arthritis, infection by a Dengue virus, as well as various
types of cancer.
9.5.1
Glycated Hemoglobin as
a Diabetes Marker
Glycated hemoglobin is a marker of diabetes.
In this disease, a
continuously elevated concentration of glucose within the blood of
patients with diabetes modifies the N-terminal valine of the β-chain
of hemoglobin. Thus, monitoring of glycated hemoglobin can give information
about a long-term progression of the disease, not influenced by a
short-term variability in the glucose concentration. The clinical
reference range is 4–20% of a glycated form of hemoglobin related
to the total amount of hemoglobin with the amount above 7% indicating
a pathological process. The 1% change in this percentage proportion
corresponds to a fluctuation in mean blood glucose concentration of
about 350 mg/L. Besides traditional methods of analysis of glycated
hemoglobin,
838
the EC way of determination
can be a viable alternative. The first study for EC detection of glycated
hemoglobin complexed with ferroceneboronic acid was published in 2008.
839
The EC assay-based method could detect nanomolar
level of glycated hemoglobin, and a 3-fold signal improvement was
achieved using glucose oxidase immobilized on the electrode to enhance
electron transfer rate between the electrode and a protein–ferrocene
complex.
839
Later, a poly(amidoamine) dendrimer
layer formed on a modified gold electrode surface was applied to detect
glycated hemoglobin with a percentage of 2.5–15% in a mixture
with hemoglobin.
840
Recently, glycated
hemoglobin was successfully detected in whole blood. A blood sample
was hemolyzed, and total hemoglobin was preconcentrated by Zn2+-induced precipitation
and centrifugation to remove interfering
glucose and glycoproteins including antibodies.
841
The study applied the same transduction mechanism as discussed
above.
839
9.5.2
Rheumatoid
Arthritis
Antibodies
(particularly immunoglobulins G, IgG) circulating in the blood contain
complex type N-glycans of a biantennary structure.
842
IgG’s glycan is in healthy individuals
often terminated with N-acetylneuraminic acid, and
in patients with rheumatoid arthritis this glycan can be terminated
in galactose or N-acetylglucosamine. The severity
of the rheumatoid arthritis correlates with the extent of the IgG’s
glycosylation change.
520,644,843
Diluted serum samples from patients suffering from rheumatoid arthritis
and healthy individual controls were analyzed by the EIS-based lectin
biosensors.
644
Three different lectins
were covalently immobilized on SAM patterned gold surfaces to prepare
three lectin biosensors for detection of sialic acid, galactose, and
mannose/N-acetylglucosamine present on the surface
of glycoproteins. The lectin biosensors could detect glycoproteins
selectively down to the femtomolar level with the ability to distinguish
between samples from healthy and sick individuals based on changes
in the glycan composition on IgGs. Moreover, the lectin biosensors
outperformed state-of-the-art lectin microarrays as a commonly used
glycoprofiling tool in terms of wider linear range response and LOD.
Thus, EIS-based lectin biosensors have a great potential for searching
for new disease biomarkers, which are often present in complex samples
at extremely low concentrations.
644
A recent
study suggests that other glycoproteins, such as ficolin 3, might
be potential biomarkers for rheumatoid arthritis.
844
9.5.3
Viral Glycoproteins
Dengue is a
widely spread disease caused by RNA dengue virus. Four different virus
serotypes can cause three clinically, pathologically, and epidemiologically
distinct symptoms.
845
EIS biosensor based
on Con A lectin immobilized on a gold nanoparticles (AuNPs)-modified
electrode exhibited different interactions with serum glycoproteins
from patients suffering from the disease and control serum samples.
845
Moreover, EIS-based Con A biosensor could distinguish
the serum of healthy individuals from the serum of people suffering
with different disease types by variations in R
CT used for data evaluation of the biosensor.
846
Moreover, when three parameters obtained from fitting EIS
response by an equivalent circuit were plotted in a 3D graph, a distinct
spatial localization of different samples within the graph was observed.
In the next two studies, the authors managed to apply new lectins,
CramoLL from Cratylia mollis(847) and BmoLL from Bauhinia monandra,
848
in the detection of various dengue
sera serotypes applicable in the analysis of an early stage of dengue
virus infection.
Hong et al. prepared a nanostructured surface
with immobilized Con A for analysis of Norovirus (causing gastroenteritis)
viral particles in a sandwich format of analysis.
849
The EC response (CV and EIS) of the biosensor was linear
in the concentration range 102–106 copies/mL
with LOD in real samples (lettuce extract) of 60 copies/mL. Analysis
was completed within 1 h, and the biosensor was able to distinguish
between the Norovirus particles and hepatitis A and E viral particles,
respectively, with selectivity of about 98%.
849
9.5.4
Cancer
An early diagnosis of any
type of cancer requires identification of cancer biomarkers, an effort
feasible to be achieved only by a multidisciplinary approach relying
on an array of strategies, technologies, and methods.
569,850,851
Such studies revealed that in
cancer tissues, glycan structures of glycoproteins are altered. There
are only few cancer glycoprotein biomarkers approved by the U.S. Food
and Drug Administration for diagnostics of various cancer types (Table 3), but the
reliability of many of these is questionable.
9.5.4.1
P-Glycoprotein
A multidrug resistance
is the main reason for the limiting efficiency of chemotherapy to
treat cancer. Resistance is the result of overexpression of few proteins,
and among them the most important appears P-glycoprotein, a protein
believed to function as an energy-dependent drug efflux pump lowering
intracellular drug level.
852
Thus, evaluation
of the P-glycoprotein level on the cell membrane can predict efficiency
of chemotherapy.
684
Huangxian Ju’s
group detected P-glycoprotein directly in an immunoassay format on
the cell surface of K562/ADM leukemic cells with a primary monoclonal
antibody followed by incubation with a secondary antibody (against
primary antibody) conjugated with AP enzyme.
684
The same experimental setup with a primary and a secondary antibody
was later applied for analysis of human gastric BGC823 carcinoma cells
via detection of P-glycoprotein.
685
In
this case, AP made an insoluble product on an electrode surface by
hydrolysis of 5-bromo-4-chloro-3-indolyl phosphate, which made the
DPV assay more sensitive. Interestingly, this approach was applied
to calculate the number of P-glycoproteins as many as 4.7 × 107 molecules per one
cell.
685
Huangxian
Ju’s group later applied EIS for a label-free detection of
P-glycoprotein directly on the surface of K562/ADM leukemic cells
by their incubation on the GCE modified by antibody against this protein.
853
All of these assay protocols for detection
of P-glycoprotein relied on antibodies, and comparison of cancer cell
lines with other cells was not done.
The first paper using immobilized
Con A lectin for attachment of cancerous HeLa cells with subsequent
biorecognition of P-glycoprotein by HRP-labeled antibody was published
in 2010.
681
The DPV signal read at −250
mV vs SCE was generated by the action of HRP in the presence of H2O2 on the thionine-adsorbed
surface, and the method
allowed estimation of 4.0 × 1010 mannose moieties
and 8.5 × 106 molecules of P-glycoprotein, respectively,
on each HeLa cell. Interestingly, reversal of a multidrug resistant
leukemia cell K562/ADM into a drug resistant one by addition of limonin
agent was monitored by EC means. After addition of limonin into suspension
of cancer cells, the efflux of an anticancer drug kaempferol from
the cells decreased. Because this drug is redox active, its extracellular
concentration was monitored by DPV at 150 mV vs SCE in a convenient
way.
854
9.5.4.2
PSA
Most EC studies on PSA detection
were performed in an immunoassay format not involving recognition
of glycan moieties (section 9.2). Recent studies,
however, suggest that PSA from sera of patients with prostate cancer
contain glycan with a higher amount of sialic acid as compared to
PSA from healthy individuals or those having benign prostatic hyperplasia.
543
Moreover, it was shown that a relative abundance
of glycosylated isoforms of PSA may provide useful additional information
for clinically relevant investigations.
855
Despite these efforts, no EC-based biosensor so far has been described
for the detection of PSA via recognition of a glycan moiety of PSA.
9.5.4.3
Carcinoembryonic Antigen
CEA
is one of the most widely used tumor biomarkers worldwide and the
most often utilized biomarker of colorectal cancer. Although it was
shown that CEA (or a CEA-like protein) is present in healthy people,
its concentration in people affected by cancer is approximately 60-fold
higher as compared to healthy individuals.
856
CEA is considered to be a biomarker also for other kinds of cancer
including breast, lung, and pancreatic cancer.
543
Again, we will discuss here methods of CEA detection
relying on or involving (at least to some extent) its glycan part.
For instance, EC immunosensor for analysis of CEA was built up on
a layer of boronic acid containing SAM layer for selective attachment
of CEA via its glycan moiety. In the next step, an anti-CEA (HRP-labeled)
antibody was added. Finally, two different detection strategies were
employed: either EIS assays or DPV technique detecting thionine via
HRP at an applied potential of −210 mV. Both assay protocols
allowed the detection of CEA down to 1 ng/mL (∼10 pM) concentration.
857
An elegant strategy on how to detect
a drug to cure a colorectal
cancer, cetuximab, together with an analysis of a disease marker CEA
in a single square wave voltammogram was based on the displacement
of QD-labeled glycans by the target glycans using two different lectins,
Con A and Euonymus europaeus lectin,
as the recognition element.
705
Upon incubation
with the sample containing both the drug and the biomarker, QDs conjugates
(ZnO/CEA and CdSe/cetuximab) were displaced from the surface, and
released Zn2+ and Cd2+ ions were analyzed by
stripping voltammetry at −1100 or −700 mV, respectively,
vs SCE on a single GCE. Both glycoproteins were detected down to a
concentration 1 order of magnitude lower than a required diagnostic
cutoff value for both proteins.
705
Even
though this suspension array was not applied in real sample analysis,
the authors suggested that this detection platform for simultaneous
analysis of 5–6 proteins in a single run is possible.
CEA as a cancer biomarker was detected together with epithelial
cell adhesion molecules directly on the surface of circulating cancer
cells with antibodies attached to gold nanoparticles, while other
circulating cells were not affected. EC detection of circulating cancer
cells expressing both antigens was performed by the aid of an excellent
electrocatalytic activity of gold nanoparticles toward hydrogen evolution
detected chronoamperometrically at a negative voltage.
858
9.5.4.4
α-Fetoprotein
The α-fetoprotein
(AFP) is present in different forms of cancer and contains biantennary
glycan chains of a complex type.
575
AFP
was detected with a label-free EIS-based biosensor with wheat-germ
agglutinin immobilized covalently on oxidized SWCNTs deposited on
a screen-printed carbon electrode down to the femtomolar level.
859
Glycan composition of AFP was probed by four
other lectins for detection of all carbohydrates present in the glycan,
that is, mannose, sialic acid, galactose, N-acetylglucosamine,
and N-acetylgalactosamine. Serum samples from healthy
individuals and those suffering from cancer were diluted 1:100, and
glycoprofiling showed changes in glycan composition as a result of
liver cancer.
859
It was concluded that
samples from patients with cancer contain a higher amount of fucose
and sialic acid on glycoproteins (such as AFP), as compared to samples
from healthy individuals. This makes the strategy applicable for clinical
diagnosis of an early stage of cancer.
859
9.5.5
Analysis of Intact Cancerous Cells
9.5.5.1
Gastric Carcinoma Cells
An extremely
sensitive lectin-based biosensor was prepared through attachment of
gastric carcinoma cells on the surface of SWCNT-coated GCE modified
by RGDS peptide, applied for nonspecific attachment of any type of
cells via integrin receptors.
860
Mannosyl
groups present on the surface of cells were profiled by incubation
with HRP-labeled Con A lectin, and the EC signal was generated by
the oxidation of o-phenylendiamine by HRP in the
presence of H2O2 using DPV (Figure 38). Both SWCNTs and HRP amplified the EC signal
considerably with an ability to detect only 620 cells/mL or as few
as 6 cells present in the 10 μL of the assayed liquid. The cells
could be detected in a concentration up to 1 × 107 cells/mL, and an estimated amount
of mannose molecules was 5.3 ×
107 mannoses/cell.
860
Figure 38
Analysis
of a gastric carcinoma cell line after a nonspecific attachment
of cells to a RGDS peptide. In the following step, a Con A–HRP
conjugate was injected to probe glycans on the cell surface. A DPV
signal was acquired by analysis of a product of oxidation of o-phenylendiamine by
HRP in the presence of hydrogen peroxide.
Adapted with permission from ref (860). Copyright 2008 American Chemical Society.
9.5.5.2
Hepatoma/Liver
Carcinoma Cells
A human hepatoma carcinoma SMMC-7721 cell
line was analyzed using
photosensitive CdS-polyamidoamine nanocomposite film prepared by an
electrodeposition method.
861
In the presence
of ascorbate, the photoexcitation of this modified electrode held
at a potential of 0 V vs Ag/AgCl led to an anodic photocurrent. Con
A lectin was immobilized on the surface of modified ITO electrode
covalently via glutaraldehyde, and after the cells were bound to Con
A, a change of the current could be applied for a signal reading with
a LOD down to 5 × 103 cells/mL.
861
A human liver Bel-7404 cancer cell line was detected
by EIS sensing with Con A immobilized directly on a gold electrode,
achieving LOD of 234 cells/mL.
862
Much
lower change of R
ct was observed when
the biosensor was incubated with normal liver cell line L02, because
these cells contain a lower amount of a membrane-associated glycoprotein
gp43. Finally, the biosensor exhibited a high reliability of sensing
with a recovery index of 97–100%.
862
9.5.5.3
Human Leukemic Cell Lines
Two
different leukemic cell lines were analyzed in numerous studies with
Con A lectin almost exclusively applied for biorecognition and quantitation
of a number of mannose residues present on the surface of such cells.
Human leukemic K562 cell line was detected by incubation with Con
A covalently labeled by ferrocene carboxylic acid.
708
These cells were detected down to 3 × 103 cells/mL, and EC assay protocol agreed with
results obtained from
flow cytometric detection.
708
Later, a
similar concept of analysis of K562 cells was based on Au nanoparticles
loaded with Con A and ferrocene moieties.
709
The Con A immobilized on a mixed SAM film recognized the cells,
and the sandwich assay protocol was completed by incubation of a cell
layer with modified gold nanoparticles. The biosensor could detect
as few as 73 cells/mL.
709
The same cell
line was analyzed on a carbon nanohorn-modified GCE interfaced with
RGDS peptide.
682
A gold nanoparticle loaded
with Con A and HRP formed a sandwich configuration for analysis of
cells down to a concentration of 1.5 × 103 cells/mL.
682
A sandwich configuration combined with electrochemiluminescent
detection was applied for analysis of the K562 cell line.
863
GCE modified by MWCNTs was applied for covalent
immobilization of Con A, and after incubation with cells, a nanoprobe
containing gold nanoparticles, Ru(bipy)3
2+-doped
silica nanoparticles, and Con A was applied for signal generation.
K562 cells could be detected down to 600 cells/mL. The biosensor was
successfully applied to monitor dynamic changes in the expression
of glycans on cell surface during a growth phase and to detect variation
in the glycan composition as a result of an inhibitor action.
863
Another sandwich configuration with electrochemiluminescent
detection protocol allowed the detection of K562 cells down to 46
cells/mL.
864
A screen-printed carbon electrode
modified by a nanoporous gold film was applied for DNA aptamer immobilization.
864
Carbon QDs deposited on ZnO nanospheres conjugated
to Con A formed a sandwich configuration after cells were bound to
the DNA aptamer. The biosensor reliability was proved by a recovery
index between 92% and 106%, and the biosensor could be regenerated.
864
EIS-based biosensor built on a GCE modified
by MWCNTs with immobilized Con A was applied for the analysis of a
K562 cell line down to a concentration of 1 × 104 cells/mL.
865
The EC method was applied for monitoring of
a dynamic glycan variation on K562 cells as a response to their exposure
to drugs.
865
In another approach,
the EIS assay of K562 cells was integrated
into a microfluidic array platform of three different immobilized
lectins.
866
ITO electrodes were used for
EIS detection of a binding event, but also for cell counting using
optical microscopy. Both methods of analysis showed that the best
lectin for binding of K562 cells is wheat germ agglutinin, followed
by Con A and PNA. The optical-EC microfluidic biochip was employed
to evaluate the composition of cell surface glycans in response to
the 3′-azido-3′-deoxythymidine drug.
866
Human leukemic HL-60 cell lines were also detected
in a sandwich
configuration.
704
Annexin V deposited on
a 3D matrix formed on GCE through a modification with nitrogen-doped
CNTs, and gold nanoparticles were utilized to capture human cells.
In the next step, silica particles modified by few layers of CdTe
QDs and Con A were injected over the cell surface for a signal generation.
A stripping voltammetry applied for the detection of Cd2+ exhibited LOD of 1 × 103
cells/mL (48 apoptotic
HL-60 cells) with a successful validation by a flow cytometry.
704
Glycoprofiling of K562 cells can be done on
lab-on-a-paper devices containing 3-D macroporous Au-paper electrode
with EC or electrochemiluminescent detection.
867
A sandwich format of analysis with application of lectins
exhibited LOD of 4 cells with linear range spanning from 550 to 2.0
× 107 cells/mL.
867
In the
second case, aptamer-modified surface and Con A-conjugated porous
AuPd alloy nanoparticles as nanolabels were applied for electrochemiluminescent
signal readout system.
868
This microfluidic
paper-based origami cyto-device was applied in the analysis of K562
cells down to a concentration of 250 cells/mL.
868
9.5.5.4
Intestinal/Colon Human
Carcinoma Cells
A novel EC method for the detection of cancer
cells was based on
deposition of a quinone derivative on a gold electrode via gold–sulfur
surface chemistry, making the electrode redox active.
869
The DNA aptamer against lectin l-selectin
then was adsorbed on the electrode, “shielding” a quinone
redox activity, which resulted in a decreased current. In the subsequent
step, incubation of the biosensor with l-selectin pushed
a DNA aptamer away from the electrode, partly restoring redox activity
of a quinone moiety. When intestinal human carcinoma LS180 cells were
incubated with the biosensor, l-selectin was withdrawn by
cells, leaving the surface of the biosensor blocked by the DNA aptamer.
Thus, a decrease of the current response was again detected after
cells were bound to l-selectin. The carcinoma cells were
detected down to 1 × 103 cells/mL.
869
9.5.5.5
Human Lung Cancer Cells
Quite
a complex assay protocol was applied to achieve LOD down to 12 cell/mL
for a lung cancer H1299 cell line.
870
The
assay procedure relied on CNT and gold nanoparticle-modified GCE interface
with immobilized Con A. Such a surface was preincubated with CNTs
loaded with gold nanoparticles, thionine (a redox probe), and mannose
units. When the device was incubated with cancerous cells, the hybrid
nanocomposite was displaced from the surface by the cells with a decrease
in the current as a result of a lower amount of thionine present on
the electrode surface (Figure 39). The 95-D
cell line containing 2.8 × 108 mannose units per cell
could be detected down to 580 cells/mL, while the H1299 cell line
having 3.1 × 1010 mannose units per cell could be
detected with unprecedented LOD of 12 cells/mL.
870
Figure 39
A displacement strategy for analysis of cells. Con A was
immobilized
on SAM layer by covalent coupling. A thionine/carbon nanotube (CNT)/gold
nanoparticles (AuNPs) nanocomposite was then incubated with the biosensor,
and displacement of the nanocomposite from the surface of the Con
A biosensor after incubation with cells resulted in a decrease of
EC response. Adapted with permission from ref (870). Copyright 2011 Royal
Society of Chemistry.
9.5.5.6
Human Gastric Cancer Cells
Human
gastric cancer cells were detected by an interesting competitive approach
down to a concentration of 1.2 × 103 cells/mL.
871
GCE was covered with CdS QDs and a mannan (mannose-containing
polysaccharide) to selectively detect Con A lectin. First, cells were
incubated with Con A in a solution, and then the solution was incubated
with the GCE-modified surface to detect freely available residual
Con A molecules not bound to the cells. After binding of Con A to
the modified surface, electrochemiluminescent signal decreased proportionally
with increasing amount of freely available Con A and thus increased
with increasing concentration of cells. These measurements suggested
that every cell contains 8.7 × 107 glycan molecules.
871
Two lectins, Sambucus
nigra agglutinin (SNA, for sialic acid) and Con A
(for mannose), were used for the monitoring of changes in the expression
of mannose and sialic acid in normal and cancer cells derived from
human lung, liver, and prostate tissues.
872
The lectins were covalently immobilized on the modified GCE, and
after attachment of cells, a sandwich was completed by incubation
of the surface with nanoparticles loaded by the lectin and thionine
as a redox probe. Analysis of three different types of lung cancerous
cells showed an increased expression of sialic acid on the cell surface,
while a decrease in the amount of mannose units on the surface of
the cell was observed at the same time, when compared to a normal
cell line. The same glycosylation changes were detected on two different
liver cancerous cell lines, as compared to normal cells. In the case
of prostate cancer, a cancerous cell line showed an increase in the
amount of both sialic acid and mannose, as compared to normal cells.
872
Analysis of cancer CCRF-CEM cell line
was possible with application
of a supersandwich strategy for the amplification of signal.
873
In this case, LOD of 50 cells/mL was achieved.
GCE was patterned by CNTs, gold nanoparticles, and Con A to complete
a receptive layer. The cells were incubated with this biosensor surface,
and in a subsequent step a supersandwich was formed by binding of
cells to DNA concatamer containing CdTe QDs. DNA concatamer with multiple
target molecules and signal probes was formed by hybridization of
three different oligonucleotide chains (Figure 40). Finally, cancer cells were detected
by anodic stripping voltammetry
of Cd2+ ions. A supersandwich biosensor configuration was
5.6-fold more sensitive as compared to a biosensor configuration using
only a DNA aptamer–QD conjugate.
873
Figure 40
A supersandwich strategy for signal amplification in cancer cell
analysis. Initially, GCE was modified by oxidized MWCNTs, dopamine,
and AuNPs to which Con A was immobilized for detection of a CCRF-CEM
cancerous cell line. DNA concatamer was first assembled from a capture
DNA-aptamer (CDNA), a CdTe QD-labeled signal DNA (SDNA), and an auxiliary
DNA (ADNA). When the cells were attached to the surface, they were
probed by a preassembled concatamer, and finally a binding event was
detected by an anodic stripping voltammetry of Cd2+. Adapted
with permission from ref (873). Copyright 2013 American Chemical Society.
CCRF-CEM cells can be detected with a LOD of 10
cells/mL on GCE
modified with a poly(amidoamine) dendrimer on a reduced graphene oxide
(rGO) nanocomposite.
874
After immobilization
of Con A lectin and blocking with BSA, cells were successfully bound
to a surface, and the DPV signal was amplified by application of AuNPs/aptamer/HRP
nanoprobe in a sandwich format of analysis. In addition, at a concentration
of 5.0 × 104 cells/mL, this biosensor clearly distinguished
the CCRF-CEM from five other types of cell lines used in the study.
874
The same cell line was detected in a sandwich
configuration with LOD of 38 cells/mL using the biosensor having aptamer
immobilized on a rGO-dendrimer-modified surface with electroluminescent
detection.
875
9.6
Active Glycoprofiling by Microengines
A remarkable
way for active glycoprofiling of bacterial species in
a wide range of environments was developed by Joseph Wang.
876
Nanowire-based microengine with attached Con
A lectin was self-propelled by the formation of oxygen bubbles from
hydrogen peroxide as a fuel. The device was able to selectively pick
up Escherichia coli cells, transport
them, and finally release them by a change of pH.
876
Alternatively, cargo could be on demand released by the
interaction with saccharide.
877
The device
can be loaded with other cargos besides bacterial cells including
polymeric drug carrying spheres having a therapeutic effect.
876
The device can actively isolate microbial cells,
when Con A lectin was substituted by boronic acid-containing film.
878
Toxicity of hydrogen peroxide as a fuel was
overcome, when magnetic field
879
or ultrasound
880
was applied to propel microengines. However,
practical use and selectivity of such active glycoprofiling has yet
to be proved.
9.7
Concluding Remarks
Although many
EC strategies were developed for highly sensitive detection of various
important biomarkers with good accuracy and selectivity, not many
of them were actually validated against patient samples. Moreover,
especially in cancer diagnostics it will be important to detect simultaneously
a panel of 4–10 biomarkers to obtain more reliable results.
53
EC methods are well-suited for point-of-care
diagnostics, requiring inexpensive and simple instrumentation, which
is easy to operate. EC sensors and assays for determination of protein
biomarkers of cancer, AIDS, diabetes, or rheumatoid arthritis were
reported mostly on the basis of application of antibodies, aptamers,
or lectins for their capturing, often combined with nanomaterials
to greatly enhance the sensitivity of the method. Despite all of these
reports, much more work will be necessary to develop a reliable and
commercially successful EC device for point-of-care diagnostics, applicable
to clinical samples.
10
Conclusions
In
this Review, we show that EC analysis of proteins is not limited
to redox signals of nonprotein components of conjugated proteins and
that some aa residues in proteins can contribute to protein reduction
or oxidation signals. The development of a label-free EC method for
analysis of practically all proteins represents a great challenge
for electrochemistry to enter wide fields of proteomics and complement
standard methods. High sensitivity of EC methods, simple miniaturization,
and formation of chips for parallel analysis are among the advantages
of EC analysis. These advantages are gradually finding ground in biomedicine.
Very recently it has been shown
899
that
glycation (section 9.5.1) of BSA results in
a decrease or disappearance of electrocatalytic peak H (section 5). Moreover, we show
that EC analysis can be applied
also in glycomics to analyze glycans directly in glycoproteins on
the cell surface or after their isolation. In this direction, perhaps
the greatest progress has been done in EIS detection of glycoprotein
interactions with specific lectins. Moreover, it has been shown that
some glucosamine-containing poly- and oligosaccharides are electroactive
under conditions close to physiological and that most polysaccharides
and glycans can be transformed into electrochemically active substances
by a simple chemical modification. Detection of protein biomarkers
is only briefly summarized, but special attention is paid to glycoprotein
biomarkers and particularly to their glycan parts. It appears that
finding differences in protein glycosylations may significantly increase
the specificity of some biomarkers. Detection of a single biomarker
usually provides only a very low specificity. Therefore, usually 4–10
biomarkers have to be detected to obtain good specificity and selectivity
of detection. EC detection appears particularly advantageous for the
preparation of low-density chips with this number of biomarkers.
An important take-home message is that, when considering protein
EC analysis, not only voltammetry and EIS but also chronopotentiometry
should be taken into account. Now it is clear that the old chronopotentiometry
combined with adsorptive stripping, thiol-modified electrodes, and
present instrumentation is particularly advantageous in protein structure-sensitive
as well in DNA–protein complexes analyses. The combination
of high current densities with high-electron yield electrocatalytic
processes allows very fast potential changes, which can be utilized
in studies of protein molecules and protein complex stability at electrode
surfaces. Further development of this rapidly growing field may show
the great usefulness of EC methods for biochemistry and molecular
biology.