Glycation from α-dicarbonyl compounds has different effects on the heat-induced aggregation of bovine serum albumin and β-casein

Small-angle scattering of X-rays (SAXS) is an established method for the low-resolution structural characterization of biological macromolecules in solution. The technique provides three-dimensional low-resolution structures, using ab initio and rigid body modeling, and allow one to assess the oligomeric state of proteins and protein complexes. In addition, SAXS is a powerful tool for structure validation and the quantitative analysis of flexible systems, and is highly complementary to the high resolution methods of X-ray crystallography and NMR. At present, SAXS analysis methods have reached an advanced state, allowing for automated and rapid characterization of protein solutions in terms of low-resolution models, quaternary structure and oligomeric composition. In this communication, main approaches to the characterization of proteins and protein complexes using SAXS are reviewed. The tools for the analysis of proteins in solution are presented, and the impact that these tools have made in modern structural biology is discussed.

Accurate calorimetric data for the thermodynamics of transfer of six liquid hydrocarbons to water have been combined with solubility data to provide a model for the temperature dependence of the hydrophobic interaction in protein folding. The model applies at temperatures for which the change in heat capacity (delta Cp) is constant. The extrapolated value of the temperature (Ts) at which the entropy of transfer (delta S degrees) reaches zero is strikingly similar (Ts = 112.8 degrees C +/- 2.2 degrees C) for the six hydrocarbons. This finding provides an interpretation for the empirical relation discovered by Sturtevant: the ratio delta S degrees/delta Cp measured at 25 degrees C is constant for the transfer of nonpolar substances from nonaqueous media to water. Constancy of this ratio is equivalent to Ts = constant. When applied to protein folding, the hydrocarbon model gives estimates of the contributions of the hydrophobic interaction to the entropy and enthalpy changes on unfolding and, by difference, estimates of the residual contributions from other sources. The major share of the large enthalpy change observed on unfolding at high temperatures comes from the hydrophobic interaction. The hydrophobic interaction changes from being entropy-driven at 22 degrees C to being enthalpy-driven at 113 degrees C. Finally, the hydrocarbon model predicts that plots of the specific entropy change on unfolding versus temperature should nearly intersect close to 113 degrees C, as observed by Privalov.

The complexity of the Maillard reaction arises partly from multiple fragmentation reactions of the sugar moiety, constituting branch points in the reaction progress and establishing many parallel reaction pathways. Reactive intermediates produced by these processes are often alpha-oxoaldehydes. The formation of alpha-oxoaldehydes enhances and redirects glycating activity in the Maillard reaction since alpha-oxoaldehydes are up to 20,000-fold more reactive than glucose in glycation processes and are predominantly arginine-directed glycating agents. alpha-Oxoaldehydes bypass a requirement for a fructosamine precursor in the formation of advanced glycation end products (AGEs) since alpha-oxoaldehydes react with proteins (also nucleotides and basic phospholipids) to form AGEs directly. The major AGE formed from alpha-oxoaldehydes is generally a hydroimidazolone with other products-although for glyoxal, N(omega)-carboxymethylarginine is a major product. alpha-Oxoaldehyde formation also occurs in the absence of an amine substrate, particularly during heat processing of sugar solutions and lipid peroxidation processes-in the latter case, the glycation adducts are advanced lipoxidation products (ALEs). Hydroimidazolones are quantitatively important AGEs in cellular and extracellular proteins in physiological systems. Hydroimidazolone free adducts are liberated by cellular proteolysis and digestion. They are released into blood plasma for urinary excretion. Modification of arginine residues by alpha-oxoaldehydes may be particularly damaging since arginine residues have high-frequency occurrence in ligand and substrate recognition sites in receptor and enzyme active sites. Along with fructosamine formation, alpha-oxoaldehyde intermediates of the Maillard reaction represent a major source of damage to the proteome and genome.