There is no author summary for this article yet. Authors can add summaries to their articles on ScienceOpen to make them more accessible to a non-specialist audience.
Abstract
The spatial distribution of the hydrophobic side chains in globular proteins is of
considerable interest. It was recognized previously that most of the alpha-helices
of myoglobin and haemoglobin are amphiphilic; that is, one surface of each helix projects
mainly hydrophilic side chains, while the opposite surface projects mainly hydrophobic
side chains. To quantify the amphiphilicity of a helix, here we define the mean helical
hydrophobic moment, (mu H) = [sigma Ni = 1Hi]/N, to be the mean vector sum of the
hydrophobicities Hi of the side chains of a helix of N residues. The length of a vector
Hi is the signed numerical hydrophobicity associated with the type of side chain,
and its direction is determined by the orientation of the side chain about the helix
axis. A large value of (mu H) means that the helix is amphiphilic perpendicular to
its axis. We have classified alpha-helices by plotting their mean helical moment versus
the mean hydrophobicity of their residues, and report that transmembrane helices,
helices from globular proteins and helices which are believed to seek surfaces between
aqueous and nonpolar phases, cluster in different regions of such a plot. We suggest
that this classification may be useful in identifying helical regions of proteins
which bind to the surface of biological membranes. The concept of the hydrophobic
moment can be generalized also to non-helical protein structures.