STRUCTURAL BASIS OF AMINO-ACID ALPHA-HELIX PROPENSITY

The propensity of an amino acid to form an alpha helix in a protein wa s determined by multiple amino substitutions at positions 44 and 131 i n T4 lysozyme. These positions are solvent-exposed sites within the al pha helices that comprise, respectively, residues 39 to 50 and 126 to 134. Except for two acidic substitutions that may be involved in salt bridges, the changes in stability at the two sites agree well. The sta bility values also agree with those observed for corresponding amino a cid substitutions in some model peptides. Thus, helix propensity value s derived from model peptides can be applicable to proteins. Among the 20 naturally occurring amino acids, proline, glycine, and alanine eac h have a structurally unique feature that helps to explain their low o r high helix propensities. For the remaining 17 amino acids, it appear s that the side chain hydrophobic surface buried against the side of t he helix contributes substantially to a helix propensity.