Influence of the hydrophilic face on the folding ability and stability of alpha-helix bundles: relevance to the peptide catalytic activity

Although not the sole feature responsible, the packing of amino acid side c hains in the interior of proteins is known to contribute to protein conform ational specificity. While a number of amphipathic peptide sequences with o ptimized hydrophobic domains has been designed to fold into a desired aggre gation state, the contribution of the amino acids located on the hydrophili c side of such peptides to the final packing has not been investigated thor oughly. A set of self-aggregating 18-mer peptides designed previously to ad opt a high level of cc-helical conformation in benign buffer is used here t o evaluate the effect of the nature of the amino acids located on the hydro philic face on the packing of a four or-helical bundle. These peptides diff er from one another by only one to four amino acid mutations on the hydroph ilic face of the helix and share the same hydrophobic core. The secondary a nd tertiary structures in the presence or absence of denaturants were deter mined by circular dichroism in the far- and near-UV regions, fluorescence a nd nuclear magnetic resonance spectroscopy. Significant differences in fold ing ability, as well as chemical and thermal stabilities, were found betwee n the peptides studied. In particular, surface salt bridges may form which would increase both the stability and extent of the tertiary structure of t he peptides. The structural behavior of the pep tides may be related to the ir ability to catalyze the decarboxylation of oxaloacetate, with peptides t hat have a well-defined tertiary structure acting as true catalysts.