PROBING THE FOLDING TRANSITION STATE VIA SIDECHAIN AND BACKBONE MUTATIONS

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Backbone-backbone H-bonds are prominent features of the structures of beta-sheets. Because of their central role in this ubiquitous secondary structure, we will determine the contribution backbone-backbone H-bond formation in general, and beta-turn formation in particular, to the kinetics of beta-sheet formation. This will be accomplished using a Phi(M)-value analysis of amide-to-ester mutants of a well-studied beta-hairpin peptide. The contribution of backbone-backbone H-bonds to the thermodynamics and kinetics of protein folding in general, and beta-sheet folding in particular, is controversial. Previously, we used amide-to-ester mutations, in which a backbone amide is replaced by an ester, to address this problem. Amide-to-ester mutation is a powerful tool for studying backbone-backbone H-bonding in proteins because (1) esters and amides have similar bond lengths and angles; (2) esters and amides both have a high barrier to rotation and favor a trans conformation about the C'-Oe or C'-NH bond ("Oe" refers to the non-carbonyl oxygen in an ester); and (3) amide-to-ester mutations do not alter the intrinsic conformational propensity or side chain interactions of the mutated residue because the side chain is not altered. Many studies, including our own work on the Pin WW domain, have suggested the hypothesis that turn formation limits the folding rate of beta-hairpins and small beta-sheet proteins. To better understand this fundamental aspect of beta-sheet folding, this hypothesis will be tested by using Phi(M)-value analysis of amide-to-ester mutations on a beta-hairpin and/or alpha-helical peptides.