Redox active tyrosine residues in biomimetic beta hairpins
Sibert, Robin S.
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Biomimetic peptides are autonomously folding secondary structural units designed to serve as models for examining processes that occur in proteins. Although de novo biomimetic peptides are not simply abbreviated versions of proteins already found in nature, designing biomimetic peptides does require an understanding of how native proteins are formed and stabilized. The discovery of autonomously folding fragments of ribonuclease A and tendamistat pioneered the use of biomimetic peptides for determining how the polypeptide sequence stabilizes formation of alpha helices and beta hairpins in aqueous and organic solutions. A set of rules for constructing stable alpha helices have now been established. There is no exact set of rules for designing beta hairpins; however, some factors that must be considered are the identity of the residues in the turn and non-covalent interactions between amino acid side chains. For example, glycine, proline, aspargine, and aspartic acid are favored in turns. Non-covalent interactions that stabilize hairpin formation include salt bridges, pi-stacked aromatic interactions, cation-pi interactions, and hydrophobic interactions. The optimal strand length for beta hairpins depends on the numbers of stabilizing non-covalent interactions and high hairpin propensity amino acids in the specific peptide being designed. Until now, de novo hairpins have not previously been used to examine biological processes aside from protein folding. This thesis uses de novo designed biomimetic peptides as tractable models to examine how non-covalent interactions control the redox properties of tyrosine in enzymes. The data in this study demonstrate that proton transfer to histidine, a hydrogen bond to arginine, and a pi-cation interaction create a peptide environment that lowers the midpoint potential of tyrosine in beta hairpins. Moreover, these interactions contribute equally to control the midpoint potential. The data also show that hydrogen bonding is not the sole determinant of the midpoint potential of tyrosine. Finally, the data suggest that the Tyr 160D2-Arg 272CP47 pi-cation interaction contributes to the differences in redox properties between Tyr 160 and Tyr 161 of photosystem II.