Quest for a turnover mechanism in peptide-based enzyme mimics

Creation of synthetic structures with an enzyme-like mechanism and turnover remains a significant challenge. In this study, peptides containing a cysteine thiol and histidine imidazole group were designed to mimic the active site of the cysteine protease papain. Ellman's reagent trapping experiments showed that rapid acetyl group exchange exists between the thiol and imidazole groups. This exchange rate increased significantly in peptides with bulky R-groups (phenylalanine) between the cysteine and histidine. A reduction of the cysteine thiol pKa and NMR results further supports closer proximity of the thiol and imidazole groups in peptides with faster acetyl group exchange.