33] Catalytic mechanism in papain family of cysteine peptidases

Abstract

Publisher Summary This chapter summarizes catalytic mechanism for cysteine peptidases of the papain family. Cysteine peptidases of the papain family catalyze the hydrolysis of peptide, amide, ester, thiol ester, and thiono ester bonds. The basic features of the mechanism include the formation of a covalent intermediate, the acyl-enzyme, resulting from nucleophilic attack of the active site thiol group on the carbonyl carbon of the scissile amide or ester bond of the bound substrate. The aspect of cysteine peptidases, which is of primary importance for their catalytic activity, is the high nucleophilicity of the active site thiol group. The existence of two ionizable groups as essential catalytic residues in papain is consistent with the bell-shape form for the pH dependency of activity with this enzyme. Papain has a large binding site and there are a number of interactions that exist between the enzyme and the substrate over an extended region. Coupling of these substrate binding interactions to the hydrolytic process occurring at the active site is an important aspect of catalysis.