Inactivation of Cysteine Proteases

The cysteine proteases papain and cathepsin B are inactivated by a Michael acceptor, a peptidyl-β-chloro-α, β-unsaturated ester[formula]Inactivation occurred concomitant with chloride release which was stoichiometric with the amount of enzyme. This result is consistent with nucleophilic attack of the active site cysteine on the β-carbon of the inhibitor, followed by expulsion of chloride ion. Inactivation by this class of compounds requires the carbon skeleton about the double bond to be in thetransconfiguration. Thecisisomer was a competitive inhibitor. The difference in the mode of inhibition between the isomers is probably due to nonproductive binding of thecisisomer due to bulky chlorine substituent in the β-position.