Papain active-site directed inhibitors

Scheme 11.18 Active-site directed inhibitors analogous to the papain substrate, N-(4-toluenesulphonyl) glycine methyl ester.

Active-site directed inhibitors have reactivity with the enzyme greatly enhanced over that of non-specific inhibitors thus phenacyl iodide inhibits papain 50-fold faster than iodoacetamide whereas the active-site directed inhibitor 4-toluenesulphonylamidomethyl chloromethyl ketone reacts some 650-fold faster. The enhanced rate is due to complexation of the inhibitor with the enzyme, and indicates that the inhibitor must be reacting at the active site. 

Alternatively, diazomethylketone substrate derivatives can be efficiently used as active-site-directed inhibitors of thiol proteases. For instance, the carbobenzoxyphenylalanine analogue reacts stoichiometrically at the active center cysteine residue of papain. 

The categorization of inhibitors of the papain super-family into five distinct groups has been reviewed in some detail and examples of each type have been cited [6]. These groups range in mechanism of action from those interacting in a purely non-covalent fashion to those that rely primarily on a group that is irreversibly reactive toward the active site thiol. A brief summary of these categories is presented here, with some illustrative examples the reader is directed to reference 6 for more detail. 

F .5.2 Inhibitors of papain family cysteine proteases. The heavy arrows indicate the direction of binding of the peptidal inhibitor relative to the active site cysteine thiol. The... 

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