A new class of competitive inhibitors for the cysteine protease papain is described. These inhibitors are based upon a 4-heterocyclohexanone ring and are designed to react with the enzyme active site nucleophile to give a reversibly formed hemithioketal. The electrophilicity of the ketone in these inhibitors is enhanced by ring strain and by through-space electrostatic repulsion with the heteroatom at the 1-position of the ring. Equilibrium constants for addition of water and 3-mercaptopropionic acid to several 4-heterocyclohexanones were measured by H-1 NMR spectroscopy. These reactions model addition of the active site nucleophile to the corresponding inhibitors. The equilibrium constants give a linear correlation with the field substituent constant F for the functional group at the 1-position of the heterocyclohexanone. These equilibrium constants also correlate well with the inhibition constants for the 4-heterocyclohexanone-based inhibitors, which range from 11 to 120 mu M. Thus, the model system can be used to predict the potency of structurally related enzyme inhibitors.