The nature of interaction between the carboxylate of substrates and the guanidinium moiety of Arg-145 in carboxypeptidase A probed by inhibitors of the enzyme

Replacement of the α-proton of 2-benzyl-3-hydoroxypropanoic acid, a competitive inhibitor of carboxypeptidase A with a fluoro group brought about a 2-fold increase in Ki value (0.61 mM → 1.19 mM), while pKa value decreased by 1.4 units (4.36 → 2.95), suggesting that the carboxylate of the inhibitor and that of substrates as well are hydrogen bonded to the guanidinium moiety of Arg-145 of the enzyme.