Mechanism of carboxypeptidase A: hydration of a ketonic substrate analogue.

The structure of the complex between carboxypeptidase A alpha (EC 3.4.17.1) and the ketonic substrate analogue 5-benzamido-2-benzyl-4-oxopentanoic acid (BOP) has been determined by x-ray crystallographic methods to a resolution of 1.7 A (final R = 0.191). Interestingly, BOP was observed to bind to the active site of carboxypeptidase A alpha as the covalent hydrate adduct. Because BOP is probably less than 0.2% hydrated in aqueous solution, this result was unexpected. One possibility is that the zinc-bound water of the native enzyme added to the ketone carbonyl. Alternatively, the enzyme may preferentially scavenge the hydrated ketone as it is continuously maintained at equilibrium in the solution in which the carboxypeptidase A alpha crystals were immersed. In either case, this mode of binding of BOP to carboxypeptidase A alpha provides an example of the preferred binding of a model of a structure along the reaction coordinate of a hydrolytic reaction.