Model studies of carboxypeptidase A

Abstract

In the action of carboxypeptidase A (CPA), the active-site Zn(II) ion and the Glu-270 carboxylate group are believed to play very crucial catalytic roles. As models of CPA, various aspects of catalysis of amide or ester hydrolysis by the carboxyl group or the metal ion as the monofunctional catalytic group have been intensively investigated. On the other hand, the cooperative catalysis by the carboxyl group and the metal ion has been achieved in few model reactions. In some of the bifunctional catalytic systems, however, the catalytic roles of the carboxyl group and the metal ion are different from those proposed for the action of CPA. The most successful model of CPA designed to date is the Ni(II)-catalyzed hydrolysis of ester 37 and the Cu(II)-catalyzed hydrolysis of amide 39 in dimethyl sulfoxide containing 5% ( v v ) water. In these reactions, the following catalytic features of CPA are reproduced: (i) both an alkyl amide and an alkyl ester are readily hydrolyzed, (ii) the catalysis is achieved by the cooperative participation of the metal ion and the carboxyl group as well as the reaction medium, (iii) the catalytic roles of the metal ion and the carboxyl group are similar to those proposed for CPA action, and (iv) the anionic form, instead of the acidic form, of the carboxyl group is catalytic. Various model studies also provide mechanistic information with which the data obtained directly with CPA can be reevaluated. Some of the mechanistic arguments previously presented in support of the general base role of the Glu-270 carboxylate, when reevaluated in the light of results from the model studies, appear to be also compatible with the nucleophilic role of Glu-270.