Chemical approaches to the mode of action of carboxypeptidase A

Our studies of the mechanism of action of carboxypeptidase A have been guided by the dual specificity of this enzyme as altered either by replacement of the native zinc atom at the active site with other metal ions or by chemical modification of amino acid side chains. In almost all instances esterase activity toward HPLA increased and peptidase activity toward CGP decreased in response to such treatments. A mechanism postulating the modification of a single catalytically active amino acid residue was designed as the simplest chemical event that could provide a unified account for such similar functional consequences in response to a variety of agents (Vallee, Riordan & Coleman 1963). The mechanism was postulated before the identification of the residues affected by chemical modifications. Similarly, the number and nature of binding groups were unknown, and thus, could not be considered. It was recognized that the scheme would have to be amplified, once specific residues were identified. Significant progress, in this regard, has occurred over the past several years and this report summarizes the present status of our understanding.