A Mechanism of Action for Carboxypeptidase A

In an attempt to gain a better understanding of the mechanism of action of carboxypeptidase A (EC 3.4.2.1), many kinetic studies have been undertaken using numerous substrates-both esters and peptides-that have exhibited substrate linearity, inhibition, activation, and sigmoid-shaped rate plots. Numerous interpretations of the kinetic data have been proposed, none of which are fully in accord both with kinetic data and x-ray crystallographic studies. Much of the kinetic data has been interpreted using multisite binding while the x-ray information seems to severely restrict these possibilities.We have examined the feasibility of a simple model with a single active site, without modifier sites, that allows only one substrate molecule to bind the enzyme at a time. A random-pathway model was identified that simultaneously accounts for the nonlinear kinetic data and meets the restrictions imposed by the x-ray crystallographic studies.