The spectrum of cobalt bovine procarboxypeptidase A, an index of catalytic function.

The spectra of functionally essential, chromophoric metal atoms of metalloenzymes are thought to reflect catalytic potential [Vallee and Williams (1968) Proc. Nat. Acad. Sci. USA 59, 498]. The spectra of cobalt procarboxypeptidase and their perturbations by substrates and inhibitors are virtually the same as those of cobalt carboxypeptidase; both are consistent with a distorted tetrahedral geometry about the cobalt atom, suggesting irregular coordination geometries and low symmetries of metal-binding sites. We have, therefore, examined the enzymatic properties of cobalt procarboxypeptidase and found that it catalyzes the hydrolysis of a series of haloacylated amino acids at rates equal to or greater than those of native zinc carboxypeptidase. These observations demonstrate the existence both of the catalytic and of the binding sites for haloacylated amino acids of the zymogen, even before activation. Hence, the magnitude of conformational changes at the catalytic site thought to accompany its activation must be very small, no matter what their magnitude might be elsewhere in the molecule. The data support the entatic state hypothesis and suggest that it may profitably guide the exploration of catalytic potential of metalloproteins and metal-protein complexes.