Zinc-Containing Metalloenzymes

Abstract

Carboxypeptidase A and B are formed by the hydrolytic action of trypsin on their respective procarboxypeptidase precursors synthesised in the pancreas¹⁸⁷. Of these two enzymes, carboxypeptidase A has been the more widely studied and has received detailed investigation on the basis of X-ray crystallographic studies^29,188,189 Bovine carboxypeptidase A_α (cpa) is an enzyme comprised of 307 amino acids in a single polypeptide chain which binds tightly one gram-ion of Zn(II) per mole of enzyme. The requirement of Zn(II) for enzymatic activity was first demonstrated by noting that cpa freed of the metal ion remained inactive but activity could be reconstituted on addition of Zn(II)^190,191. The metal-free enzyme under these conditions appeared to retain the same gross structural properties of active cpa¹⁹¹. Later the nature of the role of the Zn(II) ion by binding substrate in peptide hydrolysis was firmly established from crystallographic studies²⁹. The enzyme exhibits specific stereochemical requirements in proteolysis by cleaving the C-terminal amino acid from a peptide chain if the C-terminal carboxylate is free and if the amino acid has an l-configuration^192,193. Higher activity is usually observed if the C-terminal amino acid contains an aromatic or branched-chain residue¹⁹⁴.