Considerations of the concept of structural homology as applied to bovine carboxypeptidases A and B.

An argument is presented in favor of the hypothesis that, bovine carboxypeptidases A and B fit by and large, the concept of homology on the levels of both amino acid sequence and three-dimensional structure. Two peptide sequences from carboxypeptidases A and B, comprising 46 amino acid residues, display structural homology to the extent of 51 per cent. One of the two peptides contains two of the amino acid residues which have been identified by X-ray crystallography as zinc ligands in carboxypeptidase A. The same residues are conserved in carboxypeptidase B in a loop of comparable dimensions. The other homologous pair of amino acid sequences appears to occur in different regions of the two enzymes which include in carboxypeptidase B the cysteinyl residue proposed to function as the third zinc ligand but not the amino acid residue believed to be the analogous ligand in carboxypeptidase A. The uncertainties of this prediction are considered in the light of existing chemical and structural evidence.