Carboxypeptidase point

The evolutionary pathways leading to such a scheme are, in part, reflected in the apparent structural homology which exists between these proteases. Sequence information points to a close relationship between trypsin and chymotrypsin (8), and there are marked similarities in the structures of carboxypeptidases A and B, both to each other and to pepsin. Such relationships seem to exist but are beyond the scope of this article. 

The hrst zinc enzyme to be discovered was carbonic anhydrase in 1940, followed by carboxypeptidase A some 14 years later. They both represent the archetype of mono-zinc enzymes, with a central catalytically active Zn " " atom bound to three protein ligands, and the fourth site occupied by a water molecule. Yet, despite the overall similarity of catalytic zinc sites with regard to their common tetrahedral [(XYZ)Zn " "-OH2] structure, these mononuclear zinc enzymes catalyse a wide variety of reactions, as pointed out above. The mechanism of action of the majority of zinc enzymes centres around the zinc-bound water molecule, which is best represented as Zn -OH2. What determines the catalytic properties of each enzyme is not only the nature of the donor ligands, but also the distance that separates them in the amino acid sequence of the protein, lypically (Table 12.1), two of the ligands are separated by only 1—3 amino acids, whereas the third ligand is separated by a longer spacer of between 5 and 196 residues. 

The backbone structure of carboxypeptidase A a-helical segments are purple /3-pleated sheets are indicated by flat green arrows pointing in the N----> C direction. 

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