ZINC ENZYMES

The number of zinc enzymes for which detailed structural and mechanist ic data, including high resolution crystal structures, are available i s increasing rapidly. The new findings continue to support the conclus ion that the majority of zinc enzymes catalyze hydrolysis or closely r elated transfer reactions. In a protein environment, tetrahedral or B- coordinate Zn2+ is ideally suited to activate a coordinated water (fre quently a Zn2+--OH) as a nucleophile attacking the carbonyl carbon of a peptide bond, the carbon of carbon dioxide or the phosphorus of a ph osphate ester. Protein-bound Zn2+ can function catalytically by formin g mixed complexes with the substrate, either by expanding its coordina tion sphere or by exchanging a ligand. Formation of protein-Zn2+-subst rate bonds can position the substrate or polarize its electron distrib ution to facilitate further steps in the reaction.