The zinc(II) ion is a biologically essential element. Knowledge of its importance is increasing, as more and more enzymes are discovered as zinc(II)-containing bioinorganic catalysts. Carbonic anhydrase, carboxypeptidase, aminopeptidase, superoxide dismutase, yeast aldolase, β-lactamase, alcohol dehydrogenase, alkaline phosphatase, phospholipase C, nucleic acid polymerase, and P1 nuclease may be classified as classic zinc enzymes . More recently added are phosphotriesterase (or ‘‘organophosphorus hydrolase,’’ EC group 3.1.8), which hydrolyzes pesticides and nerve gases ; the hedgehog family of secreted signaling proteins ; collagenase (a member of the matrix metalloprotease family) ; and so on. There are now more than 300 known zinc enzymes. The x-ray crystallographic structures of a considerable number of zinc enzymes have now been established.