ABSTRACT
There are today five dozen structural references of the catalytic zinc sites encompass ing five of the six classes of enzymes (Table 1). The class III hydrolases have by far the greatest number of representatives. In these sites zinc generally forms complexes with any three nitrogen, oxygen, and sulfur donors of His, Glu, Asp, and Cys with His being the predominant amino acid chosen. Histidine may be chosen because of its capacity to disperse charge through H bonding of its nonliganding nitrogen. The ligands are generally separated by short and long amino acid spacers. The length of the short spacer is often dictated by the ligand support structure; 3 for an a helix and 1 for a (3 sheet. The coordination number for such sites is usually 4 or 5 and the geometry in the free state is frequently distorted-tetrahedral or trigonal-bipyramidal. Water is always a ligand to the catalytic zinc. The zinc-bound water is activated for ionization, polarization, or displacement by the identity and arrangement of ligands coordinated to zinc [9]. Ionization of the activated water or its polarization brought about by a base form of an active site amino acid provides hydroxide ions at neutral pH, and its displacement results in Lewis acid catalysis on the part of the catalytic zinc atom. The structure of the active site implies that the identity of the three protein ligands, their spacing, and secondary interactions with neighboring amino acids in conjunction with the vicinal properties of the active center created by protein folding are critical for the various mechanisms through which zinc can be involved in catalysis.
