Cocatalytic Zinc Sites

Abstract

AbstractCocatalytic zinc sites occur in enzymes where two or three metals are closely grouped to bring about catalysis. There are over four dozen representatives of this type of zinc site with the great majority belonging to the Class III hydrolases. A novel feature of these sites is the bridging of two of the metal sites by a side chain moiety of a single amino acid residue, such as the ring nitrogens of the imidazole group of histidine or the carboxylate oxygens of aspartic acid, glutamic acid, or of a carboxylated lysine, LysCO2−. Such an interaction requires the metals to be in close proximity to each other. The ligands to cocatalytic zinc sites often come from nearly the entire length of the protein, indicating that the metal sites may be important to the overall fold of the protein as well as to catalytic function. The secondary structure of the protein plays a major role in providing the ligands to these sites. In most cases, ligands are provided by amino acids residing within one or two residues before or after a β‐sheet or an α‐helix. The zinc ions are often penta‐coordinate and arranged in a trigonal bipyramidal geometry. The bridging amino acids and H2O probably have critical roles in catalysis. Their dissociation from either metal atom during catalysis will change the charge on the metal promoting its action as a Lewis acid or allowing interaction with an electronegative atom of the substrate. Thus, it can be envisioned for hydrolytic enzymes that substrate binding involves one zinc site acting as a template for substrate binding, while the other zinc site provides hydroxide for nucleophilic attack on the sp2center of the ester or amide bond of the substrate. In the next step, the roles of the metals can be reversed. In this manner, the metal atoms and their associated ligands play specific roles in each step of the reaction that works to bring about catalysis. The ligands in these sites, in particular the histidines, are often involved in further hydrogen‐bonding interactions with other amino acids. These interactions should effect the charge on the metal and the stability of the metal complex, thus fine‐tuning catalysis and the stability of the metal sites.