Zinc Enzymes

Abstract

AbstractA remarkable mosaic of zinc‐binding motifs influence the function of proteins involved in all aspects of metabolism, providing evidence on the molecular level for the indispensability of zinc to growth and development and transmission of the genetic message. There are now about 400 three‐dimensional structures for zinc proteins, representing all six classes of enzymes and covering a wide range of phyla and species. These structures provide standards of reference for the identity and nature of zinc ligands in other proteins for which only the primary structure is known. Three primary types of zinc sites are apparent from examination of these structures: structural, catalytic, and cocatalytic. The most common amino acids that supply ligands to these sites are His, Glu, Asp, and Cys. In catalytic sites, zinc generally forms complexes with water and any three nitrogen, oxygen, and sulfur donors with His being the predominant amino acid chosen. Water is always a ligand to such sites. Structural zinc sites have four protein ligands and no bound water molecule. Twelve of the twenty‐two permutations of four Cys, His, Glu, and Asp ligands have been observed. Cys is the preferred ligand in such sites. Cocatalytic sites contain two or three metals in close proximity with two of the metals bridged by a side‐chain moiety of a single amino acid residue, such as Asp, Glu, or His and sometimes a water molecule. Asp and His are the preferred amino acids for these sites. The properties of both H‐bonding and hydrophobic amino acid ‘outer shell ligands’ and the secondary support structure of the zinc sites is important to the function, stability, and reactivity of the bound metal. The influence of zinc on quaternary protein structure has led to the identification of a fourth type of zinc‐binding site, protein interface. In this case, zinc sites are formed from ligands supplied from amino acid residues residing in the binding surface of two proteins. The resulting zinc site usually has the coordination properties of a catalytic or structural zinc‐binding site.