Cocatalytic Zinc Enzymes

A number of zinc enzymes require two or more metal ions for full activity, but in the absence of X-ray structural data the location of these metal centres with regard to one another was often uncertain. When the first three-dimensional structures began to appear, it became clear that the metals were in close proximity. A particular feature of many of these enzymes was the presence of a bridging ligand between two of the metal sites, usually an Asp residue of the protein, which is occasionally replaced by a water molecule. Whereas some of the sites contain only Zn ions, several contain Zn together with Cu (in cytosolic superoxide dismutases), Fe (in purple acid phosphatases), or Mg (in alkahne phosphatase). 

FIGURE 12.10 The essential features of the catalytic cycle of liver alcohol dehydrogenase. (Adapted from Parkin, 2004.) 

FIGURE 12.11 Specihc attachment of a prochiral centre to an enzyme-binding site enables the enzyme to distinguish between prochiral methylene protons in ethanol. (Adapted from Voet Voet, 2004.) 

FIGURE 12.13 Repair of damaged DNA by sacrificial alkylation of one of the zinc cysteine thiolate ligands of the Ada DNA repair protein. 

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Multinuclear and Cocatalytic Zinc Enzymes.. . Zinc Fingers - DNA- and RNA-Binding Motifs References. 

The reader is referred to the following sources of further information on the specific cocatalytic enzymes superoxide dismutase, and the reduced form, alkahne phosphatases, nuclease Pl, purple acid phosphatase, amidohydrolase, leucine amtnopeptidase, general comments on the mechanisms of the phosphatases and aminopeptidases, and other cocatalytic zinc enzymes. ... 

We consider, successively, the catalytic role of several classes of mononuclear Zn2+ enzymes and then discuss enzymes with di- and tri-nuclear cocatalytic zinc centres, some of which include a metal ion other than zinc. We conclude with a presentation of some of the zinc-based motifs found in proteins involved in the regulation of nucleic acid and protein synthesis. 

The metal ion is held in place by amino acid residues, generally His, Glu, Asp, or Lys. In many metallopeptidases, which may be exopeptidases or en-dopeptidases, only one zinc ion is required. In all Co2+ or Mn2+-dependent, and in some Zn2+-dependent metallopeptidases, two metal ions are present and act cocatalytically these enzymes are exopeptidases [2][73][74],... 

Figure 12.1 Zinc-binding sites in enzymes can be catalytic, structural or cocatalytic. The protein ligands are indicated by smaller filled circles. (From Auld, 2001. With kind permission of Springer Science and Business Media.)...

Aminopeptidases are counterparts to carboxypeptidases, removing N-terminal amino acids. However, unlike the carboxypeptidases, they contain dinuclear zinc sites. They fall into two groups, the first of which includes the leucine aminopeptidase from bovine lens, while the second includes the leucine aminopeptidases AAP from Aeromonas proteolytica and SAP from Streptomyces griseus (Figure 12.15). The mechanism of the AAP enzyme has been well studied, and may well represent a general catalytic mechanism for peptide hydrolysis by metal-lopeptidases with a cocatalytic active site.ki. 

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