Zinc enzymes, classification

XAS in general, and EXAFS in particular, offer a unique and direct probe of zinc centers in proteins. So far, only three types of amino acid residues have been identified as ligands of zinc in a protein an imidazole nitrogen of histidine, the carboxylate oxygen of glutamic or aspartic acid, and the sulfur of cysteine. A tentative classification of zinc enzymes based on distinctions possible by EXAFS may be suggested (30) ... 

Carbonic anhydrases, also known as carbonate dehydratase and placed in enzyme classification 4.2.1.1, are zinc metalloproteins that catalyze the reversible hydration of carbon dioxide to bicarbonate (Eq. 1) ... 

ZINC(II) ENZYME CLASSIFICATION BY COORDINATING AMINO ACIDS. 3 PROGRESS IN UNDERSTANDING MECHANISMS OF ZN -HYDROLASES... 

ZINC(II) ENZYME CLASSIFICATION BY COORDINATING AMINO ACIDS... 

Lactamases may be chromosomal or plasmid-borne, inducible or constitutive, and for this reason their terminology can be confusing. A number of classification systems have been proposed, including classes A-D based on peptide sequence. Classes A, C and D have a serine at the active site, whereas class B enzymes have four zinc atoms at their active site and these are also called metallo-(3-lactamases. Class A enzymes are highly active against ben-zylpenicillin, class B (3-lactamases are effective against penicillins and cephalosporins. Class C en-... 

Fig. 8.1 Classification of metallopeptidases by zinc-binding motifs. The major amino acid motif in zincins has two histidine residues that coordinate with the metal ion and a glutamate residue (E) for catalysis. A third residue that coordinates with the metal may be glutamate, aspartate (D), or histidine. In metzincins, the third coordinating residue is histidine or aspartate (H/D), but the name is taken from the presence of a downstream invariant methionine residue (see Fig. 8.2 and text). The red type indicates enzymes or enzyme subfamilies encoded in the human genome (Slightly modified from Fig. 1A of F.X. Gomis-Ruth, Structural aspects of the metzincin clan of metal-loendopeptidases. Mol. Biotechnol. 24(2) 157-202, 2003)...

The structural classification of the zincins discussed above has been based on the three-dimensional data on zinc proteases currently available. These data (Fig. 2) are from zinc-dependent endopeptidases. It is therefore not mandatory for these enzymes to interact with the ter-minii of their protein substrates. However, for the zinc-dependent amino- and carboxypepti-dases that possess the HexxH consensus [3-5], this additional requirement for substrate recognition may result in a three-dimensional structure which is considerably modified from the astacin scaffold observed for the endoproteases. 

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