Aims of the Review
The main role of transition metals in organisms depends on their catalytic properties. Many enzymes consist only of protein (that is, polypeptides), but others consist of a protein (called the ‘apoenzyme’) and one or more smaller molecules or ions (‘cofactor’, ‘coenzyme’ or ‘prosthetic group’), which together form the complete enzyme or ‘holoenzyme’. The cofactor may be an organic molecule such as flavin, pyridoxal, pyridine nucleotide, etc., attached to the protein by covalent bonds, hydrogen bonds or van der Waals interaction; or it may be a simple metal ion (for example, Cu) or a metal complex with one or more ligands (for example, Fe porphyrins, Co corrinoids). Where one or more of the amino-acid residues is co-ordinated to the metal, the protein may be regarded as the ligand, albeit a rather unusual one. Obviously these metallo-enzymes can be considered either as a special group of enzymes or as a special group of metal complexes, and hence compare the catalytic activity either of enzymes with and without transition metals, or of transition-metal complexes with and without a protein. In this review we shall neglect those metallo-enzymes where the metal functions merely as a Lewis acid (as in certain hydrolytic enzymes—see, for example reference 59) and include only those metallo-enzymes and metal-containing proteins that undergo reactions (for example, redox reactions) and deal with ligands (for example, O2) that are usually considered as more typical of transition-metal complexes.
KeywordsMetal Complex Nitrogen Fixation Individual Step Pyridine Nucleotide Anaerobic Fermentation
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