1.2- Dithiolenes molybdenum enzyme families

Figure 1. The four families of dithiolene-containing molybdenum and tungsten enzymes based on the structures of the catalytic reaction centers.

The term molybdenum cofactor (or Moco) refers to the metal center and its inner coordination sphere. Moco is not a single, unique, moiety, rather it is a diverse collection of protein-bound sites that have certain common features. Thus, one or two MPTs are coordinated to the metal via the dithiolene group and the remainder of the metal s coordination sphere is taken up by non-protein ligands (e.g., oxo, hydroxo, water, or sulfido groups) and, in some cases, an amino acid side chain is coordinated. Hille (1) has shown that the mononuclear Mo—MPT enzymes of molybdenum can be classified into three families, on the basis of the nature of the inner coordination sphere of the oxidized form of the enzyme (Fig. 2). 

The N-terminal sequence of ArrA suggests that Arr is a member of the DMSO reductase family. These enzymes feature molybdenum coordinated by two bidentate dithiolene ligands contributed by two equivalents of the molybdopt-erin cofactor (16,18). 

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