Enzymes oxomolybdenum

Complexes of molybdenum and tungsten with bidentate sulfur ligands have been investigated extensively. In recent years, the work in this field has been escalated by the impetus of designing models of such bioinorganic enzymes as nitrogenase and xanthine oxidase (125). The early work reviewed by Coucouvanis (1) dealt exclusively with the isolation of oxomolybdenum(V) and -(VI) species. 

Hille, R., 1994, The reaction mechanism of oxomolybdenum enzymes, Biochim. Biophys. Acta 1184 143nl69. 

The use of ESR spectroscopy to probe Mo centres in biological systems has stimulated attempts to reproduce the ESR spectra of molybdoenzymes using well-defined chemical systems. The similarity between ESR parameters of species formed by reacting Na2[Mo04] with S-donor ligands and those of xanthine oxidase imply that in the enzyme (cysteinyl)S ligation of molybdenum may occur. However, no well-characterized Mo complex has an ESR spectrum which reproduces all the aspects of one observed for the Mo centre of an oxomolybdenum enzyme. A particular problem is the absence of superhyperfine coupling constants for well-defined Mo systems a notable exception is the observation of... 

Oxomolybdenum centers are present in a series of enzymes known as molybdenum hydroxylases [4] which catalyze oxygen atom transfer reactions of a variety of small molecules. Examples include the oxidation of xanthine to uric acid and the reduction of N03 to N02"- The reactions are equivalent to two-electron transfers, and the molybdenum atom cycles between oxidation states VI and IV during these processes. Molybdenum (VI) typically contains two multiply bonded oxo (or sulfido) groups, and molybdenum (IV) contains one. Therefore, it is believed that Mo02 and MoO +centers intercovert by coupled transfer of protons and electrons over a narrow potential interval during turnover of these enzymes. However, other explanations are possible. 

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