Chemical keys to molybdenum enzymes *

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Abstract
Nitrogenase, in which molybdenum is part of a unique multinuclear iron–molybdenum–sulfide cluster, has garnered much deserved attention. However, about 30 other distinct enzymes use molybdenum (and several more use tungsten) in a mononuclear site involving special pterin–ene-dithiolate [pterin = 2-amino-4(1H)-pteridinone, ene-dithiolate = dithiolene] ligation. These molybdenum enzymes have environmental, agronomic, and health relevance. Their ‘molybdenum cofactor’, Moco, is now viewed as a generic term for a family of prosthetic groups, although there are unresolved issues involving nomenclature. Oxygen-atom transfer and proton–electron transfer are convenient ways to stoichiometrically formulate the two-electron substrate and active-site regeneration reactions. Such processes are common in molybdenum chemistry and may have mechanistic relevance. Dithiolene ligands of the type present in Moco are redox active in free or complexed forms. Molybdenum–sulfur systems show facile internal electron–transfer chemistry and partial redox states that have potential structural and mechanistic implications for molybdenum enzymes. Chemical differences between molybdenum and tungsten may influence the respective presence or absence of these elements in enzymes from organisms that occupy diverse habitats.