Nitrogenase characteristics

B. Electron Allocation Coefficient (EAC) of Nitrogenase Characteristics and Regulation... 

Characteristic Catalyst and Nitrogenase Co-Cysteine Co-Cysteamine Mo-Cysteine Nitrogenase... 

Current concepts of the chemical nature and role of molybdenum-containing enzymes are reviewed. Methods for molybdenum in enzymes, spectroscopic manifestations of the metal, and the characteristics of molybdenum-deficient enzymes are discussed, with particular attention to xanthine oxidase, sulfite oxidase, and nitrate reductase, in which Mo5 (and Mo3 in some cases) species are readily demonstrated. Nitrogenase is presumed to use molybdenum in a catalytic step, but no direct evidence for its participation in catalysis is yet available. 

Enzymes containing molybdenum are of two types (1) Nitrogenases, which are required for converting atmospheric nitrogen to nitrogen compounds (NH3, for example) nitrogenases contain a characteristic polymetal atom cluster species called the iron-molybdenum cofactor, FeMoco (Section 17-E-10). (2) The other Mo enzymes, all of which have some variant of a characteristic molybdenum cofactor, Moco. 

Molybdenum is found in the active site of nitrogenase as part of a multinuclear cluster with iron (the so-called M-cluster), but in all other enzymes it comprises a mononuclear center (Hille, 1996 Kisker et al., 1997a). The distinguishing characteristic of this latter group of enzymes is that, while other redox-active centers are often present, the molybdenum center itself possesses only a single equivalent of the transition metal. This large and diverse group of enzymes is the subject of the present account. 

When FeMoco extracted from MoFe protein purified from a nifV mutant is recombined with apo-MoFe protein, the activated protein has the substrate-reducing characteristics of the nifV enzyme (reduces C2H2 effectively but N2 only poorly). This observation provides the most compelling evidence that FeMoco is, or forms part of, the active site of nitrogenase. Site-directed mutagenesis has implicated one of the conserved Cys residues of the a subunit Cys 275 in binding FeMoco, and also His 196 and Gin 192 (see Refs. 17 and 38 for discussion). 

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