Molybdenum in nitrogenases

Although the hypothesis of Egumi may be an oversimplification, it is certainly true that Fe /Feu is widely used in redox systems. Zn " in hydrolysis, esterification, and similar reactions, and molybdenum in nitrogenase. xanthine oxkkise. nitnite reductase, etc Putting abundance aside, discuss the specific chemical properties of these metals this make them well suited for their tasks. 

Using your knowledge of periodic relationships, predict which element might come closest to reproducing the behavior of molybdenum in nitrogenase. Recall that nitrogen fixation involves both complexalion and redox reactions.l5h... 

Figure 4. Possible reduction cycle of dinitrogen on molybdenum in nitrogenase (Thorneley et al. 1979)...

Molybdenum. Molybdenum is a component of the metaHoen2ymes xanthine oxidase, aldehyde oxidase, and sulfite oxidase in mammals (130). Two other molybdenum metaHoen2ymes present in nitrifying bacteria have been characteri2ed nitrogenase and nitrate reductase (131). The molybdenum in the oxidases, is involved in redox reactions. The heme iron in sulfite oxidase also is involved in electron transfer (132). 

So little is known about molybdenum enzymes other than milk xanthine oxidase that there is little to be said by way of general conclusions. In all cases where there is direct evidence (except possibly for xanthine dehydrogenase from Micrococcus lactilyticus) it seems that molybdenum in the enzymes does have a redox function in catalysis. For the xanthine oxidases and dehydrogenases and for aldehyde oxidase, the metal is concerned in interaction of the enzymes with reducing substrates. However, for nitrate reductase it is apparently in interaction with the oxidizing substrate that the metal is involved. In nitrogenase the role of molybdenum is still quite uncertain. 

There has been considerable interest in the chemistry of molybdenum in recent years, in part due to its occurrence in nitrogenase which catalyzes the reaction ... 

Stmcmre Elucidation in Unknown Systems. Certainly one of the most exciting uses of XAS is as a tool for characterizing stmcturally unknown systems. A dramatic example of this ability is provided by studies of the molybdenum site in nitrogenase. 

Monomeric non-oxo molybdenum(V) complexes have been isolated with nitride (N3 ), imide (NR2-) and amide (NRR -) ligands, which may act as models for intermediates in the degradation of dinitrogen in nitrogenase. However, no molybdenum(V) compounds are yet known which correspond to the diazenide or hydrazide analogues of the molybdenum(IV) complexes described in Section 36.4.2.3. 

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