Nitrogenase, nitrogen reduction molybdenum site

Nitrogen Fixation in Nature The nitrogenase enzyme is a two-component protein that consists of an electron-transfer Fe protein and a catalytic protein [85]. Three different nitrogenase enzymes are known, which differ primarily in the nature of the putative active site within the catalytic protein. The most common form is the MoFe protein, in which the active site for nitrogen reduction, the so-called FeMo cofactor (FeMoco), is composed of seven irons, one molybdenum, and nine sulfides... 

Figure 4. Proposed mechanism for nitrogen reduction at a molybdenum site in nitrogenase. 1980, used with permission of the American Chemical Society [10].

The electrosynthesis of hydride complexes directly from molecular hydrogen at atmospheric pressure by reduction of Mo(II) and W(II) tertiary phosphine precursors in moderate yield has been described as also the electrosynthesis of trihydride complexes of these metals by reduction of M(IV) dihydride precursors [101,102]. Hydrogen evolution at the active site of molybdenum nitrogenases [103] is intimately linked with biological nitrogen fixation and the electrochemistry of certain well-defined mononuclear molybdenum and tungsten hydrido species has been discussed in this context [104,105]. 

The proteins involved in the reduction of nitrogen to ammonia and other accessible forms contain several such clusters coupled with molybdenum centres. The structure of the central iron-molybdenum cluster at the centre of nitrogenase is shown in Fig. 10-9. Even with the detailed knowledge of this reaction site, the mode of action of nitrogenase is not understood. 

The mechanism of the reduction of N2 to ammonia catalyzed by the nitrogenase complex is one of the continuing mysteries of chemistry and biology. Since the first N2 complex was discovered [55] and the basic structure of the active site of conventional molybdenum nitrogenases was unraveled [43,44,48,53], efforts have been made to combine chemistry and biology to explain the mechanism of biological nitrogen fixation at the atomic level [56,57]. 

Figure 12.4. A representation of the MoFe cofactor of the nitrogenase enzyme isolated from A. vinelandii. It is argued that the cysteine residue on the one end and the imidazole on the other (which also bears a homocitrate bound to molybdenum) immobilize this in the active site where the reduction of nitrogen actually occurs.

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