Molybdenum nitrogenase structure

Information, particularly structural, concerning vanadium-dependent nitrogenases, is relatively limited. The consensus is that they resemble the molybdenum nitrogenase in most aspects except for the presence of a FeV cofactor, and they will not be discussed further. 

B. K. Burgess, Nitrogenase structure, function and genetics, in Molybdenum Enzymes, Cofactors and Model Systems , eds. 

Iron-vanadium nitrogenase was isolated in 1986. Its nitrogen-reducing activity is 1/3-1/2 of that for molybdenum nitrogenase, and the VFe protein is spectroscopically different from the analogous MoFe protein. There is no X-ray crystal structure of any component of iron-vanadium nitrogenase. 

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]. 

However, when the X-ray crystal structure of the MoFe protein was examined, it was clear that homocitrate could not directly hydrogen bond to the histidine, since the carboxylate group and imidazole are stacked parallel to each other in the crystal. Nevertheless, as noted in the previous section, studies on model complexes have suggested that homocitrate can become monodentate during nitrogenase turnover, with the molybdenum carboxylate bond breaking to open up a vacant site at molybdenum suitable for binding N2. 

A preparation of the third nitrogenase from A. vinelandii, isolated from a molybdenum-tolerant strain but lacking the structural genes for the molybdenum and vanadium nitrogenases, was discovered to contain FeMoco 194). The 8 subunit encoded by anfG was identified in this preparation, which contained 24 Fe atoms and 1 Mo atom per mol. EPR spectroscopy and extraction of the cofactor identified it as FeMoco. The hybrid enzyme could reduce N2 to ammonia and reduced acetylene to ethylene and ethane. The rate of formation of ethane was nonlinear and the ethane ethylene ratio was strongly dependent on the ratio of nitrogenase components. 

Kim, J. and D.C. Rees. Structural models for the metal centers in the nitrogenase molybdenum-iron protein. Science 257,1677-1682 (1992). 

Eady RR (1996) Structure-function relationships of alternative nitrogenases. Chem Rev 96 3013-3030 Emerson SR, Huested SS (1991) Ocean anoxia and the concentrations of molybdenum and vanadium in seawater. Mar Chem 34 177-196... 

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