Nitrogenase iron efficiency

Figure 24.2. Nitrogen Fixation. Electrons flow from ferredoxin to the reductase (iron protein, or Fe protein) to nitrogenase (molybdenum-iron protein, or MoFe protein) to reduce nitrogen to ammonia. ATP hydrolysis within the reductase drives conformational changes necessary for the efficient transfer of electrons.

Molybdenum occurs with iron in the enzymes nitrate reductase and nitrite reductase and in nitro-genase, and consequently is utilized in nitrate assimilation and N2 fixation. Along with the Fe-Mo enzyme, there are two other isoforms of nitrogenase, a primitive less-efficient form containing only iron in its active center, and another which contains iron and vanadium. Thus molybdenum is not absolutely essential for dinitrogen fixation, although the predominance of the more efficient Fe-Mo isoform in the modern ocean helps to minimize iron limitation of... 

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