Abstract
Nitrogen fixation, the reduction of atmospheric dinitrogen (N2) to biologically available ammonium, has been important in the balance of biologically available nitrogen since early in the evolution of life on Earth. The nitrogen fixation reaction requires ATP and reductant and also reduces H+ to H2. Nitrogenase is composed of two multi-subunit metalloproteins. There are at least three evolutionarily related nitrogenase gene families that require different metals (Mo, V or Fe) in the cofactor for the protein component (Component I) that contains the active site for N2 reduction. The requirements for metals probably played an important role in the evolution of nitrogenases, as the oceans progressed from anoxic waters containing relatively high concentrations of reduced Fe, to low-Fe oxic waters.