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In this article we will discuss about the enzyme system involved in the fixation of atmospheric nitrogen.
The fixation of atmospheric nitrogen relies on nitrogenase enzyme system, which comprise of nitrogenase and nitrogenase reductase. The electrons are transferred through ferredoxin or flavodoxin to nitrogenase reductase and then to nitrogenase where they are utilized to reduce N2 and H+ to NH3 and H2 as shown in the equation below:
N2 + 8e– + 8H+ + 16 Mg ATP → H2 + 16 Mg ADP + 16 Pi
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Two co-proteins are also present in nitrogenase enzyme system. These are a MoFe protein containing molybdenum plus iron and a Fe protein having iron only. The active site of nitrogenase where reduction of nitrogen takes place is dependent on a iron and molybdenum containing cofactor (Fe MO CO). The production of H2 which accompanies the reduction of nitrogen adds to ATP requirements of nitrogen fixation. Evolution of H2 accompanies biochemical nitrogen fixation.
However, only some strains of Rhizobium and Bradyrhizobium possess hydrogenase and can utilize the hydrogen, and other nitrogen fixing bacteria wastefully evolve H2 gas.Nodulated root forming systems which evolve hydrogen often are colonized by hydrogen- oxidizing Acinetobacter strains which grow on the hydrogen released by nitrogen fixing bacteria.
It is remarkable that nitrogenase is very sensitive to oxygen and is irreversibly inactivated on exposure to even low concentration. That is why, nitrogen fixation, usually is restricted to habitats in which nitrogenase is protected from exposure to molecular oxygen.
