The key enzymatic reactions in nitrogen fixation are catalyzed by the nitrogenase complex, which has 2 components: the iron protein (component 2) and a component 1 which is either a molybdenum-iron protein, a vanadium-iron, or an iron-iron protein. 16 ATP + 16 H2O + N2 + 8 reduced [2Fe-2S]-[ferredoxin] = 16 ADP + 6 H(+) + H2 + 2 NH4(+) + 8 oxidized [2Fe-2S]-[ferredoxin] + 16 phosphate Binds 1 [4Fe-4S] cluster per dimer. Nitrogenase holoenzyme is subject to 'conformational protection' by FeSII; under oxidizing conditions FeSII binds to the holoenzyme and reversibly protects it from oxidation (PubMed:7830548). Homodimer. Constitutively expressed during log and stationary phase in sucrose-limited cultures, its levels decrease during stationary phase (at protein level). The reversible ADP-ribosylation of Arg-101 inactivates the nitrogenase reductase and regulates nitrogenase activity. This subunit is associated with the molybdenum-iron nitrogenase component 2. Belongs to the NifH/BchL/ChlL family.