Nitrogenase (EC 1.18.6.1)is the enzyme used by organisms to fix atmospheric nitrogen gas (N₂). Dinitrogen is relatively inert due to its triple bond. The enzyme therefore requires uses a lot of chemical energy in the form of ATP and reducing agents, such as NADH. The product of the reaction is ammonia, which is normally incorporated into glutamate to form glutamine. The enzyme is composed of a dimer of "MoFe" associated with a dimers of "Fe" each. Nitrogenase is supplied reducing power when it associates with the reduced, nucleotide bound Fe homodimer. The heterocomplex undergoes cycles of association and disassociation to transfer one electron, which is the limiting step in the process. The process itself is very energy intensive requiring electron donors and ATP to provide reducing power. The exact mechanism of catalysis is unknown due to the difficulty in obtaining crystals of nitrogenase bound to nitrogen. This is because the resting state of MoFe doesn’t bind nitrogen and also requires at least three electron transfers to perform catalysis. Nitrogenase is able to bind acetylene and carbon monoxide, which are noncompetitive substrates and inhibitors respectively. Dinitrogen, however, is a competitive substrate for acetylene. This is because binding of dinitrogen prevents acetylene binding, and acetylene requires only one electron to be reduced and it doesn’t inhibit .

All nitrogenases have an iron- and sulfur-containing cofactor which includes a metal atom assumed to be the active site. In most, this is molybdenum, though in some species it is replaced by vanadium or iron.

Due to the similarity between oxygen and nitrogen gas, most nitrogenases are irreversibly inhibited by dioxygen in vitro. This requires mechanisms for nitrogen fixers to avoid oxygen in vivo. Despite this problem, many use oxygen as a terminal electron acceptor for respiration. One known exception, a recently discovered nitrogenase of Streptomyces thermoautotrophicus, is unaffected by the presence of oxygen. The Azotobacteraceae are unique in their ability to employ an oxygen liable nitrogenase under aerobic conditions. This ability has been attributed to a high metabolic rate allowing oxygen reduction at the membrane, but this idea has been shown to be unfounded and impossible at oxygen concentrations above 70 uM (ambient concentration is 230uM O2) as well as during additional nutrient limitations ¹.
Oelze J. 2000. Respiratory protection of nitrogenase in Azotobacter species: is a widely held hypothesis unequivocally supported by experimental evidence? FEMS Microbiol Rev. 24(4):321-33.

Note 2: Seefeldt LC, Dance IG, Dean DR. 2004. Substrate interactions with nitrogenase: Fe versus Mo. Biochemistry. 43(6):1401-9.

The reaction that this enzyme performs is:

N₂ + 8H⁺ + 8e^- + 16 ATP → 2NH₃ + H₂ + 16ADP + 16 Pi

Organisms that synthesise nitrogenase

• Diazotrophs
• Heterocysts
• Azotobacteraceae
• Rhizobia

See also

• Nitrogen fixation