Nitrogenase

Nitrogenase is an enzyme complex that catalyzes the conversion of atmospheric nitrogen (N2) to ammonia (NH3), a form usable for biosynthesis. This biological nitrogen fixation is carried out by certain bacteria and archaea, including free-living diazotrophs and symbiotic partners such as rhizobia in legume roots. The reaction is energetically demanding and highly oxygen-sensitive.

Most well-studied nitrogenases are the molybdenum (Mo) nitrogenases, which consist of two component proteins: dinitrogenase (the MoFe protein) and dinitrogenase reductase (the Fe protein). The MoFe protein is a heterotetramer containing the active site FeMo-cofactor, a complex metal cluster where N2 binding and reduction occur. The Fe protein is a homodimer that contains a [4Fe-4S] cluster and provides electrons to the MoFe protein in an ATP-dependent manner.

Mechanistically, electrons are donated from an electron donor to the Fe protein, which transfers them to the MoFe protein with the consumption of ATP. The overall electron flow reduces N2 at the FeMo-cofactor to NH3, with H2 produced as a by-product. The overall stoichiometry is N2 + 8 H+ + 8 e- + 16 ATP → 2 NH3 + H2 + 16 ADP + 16 Pi, reflecting the energy-intensive nature of the process.

In addition to Mo-dependent nitrogenases, some organisms possess alternative nitrogenases containing vanadium (V) or iron only (Fe-only) cofactors (vnf and anf gene clusters). These variants operate under Mo limitation and have different metal cofactors and somewhat different properties but accomplish the same overall goal of reducing N2 to NH3.

Nitrogenase is highly sensitive to oxygen, which inhibits its activity. Diazotrophs employ various strategies, such as microaerobic conditions or specialized cell types, to protect the enzyme during nitrogen fixation.