Nitrogenases

Nitrogenases are a family of enzymes that catalyze the reduction of atmospheric dinitrogen (N2) to ammonia (NH3), a key step in biological nitrogen fixation. In nature, nitrogen fixation provides usable nitrogen to ecosystems and supports growth of many plants and microbes. The reactions are carried out by bacteria and archaea, either free-living in soil or aquatic environments, or in symbiotic associations with plants such as legumes.

Most nitrogenases consist of two components: the MoFe protein, which carries the catalytic site for N2 reduction, and the Fe protein, which transfers electrons from cellular donors. The MoFe protein contains the FeMo-cofactor at its active site and the P-cluster that connects to the FeMo-co. The Fe protein harbors a [4Fe-4S] cluster and is coupled to flavin adenine dinucleotide (FAD). Electron transfer from the Fe protein to the MoFe protein is driven by ATP hydrolysis, with about 8 electrons and 16 ATP molecules consumed per N2 reduced to two NH3 (and H2 is produced as a byproduct).

Oxygen sensitivity is a defining feature: nitrogenases rapidly inactivate in the presence of O2. In legumes, nodules with leghemoglobin maintain a microaerobic environment; in cyanobacteria, heterocysts provide low-oxygen conditions.

Variants: some bacteria possess alternative nitrogenases that use vanadium (V) or iron-only (Fe-only) cofactors when molybdenum is scarce. These enzymes can also reduce N2 though with different efficiencies.

Significance: nitrogenases are central to the global nitrogen cycle and have potential applications in sustainable agriculture and biotechnology.