Nitrogenases are enzymes () that are produced by certain bacteria, such as cyanobacteria (blue-green bacteria) and rhizobacteria. These enzymes are responsible for the reduction of nitrogen (N2) to ammonia (NH3). Nitrogenases are the only family of enzymes known to catalyze this reaction, which is a key step in the process of nitrogen fixation. Nitrogen fixation is required for all forms of life, with nitrogen being essential for the biosynthesis of molecules (nucleotides, amino acids) that create plants, animals and other organisms. They are encoded by the Nif genes or homologs. They are related to protochlorophyllide reductase. Classification and structure Although the equilibrium formation of ammonia from molecular hydrogen and nitrogen has an overall negative enthalpy of reaction (), the activation energy is very high (). Nitrogenase acts as a catalyst, reducing this energy barrier such that the reaction can take place at ambient temperatures. A usual assembly consists of two components: The heterotetrameric MoFe protein, a nitrogenase which uses the electrons provided to reduce N2 to NH3. In some assemblies it is replaced by a homologous alternative. The homodimeric Fe-only protein, the reductase which has a high reducing power and is responsible for the supply of electrons. Reductase The Fe protein, the dinitrogenase reductase or NifH, is a dimer of identical subunits which contains one [Fe4S4] cluster and has a mass of approximately 60-64kDa. The function of the Fe protein is to transfer electrons from a reducing agent, such as ferredoxin or flavodoxin to the nitrogenase protein. The transfer of electrons requires an input of chemical energy which comes from the binding and hydrolysis of ATP. The hydrolysis of ATP also causes a conformational change within the nitrogenase complex, bringing the Fe protein and MoFe protein closer together for easier electron transfer. Nitrogenase The MoFe protein is a heterotetramer consisting of two α subunits and two β subunits, with a mass of approximately 240-250kDa. The MoFe protein also contains two iron–sulfur clusters, known as P-clusters, located at the interface between the α and β subunits and two FeMo cofactors, within the α subunits. The oxidation state of Mo in these nitrogenases was formerly thought Mo(V), but more recent evidence is for Mo(III). (Molybdenum in other enzymes is generally bound to molybdopterin as fully oxidized Mo(VI)). The core (Fe8S7) of the P-cluster takes the form of two [Fe4S3] cubes linked by a central sulfur atom. Each P-cluster is linked to the