MoFe protein by six cysteine residues. Each FeMo cofactor (Fe7MoS9C) consists of two non-identical clusters: [Fe4S3] and [MoFe3S3], which are linked by three sulfide ions. Each FeMo cofactor is covalently linked to the α subunit of the protein by one cysteine residue and one histidine residue. Electrons from the Fe protein enter the MoFe protein at the P-clusters, which then transfer the electrons to the FeMo cofactors. Each FeMo cofactor then acts as a site for nitrogen fixation, with N2 binding in the central cavity of the cofactor. Variations The MoFe protein can be replaced by alternative nitrogenases in environments low in the Mo cofactor. Two types of such nitrogenases are known: the vanadium–iron (VFe; Vnf) type and the iron–iron (FeFe; Anf) type. Both form an assembly of two α subunits, two β subunits, and two δ (sometimes γ) subunits. The delta subunits are homologous to each other, and the alpha and beta subunits themselves are homologous to the ones found in MoFe nitrogenase. The gene clusters are also homologous, and these subunits are interchangeable to some degree. All nitrogenases use a similar Fe-S core cluster, and the variations come in the cofactor metal. The Anf nitrogenase in Azotobacter vinelandii is organized in an anfHDGKOR operon. This operon still requires some of the Nif genes to function. An engineered minimal 10-gene operon that incorporates these additional essential genes has been constructed. Mechanism General mechanism Nitrogenase is an enzyme responsible for catalyzing nitrogen fixation, which is the reduction of nitrogen (N2) to ammonia (NH3) and a process vital to sustaining life on Earth. There are three types of nitrogenase found in various nitrogen-fixing bacteria: molybdenum (Mo) nitrogenase, vanadium (V) nitrogenase, and iron-only (Fe) nitrogenase. Molybdenum nitrogenase, which can be found in diazotrophs such as legume-associated rhizobia, is the nitrogenase that has been studied the most extensively and thus is the most well characterized. Figures 1-2 display the crystal structure and key catalytic components of molybdenum nitrogenase extracted from Azotobacter vinelandii. Vanadium nitrogenase and iron-only nitrogenase can both be found in select species of Azotobacter as an alternative nitrogenase. Equations 1 and 2 show the balanced reactions of nitrogen fixation in molybdenum nitrogenase and vanadium nitrogenase respectively. All nitrogenases are two-component systems made up of Component I (also known as dinitrogenase) and Component II (also known as dinitrogenase reductase). Component I is a MoFe protein in molybdenum nitrogenase, a VFe protein in