Iron-molybdenum cofactor
FeMoco is the primary cofactor of nitrogenase. Nitrogenase is the enzyme that catalyzes the conversion of atmospheric N2 into ammonia (NH3), through the process known as nitrogen fixation. Containing iron and molybdenum, the cofactor is called FeMoco. Its stoichiometry is Fe7MoS9C.
The FeMo cofactor is a cluster with composition Fe7MoS9C. This large cluster can be viewed as two subunits composed of one Fe4S3 cluster and one MoFe3S3 cluster. The two clusters are linked by three sulfide ligands. The unique Fe is anchored to the protein by a cysteine. It is also bound to three sulfides, resulting in tetrahedral molecular geometry. The additional six Fe centers in the cluster are each bonded to three sulfides. These six internal Fe centers define a trigonal prismatic arrangement around a central carbide center. The molybdenum is attached to three sulfides and is anchored to the protein by the imidazole group of a histidine residue. Also bound to Mo is a bidentate homocitrate cofactor, leading to octahedral geometry.Crystallographic analysis of the MoFe protein initially proposed the geometry of FeMoco, which was confirmed by extended X-ray absorption fine-structure (EXAFS) studies. Distances for Fe-S, Fe-Fe and Fe-Mo distances were determined to be 2.32, 2.64, and 2.73 Å respectively3.
According to the analysis by electron paramagnetic resonance spectroscopy, the resting state of the FeMo cofactor has a spin state of S=3/2. Upon one-electron reduction, the cofactor becomes EPR silent. Understanding the process in which an electron is transferred in the protein adduct shows a more precise kinetic model of the FeMo cofactor. The formal oxidation state is MoIV-2FeII-5FeIII-C4−-H+, including the cysteine proton.
Biosynthesis of FeMoco is a complicated process that requires nifS, nifQ, nifB, nifE, nifN, nifV, nifH, nifD, and nifK gene products (expressed as the proteins NifS, NifU, etc.). FeMoco assembly is proposed to be initiated by NifS and NifU which mobilize Fe and sulfide into small Fe-S fragments. These fragments are transferred to the NifB scaffold and arranged into a Fe7MoS9C cluster before transfer to the NifEN protein (encoded by nifE and nifN) and rearranged before delivery to the MoFe protein. Several other factors participate in the biosynthesis. For example, NifV is the homocitrate synthase that supplies homocitrate to FeMoco. NifV, a protein factor, is proposed to be involved in the storage and/or mobilization of Mo. Fe protein is the electron donor for MoFe protein6. These biosynthetic factors have been elucidated and characterized with the exact functions and sequence confirmed by biochemical, spectroscopic, and structural analyses.
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