An inorganic clusterthat is found in the FeMo protein of the molybdenum-nitrogenase and is essential for the catalytic reduction of N2 to ammonia. This cluster contains Fe, Mo and S in a 7:1:9 ratio. The structure of the cofactor within the FeMo protein can be described in terms of two cuboidal subunits, Fe4S3 and MoFe3S3 bridged by three S2- ions and "anchored" to the protein by a histidine bound via an imidazole group to the Mo atom and by a cysteine bound via a deprotonated SH group to an Fe atom of the Fe4S3 subunit. The Mo atom at the periphery of the molecule is six-coordinate and in addition to the three sulfido ligands and the histidine imidazole is also bound to two oxygen atoms from an (R)-homocitrate molecule.
