Biosynthesis of the Iron-Molybdenum and Iron-Vanadium Cofactors

Abstract

The iron-molybdenum cofactor (FeMo-co) (Shah, Brill, 1977) is the prototype of a small family of cofactors that constitute the active sites of the known nitrogenases. No other Mo, V or Fe-containing enzyme is known to employ FeMo-co or its analogs and all known nitrogenases contain one of these cofactors. FeMo-co (MoFe₇S₉homocitrate) is found at the active site of the nif-encoded, molybdenum nitrogenase, and its structure (Fig 1) was determined as a component of the dinitrogenase protein (NifDK) of Azotobacter vinelandii (Kim, Rees, 1992; Chan et al, 1993). The vnf-encoded nitrogenase-2 contains FeV-co (Smith et al, 1988) and the anf-encoded nitrogenase-3 contains FeFe-co (Davis et al, 1996) as dissociable cofactors that are thought to have structures that differ from FeMo-co only at the position of the heteroatom (Mo, V or Fe) as shown in Fig 1. The structures of FeV-co and FeFe-co have not been determined and the argument that their structures are similar to that of FeMo-co is based on the ability of isolated FeV-co and FeFe-co to replace FeMo-co in the nif-encoded dinitrogenase proteins When apodinitrogenase protein (NifDK, lacking FeMo-co but containing the P clusters) is reconstituted with FeV-co or FeFe-co in vitro, it functions to reduce acetylene and protons effectively, but not N₂. Furthermore, FeMo-co is found associated with the anf-encoded dinitrogenase protein (AnfDGK) when cells are supplied with Mo (Gollan et al, 1993; Pau et al, 1993).