Biosynthesis of the Iron-Molydbenum and Iron-Vanadium Cofactors of the nif- and vnf-Encoded Nitrogenases

Abstract

The Iron-Molybdenum cofactor (FeMo-co) of the nif-encoded nitrogenase constitutes the active site of the enzyme. The cofactor consists of homocitrate bound to an inorganic lattice of metal atoms with the ratio Fe7S9MoX as shown in Figure 1. MoFe protein contains an additional metallocluster with a Fe8S7 composition known as the P-cluster. Very little is known about the biosynthesis of the P-cluster, and it will not be reviewed here. This chapter will describe the biosynthesis of FeMo-co and its analogs, the Iron-Vanadium cofactor (FeV-co) and the Iron-Iron cofactor (FeFe-co). Most of the information comes from study of the FeMo-co biosynthesis in Azotobacter vinelandii, but significant insights have come from the analysis of cofactor synthesis for the vnf- and anf- encoded systems. The relevant properties of FeMo-co and its analogs will be presented here, but greater detail about the properties of this family of cofactors will be presented in other chapters of this volume. Additional insight into the structure and reactivity of FeMo-co is provided in previous reviews (Smith et al., 1985; Burgess, 1990; Ludden et al., 1993; Allen et al., 1994).