Identification of a Mo-Fe-S Cluster on NifEN by Mo K-Edge Extended X-ray Absorption Fine Structure

The iron-molybdenum cofactor (FeMo-co) is the complex metal-sulfur cofactor found within the active site of the molyb-denum-iron protein (MoFe protein) component of the nitrogenase enzyme complex.1 FeMo-co is among the most complex metal-sulfur cofactors known in biology and is composed of 7 Fe, 9 S, Mo, X (C, N, or O) and R-homocitrate.2 This cofactor is required for the nitrogenase MoFe protein to catalyze the reduction of N2 to NH3 in nitrogen-fixing organisms, such as Azotobacter Vinelandii or Rhodospirillum rubrum, and thus is essential for the input of nitrogen into the biosphere. Compared to simpler clusters, the large size and high complexity of FeMo-co is supported by a biosynthetic pathway that is more complex than that required for building simpler clusters such as [Fe2S2], [Fe3S4], or [Fe4S4]. A series of combined genetic and biochemical studies have shown that multiple proteins, mostl