The iron-molybdenum cofactor of the enzyme nitrogenase is the actual center of nitrogen reduction. Due to the immense significance of the reaction (c.f. Haber-Bosch process), this protein has long been the focus of intense bioinorganic research. Originally it was assumed that molybdenum, the presence of which could be confirmed by classical analytical methods, was the catalytically active species (indeed, Mo complexes with dinitrogen ligands and with all intermediates of nitrogen reduction were synthesized and structurally characterized).
The actual structure of the active site, however, was first solved in 1992 with the help of protein crystallography. It is made up of iron-sulfur clusters and a Mo-homocitrate unit. The spatial orientation of the metal components, as well as the crystallographically determined coordination numbers of the involved metal atoms (coordinatively saturated Mo and free coordination sites on some of the iron atoms of the iron-sulfur clusters) suggested the iron centers as binding sites for the nitrogen reduction.
In 2002, a crystallographic investigation suggested a nitrogen atom in the center of the six coordinatively unsaturated iron atoms, thus confirming the assumption that reduction takes place at the iron centers. A reinvestigation of the protein in 2011, however, found some crystallographic and spectroscopic evidence, that the corresponding atom is not nitrogen but carbon. This reopens the question of the catalytic parts of the nitrogenases.Note! Functional nitrogenases without molybdenum have also been isolated.