The active site of the Molybdenum-dependent nitrogenase enzyme complex is a molybdenum and iron-containing cluster known as the FeMo-cofactor. This is represented as being a [7Fe-9S-Mo-X-homocitrate] cluster, where X denotes an unidentified small central atom. Previous studies have shown this to be carbon, nitrogen, or oxygen, and has been considered most likely to be nitrogen. To investigate the identity of X, spectroscopic analyses were performed on 15N-labeled or 13C-labeled MoFe protein and FeMo-cofactor which was isolated from the labeled protein. Spectroscopy was also performed on FeMo-cofactor as well as 15N-labeled FeMo-cofactor bound to nifX protein. These studies did not positively identify the central atom, but did provide some new information about the nature of FeMo-cofactor.
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