Nitrogenase FeMo cofactor: an atomic structure in three simple steps.

The enzyme nitrogenase catalyzes the six-electron reduction of molecular dinitrogen to ammonium, concomitant with the reduction of protons to yield hydrogen gas. In the MoFe protein component of the nitrogenase system, the unique FeMo cofactor is the active site of catalysis, but its exact mechanism remains under debate. This review focuses on the history of the structure determination of FeMo cofactor, a process that extended over two decades and involved several iterations and corrections that are unusual and unexpected within the established field of X-ray crystallography. However, FeMo cofactor has defied expectations on several occasions, and besides being the largest single iron-sulfur cluster known to bioinorganic chemistry, this [Mo:7Fe:9S:C]:homocitrate moiety is unique in some aspects that have misled researchers several times. We have now arrived at a final and complete description of the atomic structure of FeMo cofactor, and yet many questions regarding various aspects of the catalytic mechanism of this enzyme remain to be answered.