X‑ray Spectroscopic Observation of an Interstitial Carbide in NifEN-Bound FeMoco Precursor

The iron–molybdenum cofactor (FeMoco) of nitrogenase contains a biologically unprecedented μ6-coordinated C4– ion. Although the role of this interstitial atom in nitrogenase catalysis is unknown, progress in understanding its biosynthetic origins has been made. Here we report valence-to-core Fe Kβ X-ray emission spectroscopy data to show that this C4– ion is present in the Fe8S9 “L-cluster,” which is the immediate precursor to FeMoco prior to the insertion of molybdenum and coordination by homocitrate. These results accord with recent evidence supporting a role for the S-adenosylmethionine-dependent enzyme NifB in the incorporation of carbon into the FeMoco center of nitrogenase.