The Fe-V Cofactor of Vanadium Nitrogenase Contains an Interstitial Carbon Atom

The first direct evidence is provided for the presence of an interstitial carbide in the FeV cofactor of Azotobacter vinelandii vanadium nitrogenase. As for our identification of the central carbide in the FeMo cofactor, we employed Fe K valence-to-core X-ray emission spectroscopy and density functional theory calculations, and herein report the highly similar spectra of both variants of the cofactor-containing protein. The identification of an analogous carbide, and thus an atomically homologous active site in vanadium nitrogenase, highlights the importance and influence of both the interstitial carbide and the identity of the heteroatom on the electronic structure and catalytic activity of the enzyme.