Structural Characterization of Two CO Molecules Bound to the Nitrogenase Active Site.

Trixia M Buscagan,Thomas Spatzal,Kathryn A Perez,Douglas C Rees,Ailiena O Maggiolo

doi:10.1002/anie.202015751

Abstract

As an approach towards unraveling the nitrogenase mechanism, we have studied the binding of CO to the active‐site FeMo‐cofactor. CO is not only an inhibitor of nitrogenase, but it is also a substrate, undergoing reduction to hydrocarbons (Fischer–Tropsch‐type chemistry). The C−C bond forming capabilities of nitrogenase suggest that multiple CO or CO‐derived ligands bind to the active site. Herein, we report a crystal structure with two CO ligands coordinated to the FeMo‐cofactor of the molybdenum nitrogenase at 1.33 Å resolution. In addition to the previously observed bridging CO ligand between Fe2 and Fe6 of the FeMo‐cofactor, a new ligand binding mode is revealed through a second CO ligand coordinated terminally to Fe6. While the relevance of this state to nitrogenase‐catalyzed reactions remains to be established, it highlights the privileged roles for Fe2 and Fe6 in ligand binding, with multiple coordination modes available depending on the ligand and reaction conditions.

Highlights

As an approach towards unraveling the nitrogenase mechanism, we have studied the binding of carbon monoxide (CO) to the activesite FeMo-cofactor
It has long been proposed that the N2ase active site features multiple substrate binding sites and that the formation of these binding sites requires the particular substrate under turnover conditions; that is, the cofactor is dynamic during catalysis.[11,12]
We reported the initial crystal structure of a ligand bound form of Mo N2ase from Azotobacter vinelandii (Av) in which one of the belt sulfides, S2B, of the cofactor is displaced by a carbon monoxide (CO) molecule (Av1-CO);[10] a similar binding mode was subsequently demonstrated for the Av vanadium nitrogenase.[16]