アイテム詳細

要旨

Nitrogenase is a complex enzyme that catalyzes the formation of ammonia through the reduction of molecular nitrogen. The molecular structure of the iron-molybdenum cofactor was only recently fully characterized with the identification of the carbide in the middle of the cluster (1,2). The electronic structure is still not fully understood, however, and the charge of the cofactor, the exact nitrogen binding site and the reaction mechanism of ammonia formation are still unanswered questions. Here we present theoretical calculations of the Fe-Mo cofactor of nitrogenase, as well as synthetic MoFe₃S₄ model complexes that give direct insight into the electronic structure of the molybdenum in this complex enzyme cluster. To this end, a large quantum cluster model incorporating the complete cofactor, bound ligands and a large part of the protein environment and crystal waters, was used. Computations are compared to recently obtained Mo K-edge high-energy resolution X-ray absorption spectra of the enzyme and model complexes. Different Mo oxidation state assignments are discussed, as well as different cofactor charge states, possible spin coupling scenarios and implications for future spectroscopic studies.