The unusual metal clusters of nitrogenase: structural features revealed by x-ray anomalous diffraction studies of the MoFe protein from Clostridium pasteurianum.

Abstract

Nitrogenase (EC 1.18.6.1) catalyzes the conversion of dinitrogen to ammonia, the central reaction of biological nitrogen fixation. X-ray anomalous diffraction data were analyzed to probe the structures of the metal clusters bound by nitrogenase MoFe protein. In addition to one FeMo cofactor, each half-molecule of MoFe protein binds one large FeS cluster of a type not previously observed in a protein. The FeS cluster contains roughly eight Fe atoms, comprises two subclusters, and is separated from the FeMo cofactor by an edge-to-edge distance of 14 A. The inorganic framework of the FeMo cofactor is not resolved into subclusters, but the Mo atom is located at its periphery. FeMo cofactors in different half-molecules are 70 A apart and cannot promote binuclear activation of dinitrogen by two Mo atoms.

Selenol binds to iron in nitrogenase iron-molybdenumcofactor: an extended x-ray absorption fine structure study.

“…In 1978, Rawlings et al (6) (11)(12)(13). This prediction was confirmed by recent x-ray crystallographic results (14)(15)(16), which showed that aCys-275 is a ligand of one of the Fe atoms in FeMoco.…”

“…A model, which included two Fe atoms at the same distances such as would exist if Se bridged two Fe atoms, was also evaluated, but these fits (also not shown) were also significantly worse, being comparable to the C + Mo fits in quality. These the present data, it is not possible to associate this site with a specific Fe atom in the structural model proposed for FeMoco from crystallography (14)(15)(16) …”

Maturation of Nitrogenase: a Biochemical Puzzle

“…It will become evident that large (21,43,44), K. pneumoniae (52), and Clostridium pasteurianum (7,45) have been solved and in all cases shown to exist as ␣ 2 ␤ 2 tetramers composed of two pairs of ␣␤ dimers related by a twofold symmetry (Fig. 2).…”

Catalytic Reduction of Dinitrogen to Ammonia at a Single Molybdenum Center

“…It also has been purified and studied for decades. Its structure has been determined in x-ray studies, and that structure has elicited a great deal of discussion concerning precisely where dinitrogen is reduced (8)(9)(10)(11). Although a huge effort to understand how dinitrogen is reduced by various nitrogenases has been made over a period of Ͼ40 years, definitive conclusions concerning the site and mechanism of dinitrogen reduction in nitrogenase(s) remain elusive (12).…”