The molybdenum site of nitrogenase. 2. A comparative study of molybdenum-iron proteins and the iron-molybdenum cofactor by x-ray absorption spectroscopy

105

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.

Section: Inasmuch As Thefj Values Of Mo and Fe Are Nearly Equivalentmentioning

confidence: 99%

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

Bolin

Ronco

Morgan

et al. 1993

105

“…Among these are the findings that six Fe atoms and the Mo atom form a spin-coupled cluster having S = 3/2 ground state with an EPR spectrum unique in biology (4)(5)(6)(7). Analysis of Mo atom extended x-ray absorption fine structure (EXAFS) substantiates a cluster structure (8), the most recent results indicating three Fe atoms at 2.68 A, three S atoms at 2.36 A, and three O,N atoms at 2.09 A from the Mo atom (K. 0. Hodgson, personal communication).…”

mentioning

confidence: 97%

“…Our synthetic analogue approach to the investigation ofclusters in metallobiomolecules (10,11) has resulted in the preparation ofcomplexes [1][2][3][4][5][6][7][8][9][10][11] shown in Fig. 1.…”

mentioning

confidence: 99%

Fluorine-19 chemical shifts as structural probes of metal-sulfur clusters and the cofactor of nitrogenase.

Mascharak¹,

Smith²,

Armstrong³

et al. 1982

Several properties of the FeMo-cofactor (co) of nitrogenase in N-methylformamide solution at ambient temperature have been investigated by means of 19F NMR'spectroscopy. With C6H5CF3 reference signals the magnetic moment per Mo atom was found to be =3.9 BM, consistent with S = 3/2 ground state identified by other spectroscopic methods at low temperature. Reaction of FeMo-co with 1.0 eq of RFS (RF = P-C6H4CF3, p-C6H4F) afforded isotropically shifted signals indicative of binding to a paramagnetic cluster. By comparison with the spectra of Fe-S and Fe-Mo-S species derivatized with RFS_, including the cubane-type MoFe3S4 clusters with S = 3/a ground states, it was concluded that the essential FeMo-co cluster structure remains intact and a Fe-atom is the probable thiolate binding site. An interaction of FeMo-co with C6H5S-had been detected earlier by low temperature EPR spectroscopy. The binding site assignment is based on large observed isotropic shifts (ca.

“…Addition of excess purified FeMo-co to UW45 extracts used in these experiments resulted in an activity of 44.9 nmol of ethylene formed per minute per assay. *Homocitrate lactone (Sigma) was converted to the free acid by adjusting the solution to pH 10 (20,22).…”

Section: Resultsmentioning

confidence: 99%

“…FeMo-co has been subjected to spectroscopic analyses, both in dinitrogenase and in isolated forms (8)(9)(10)(11)(12)(13)(14)(15)(16). Extended x-ray absorption fine structure (EXAFS) data indicate that FeMo-co has two or three iron atoms linked to molybdenum via bridging sulfur atoms and that the remaining iron atoms are more distant (10)(11)(12)(13)(14).…”

mentioning

confidence: 99%

Plausible structure of the iron-molybdenum cofactor of nitrogenase.

Madden

Krezel

Allen

et al. 1992

A plausible structure of the iron-molybdenum cofactor of nitrogenase [reduced ferredoxin:dinitrogen oxidoreductase (ATP-hydrolyzing), EC 1.18.6.1] is presented based on altered substrate reduction properties of dinitrogenase containing homocitrate analogs within the cofactor. Alterations on each carbon of the four-carbon homocitrate backbone were correlated with altered substrate reduction properties of dinitrogenase containing these analogs. Altered substrate reduction properties are the basis for a model in which homocitrate is oriented about two cubane metal clusters.