The molybdenum site of nitrogenase. Preliminary structural evidence from x-ray absorption spectroscopy

Abstract

One of the two protein components of the nitrogenase enzyme system (the so-called MoFe protein) contains two Mo atoms and 24 -32 Fe atoms per 220-270 000 molecular weight. Despite many hypotheses about the Mo site and its involvement in dinitrogen reduction, there has been no spectroscopic means of unambiguously probing the state of Mo. In this paper, the results of x-ray. absorption spectroscopy studies are described and the analysis of these results provides the first direct evidence of the Mo coordination environment in the MoFe protein from Clostridium pasteurianum nitrogenase. The Mo is found to have primarily S ligation and in the resting state ("semireduced form") of the protein there appears to be no Mo=O species present. On air oxidation, however, clear evidence is found for the presence of Mo=O. The interaction of the Mo with one other metal, most probably Fe and most certainly not Ma, is also seen and it is suggested that the Mo is present in a Mo-Fe-Scluster not yet adequately modeled by any synthetic inorganic system.

Characterization of the blue copper site in oxidized azurin by extended x-ray absorption fine structure: Determination of a short Cu—S distance

“…The method of curve fitting (8,10,12) is very useful to more fully resolve the frequency components of EXAFS data, especially in cases of multiple shells of scatterers (12). The procedure for carrying out a curve-fitting analysis is first to fit the data with one shell of a chemically reasonable atom type.…”

“…If this single shell does not adequately describe the EXAFS, then a second shell is added. Experience has shown that up to four or so different shells may be fit to good data on a k range of [4][5][6][7][8][9][10][11][12] A-' before correlation problems become so severe as to render the results unreliable. The type of scatterer atom chosen for a given shell can be tested by using the pairwise phase parameters for a different scatterer type.…”

“…The fine structure that occurs at energies above the x-ray absorption edge (called EXAFS) provides a way of determining distances, numbers, and h-ypes of ligands coordinating a metal. Since the EXAFS method does not require crystalline samples, it can be applied to the study of metalloproteins under noncrystalline or physiological conditions (8)(9)(10)(11). In contrast to macromolecular crystallographic results, where it is difficult to obtain high accuracy except in the few cases of very well refined structures, EXAFS can provide accurate metal-to-ligand distances (12).…”

A pyridine-2-thiolato molybdenum complex

“…Molybdenum complexes containing Mo--S and Mo--N bonds are of special interest because of their relevance to a variety of molybdenum-containing enzymes (Cramer, Hodgson, Gillum & Mortenson, 1978) and hydrodesulfurization catalysts (Anzenhofer & de Boer, 1969 Tiripicchio-Camellini & Lahoz, 1986). During our studies of molybdenum-sulfur complexes with molybdenum in a low oxidation state, we obtained several complexes with pyS in different coordination modes.…”

“…Molybdenum complexes containing Mo--S and Mo--N bonds are of special interest because of their relevance to a variety of molybdenum-containing enzymes (Cramer, Hodgson, Gillum & Mortenson, 1978) and hydrodesulfurization catalysts (Anzenhofer & de Boer, 1969). To our knowledge, (C8H20N)[Mo2(C5H4NS)(CO)9] molybdenum complexes containing Mo--S and Mo--N bonds involving a pyridine-2-thiolato (pyS) moiety as a mixed bidentate (N,S) ligand have not been reported in the literature, although many Group VII and Group VIII metal complexes with pyS ligands are known (Demming, Meah, Bates & Hursthouse, 1988;Oro, Ciriano, Viguri, Tiripicchio, Tiripicchio-Camellini & Lahoz, 1986).…”