Fe-Mo cofactor

A relationship between the redox state of an iron—sulfur center and the conformation of the host protein was furthermore established in an X-ray crystal study on center P in Azotobacter vinelandii nitroge-nase (270). In this enzyme, the two-electron oxidation of center P was found to be accompanied by a significant displacement of about 1 A of two iron atoms. In both cases, this displacement was associated with an additional ligation provided by a serine residue and the amide nitrogen of a cysteine residue, respectively. Since these two residues are protonable, it has been suggested that this structural change might help to synchronize the transfer of electrons and protons to the Fe-Mo cofactor of the enzyme (270). 

Nitrogenase is another big target of cluster synthesis. The X-ray elucidation of the active center of the Fe-Mo cofactor and P-cluster... 

Otsuka, S., Kamata, M. Molybdo-iron mixed cluster as Fe-Mo cofactor model, in Ref. 7, p. 229... 

No(e added in proof The structure of the Fe-Mo cofactor cited in Footnote 97 has led tho.se authors to suggest that the molybdenum docs not directly participate in binding the dinilrogen molecule. The Mo is already six-coordinutc with three S atoiivs. two O atoms from u homocitraic anion, and one N atom from a histidine in the protein chain. Therefore, in Eq. 19.38 Ihe N is probably bound to an Fe—S cluster in place of Mo. 

Chapter 17), which are typically composed of two proteins, the Fe-protein, a homodimer with one [4Fe—4S] cluster bridged between the subunits and one ATP-binding site in each subunit, and the MoFe-protein. The MoFe-protein, an a2P2 heterotetramer, contains two complex metallo-clusters, each containing a total of eight metal ions the P-cluster is an [8Fe—7S] cluster located between the [4Fe—4S] cluster of the Fe protein and the FeMo cofactor, whereas the Fe—Mo cofactor is a [Mo—7Fe—9S—X—homocitrate] cluster (where X is C, N or O), buried within each a-subunit. The flow of electrons is from the [4Fe—4S] cluster to the P-cluster and then to the Fe—Mo cofactor. 

In biological systems Mo is present as the Fe/Mo cofactor of the nitrogenase enzymes (2) and of the multitude of oxidoreductases (3). In the latter the common molybdopterin cofactor (4), in addition to a dithiolene functionalized pyranopterin (5) ligand (Fig. 1), contains terminal oxo ligands and in the case of xanthine oxidase both oxo and thio ligands. Some aspects of molybdenum sulfur chemistry discussed in this work may be relevant to the biosynthesis of the molybdopterin cofactor and the function of xanthine oxidase (6). 

The isolated Fe-Mo cofactor thought to be the active site of N-ase catalyzes hydrogen evolution at high potentials,53 and H2 is in fact an inhibitor of N2 reduction. H2 reduction is also proven by the formation of HD from D2 gas and protons derived from H20, which occurs only in the presence of N254 ... 

Fe-Mo cofactor of nitrogenase full width at half maximum... 

Figure 4.2. Schematic models of Fe-Mo cofactor with possible binding of dinitrogen substrate (a) and P cluster (b). Y indicates a bridging ligand. (Part a adapted, with permission, from Kim, J., and Rees, D. C. Structural models for metal centres in the nitrogenase molybdenum-iron protein. Science 257 1677-82. Copyright 1992, American Association for the Advancement of Science. Part b adapted, with permission, from Chan, M. K., Kim, J., and Rees, D. C. The nitrogenase Fe-Mo-cofactor and P-cluster pair 2.2A resolution structures. Science 260 792-4. Copyright 1993, American Association for the Advancement of Science.)...

Nitrogenase is another big target of cluster synthesis. The X-ray elucidation of the active center of the Fe-Mo cofactor and P-cluster (5) has accelerated the efforts to find rational preparative methods of trinuclear or cubane-type clusters containing molybdenum (6-9). The raft cores in these cluster complexes are one of the general structural units also in solid-state compounds, and the mutual relationships are very important. A number of review articles are now available on the syntheses, structures, and other properties of metal chalcogenide cluster compounds (6, 7,10-24). 

The isolated Fe-Mo cofactor thought to be the active site of N-ase catalyzes... 

Thiol extrusion of the FeS centres from Azobacter vinelandii and Clostridium pasteurianum with alkyl fluorothiols allows identification of at least two Fe4S4 and one FegSj cluster in the (Fe-Mo) protein. A cluster model in which two Fe4S4 units are bridged through a comer iron on each cube by a Mo S4 unit is proposed for the active site of the (Fe-Mo) cofactor. Molecular nitrogen binds axially to the central molybdenum and is Ji-bonded to the iron in the cubes. This weakens and activates the N=N bond and electrons are injected stepwise via the two iron-sulphur cubes with successive protonation at the terminal N. This latter point is consistent with a report that N—NHg is important in nitrogen fixation. 

Nitrogenases containing molybdenum are an intermolecular complex consisting of two parts called MoFe protein and Fe protein, respectively. The MoFe protein (230 kDa) has a heterotetrameric structure with an Fe-Mo cofactor and [Fe-S] cluster, while the Fe protein (64 kDa) has a homodimeric arrangement of sub-units bound by an [4Fe-4S] cluster. In some nitrogenises, molybdenum can be replaced by vanadium or iron. 

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