Vanadium nitrogenase cluster structure

The next two entries to Table 3 are cited for completeness. Nitrogenase is treated in Chapter 7 and CO dehydrogenase in Chapter 9. Nitrogenase contains a very complex iron-sulfur cluster that includes another metal, molybdenum or vanadium. The crystal structure of the Mo variant has been determined. There is a third variant, alternative nitrogenase [92], whose cluster apparently does not contain any heterometal. That cluster would thus be a perfect candidate for our definition of a redox-catalytic iron-sulfur cluster. Unfortunately, this third nitrogenase has thus far been characterized to a much lesser extent than the other two forms. For all nitrogenases holds that the binding of N2 to the cluster has not been established [53] therefore, formally these enzymes have not yet been positively identified as redox iron-sulfur catalysts. 

The vanadium nitrogenase from A. vinelandii has the same a2 2 subunit structure as the Mo enzyme. Interestingly, a second VNase has been isolated from A. vinelandii, lacking one of the a subunits and consequently one of the M clusters and half of one of the P clusters. This incomplete a/ 2 variant is still active.I ] VNases in general are less stable than their Mo counterparts, leading to a larger variability and pronounced problems when it comes to crystallisation. 

Fig. 5. Buildup of the vanadium-nitrogenase from A. chroococcum, and the proposed structure of the M cluster (FeVco). The light scatterer X has been proposed to be N.

Recently, a second or alternative nitrogenase has been isolated from Azotobacter vinelandii (21) and Azotobacter chroococcum (22) that contains vanadium as opposed to molybdenum. The MoFe and VFe nitrogenase proteins from A. vinelandii (called Av and. 4vl , respectively) are known to have different polypeptide structures and it obviously of interest to know to what extent the cluster composition is conserved. Variable temperature MCD studies of the as isolated and thionine oxidized proteins provided a convenient means of addressing this question. 

Contents E. I. Solomon, K. W.Penfield, D.E. Wilcox Active Sites in Copper Proteins. An Electronic Structure Overview. -B.A.Averill Fe-S and Mo-Fe-S Clusters as Models for the Active Site of Nitrogenase. - N.D. Chasteen The Biochemistry of Vanadium. -KKustin, G.C. McLeod, T.R. Gilbert,... 

All of these beautiful model complexes for nitrogen fixation have one disadvantage in common they do not represent any of the structural features of the iron-vanadium cofactor in the nitrogenase. In contrast, cubane clusters constimting the vanadium-iron-sulfur moiety of the FeMoco are expedient structural models, with the disadvantage, however, that they do not catalyse the reduction of N2. 

Synthetic structural models for clusters within the nitrogenase proteins or of the molybdenum or vanadium iron - sulphur cofactors isolated from them, are at an eaily stage of development and there are as yet no well - defined systems which model electron - transfer chemistry involving the binding and reduction of molecular nitrogen or other substrates dinitrogen binding to synthetic iron - sulphur clusters or heterometallic iron - sulphur clusters of any type has not been established structurally nor spectroscopically, [44- 46]. 

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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