Vanadium, alternative nitrogenase containing

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. 

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 and iron-only nitrogenases, also known as alternative nitrogenases, are related to the molybdenum nitrogenase but are all genetically distinct. A fourth and completely different nonconventional type of nitrogenase is known it also contains iron and molybdenum, but couples dinitrogen reduction to the oxidation of superoxide and carbon monoxide ]17]. 

Alternative nitrogenases also exist that lack molybdenum and instead contain either vanadium or no heterometal at all. These nitrogenases are generally less active, less selective and less understood than the Mo-containing nitrogenases. Biophysical and biochemical evidence, ineluding, in particular, cofactor isolation and re-incorporation experiments using MoFe-apoprotein, indicate that the iron-vanadium-eofactor is isostruetural with the well-studied FeMo-cofactor. 

Kentemich, T., Danneberg, G., Hundeshagen, B. and Bothe, H. (1988) Evidence for the occurrence of the alternative, vanadium-containing nitrogenase in the cyanohacterium Anabaena variabilis. PEMS Microbiol. Lett., 51, 19-24. 

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