Vanadium nitrogenase

NifM is required for maturation of VnfH, and NifS and U seem to be important for provision of sulfide and probably iron for the biosynthesis of the vanadium nitrogenase. The apo VFe protein has been isolated from an A. vinelandii strain deleted for nifKD and nifB (169). It was an hexamer that could be activated in vitro by the addi-... 

Substrate reduction by vanadium nitrogenase has not been investigated as extensively as has molybdenum nitrogenase, but there are clear differences. Acetylene is a poor substrate and N2 does not compete as effectively with protons for the electrons available during turnover. Therefore, high rates of H2 evolution are observed in the presence of these substrates. Furthermore, acetylene is reduced to both ethylene and a minor product, ethane (172). Equation (2) summarizes the most efficient N2 reduction data yet observed for vanadium nitrogenase. 

In contrast with the molybdenum enzyme, hydrazine (N2H4) is a minor product from the reduction of N2 by vanadium nitrogenase (186). The production of N2H4 increases with increasing temperature up to 50°C, at which temperature the ability of vanadium nitrogenase to produce to NH3 ceases, although the production of N2H4 increases... 

Substrate reduction by the iron nitrogenase is very similar to that observed with vanadium nitrogenases. Acetylene is a relatively poor substrate, and N2 reduction is accompanied by considerable H2 evolution. Acetylene reduction leads to the production of some ethane as well as ethylene. Beyond this, little has been investigated. Under optimal conditions for N2 reduction, the ratio of N2 reduced to H2 produced was 1 7.5 compared with 1 1 for molybdenum nitrogenase 192). 

A preparation of the third nitrogenase from A. vinelandii, isolated from a molybdenum-tolerant strain but lacking the structural genes for the molybdenum and vanadium nitrogenases, was discovered to contain FeMoco 194). The 8 subunit encoded by anfG was identified in this preparation, which contained 24 Fe atoms and 1 Mo atom per mol. EPR spectroscopy and extraction of the cofactor identified it as FeMoco. The hybrid enzyme could reduce N2 to ammonia and reduced acetylene to ethylene and ethane. The rate of formation of ethane was nonlinear and the ethane ethylene ratio was strongly dependent on the ratio of nitrogenase components. 

Dilworth MJ, RR Eady (1991) Hydrazine is a product of dinitrogen reduction by the vanadium-nitrogenase from Azotobacter chroococcum. Biochem J 277 465 68. 

Miller RW, Eady RR (1988) Molybdenum and vanadium nitrogenases of Azotobacter chroococcum. Biochem J 256 429-432... 

Eady RR (2003) Current status of structure function relationships of vanadium nitrogenase, Coord Chem Rev 237 23-30... 

The isolation of a vanadium nitrogenase suggests that vanadium may have a more important role in the nitrogen cycle than was recognized. This vanadium enzyme may have similarities with the chemical systems described in Section 33.3.8.1. 

Eady, R.R. 1990. Vanadium nitrogenases. In Vanadium in biological systems. N.D. Chasteen (Ed.), Kluwer Academic Publishers, Dordrecht, pp. 99-127. 

Eady, R.R. 1995. Vanadium nitrogenases of Azotobacter. Metal Ions in Biological Systems 31 363 -05. 

Lie, S., Pulakat, L., and Gavini, N. (2000) Activation of vanadium nitrogenase expression in DJ54 revertant in the presence of molybdenum, FEBS Lett 482, 149-153. 

The two most important vanadium enzymes described to date are the vanadium nitrogenase (V—Nase) and haloperoxidase. 

Vanadium nitrogenase is produced by certain bacteria grown in molybdenum-deficient environments. It is effective in the reduction of N2 and other nitrogenase substrates, although with less activity than the Mo—Nase. The enzyme resembles the Mo analogue (see Sections 17-E-10 and 18-C-13) in the construction and structure of the prosthetic groups, as well as in its functions.101 It consists of a FeV protein, FeVco, and an iron protein (a 4Fe—4S ferredoxin). 

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