V nitrogenase

The only kinetic study on a V nitrogenase [74] reports that the rates of binding of the reduced Fe protein with the VFe protein and the subsequent electron-transfer rates are very similar to those for Mo nitrogenase, as the components of the V nitrogenase form a slightly weaker electron-transfer complex. 

Mutant strains of A. chroococcum carrying deletions of both Mo and V nitrogenase structural genes are unable to grow on N2 ill). In contrast, comparable strains of A. vinelandii can grow provided neither Mo nor V is added to the growth medium (12). This ability is due to the presence of the third nitrogenase in A. vinelandii. 

Fe component property Mo nitrogenase V nitrogenase Third nitrogenase... 

The structural genes of the Fe proteins of the V nitrogenase of A. chroococcum and A. vinelandii and of the third nitrogenase of A. vine-landii have been cloned and sequenced 13a, 14,22). Comparison of the derived amino acid sequences shows the Fe proteins of Mo and V nitro-genases to be very similar (91% identical) and that of the third nitrogenase to be 61% identical. All sequences show the spacing of the five invariant Cys residues, a conserved Arg residue near position 100, and a consensus nucleotide-binding sequence. 

In the case of V nitrogenase the efficiency with which N2 competes with H+ as reducible substrate increases as the temperature is decreased from 30°C (47). Within the temperature range 30-5°C, assays with purified enzymes showed that the activity of Mo nitrogenase toward N2 as a substrate decreased 10-fold more than for V nitrogenase. This difference may account for the persistence in nature of what, under some conditions, appears to be an inefficient system. 

For Mo-independent nitrogenases, the pre-steady-state rates are restricted to studies of the initial electron transfer reactions of V nitrogenase (52). The rate of electron transfer is measured from the increase in absorbance of the Fe protein as it becomes oxidized during turnover. 

Scheme 2. MgATP-dependent electron transfer between components of V nitrogenase, The pre-steady-state electron transfer reactions between the Fe protein and the VFe protein of V nitrogenase of A, chroococcum have been analyzed in terms of this scheme (52). Ac2 represents the Fe protein and Acl represents the VFe protein of this system. This scheme is analogous to that used in the detailed study of Mo nitrogenase (see Hef. 50).

With regard to C2H2 as a substrate, the stereospecificity in producing [cjs- H2]ethylene as a product and small amounts of C2H6 provides a parallel with the reactions of V nitrogenase. 

The discovery of Mo-independent nitrogenases is so recent that their contribution to the global cycling of N is unknown. There is good evidence that they are widely distributed in organisms other than in Azotobacter. Recently a nifHDK deletion strain of the photosynthetic bacteria Rhodobacter capsulatus was shown to grow on N2 in medium deficient in Mo and V. Nitrogenase with properties similar to the third... 

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