Mo-independent nitrogenases

Mo-independent nitrogenases, 36 83-84 nitrogen fixation, 36 81-84 physicochemical properties, 36 82 Fepr protein, 47 245-247 crystallography, 47 232-233 from Desulfovibrio desulfuricans, 47 227-228, 229, 232... 

Whereas alternative, older nitrogenases exist, in which Fe or V were replaced by Mo, these forms are catalyticaUy less elEcient. Interestingly, Mo-independent nitrogenases have been found only in heterocystous diazotrophs (phylogenetically the most recently diverging group—see discussion below) but not in non-heterocystous species (Bergman et ai, 1997). 

In this article the intention is to indicate that despite the differences between these nitrogenase systems there is an underlying functional and structural similarity among them. The discovery and historical development of the work on Mo-independent nitrogenases have been reviewed recently (1) and will not be reiterated. 

These genetic data support the suggestion of a parallel route for the synthesis of cofactors of Mo-independent nitrogenase function involving some early steps in common with FeMoco biosynthesis. 

It is clear from the data discussed above that the Fe proteins associated with Mo-independent nitrogenases of Azotobacter are typical members of the highly conserved family of Fe proteins associated with Mo nitrogenases. 

This inefficiency is not likely to be due to inappropriate suboptimal assay conditions because it occurs in vivo cultures of Azotobacter, when growing on N2 using Mo-independent nitrogenases, evolve considerably more H2 than when growth depends on Mo nitrogenase (48). 

In addition to the reduction of N2 and H+, Mo nitrogenase has been shown to reduce a wide range of small molecules that contain triple bonds (e.g., C2H2, CN, and N3). Many of these compounds have yet to be tested as substrates with Mo-independent nitrogenases, but the reduction pattern of one such analog (C2H2) has proved interesting. 

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. 

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... 

Loveless TM and Bishop PE. (1999). Identification of genes unique to Mo-independent nitrogenase sysems in diverse diazotrophs. Can. J. Microbiol. 45, 312-317. 

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