The alternative nitrogenase of Azotobacter chroococcum is a vanadium enzyme

@article{Robson1986TheAN,
  title={The alternative nitrogenase of Azotobacter chroococcum is a vanadium enzyme},
  author={Robert L. Robson and Robert R. Eady and Toby H. Richardson and Richard W. Miller and Mary E. Hawkins and John R. Postgate},
  journal={Nature},
  year={1986},
  volume={322},
  pages={388-390}
}
The requirement for molybdenum in biological dinitrogen fixation, first reported by Bortels1, is due to its involvement at or near the site of reduction of N2 in conventional nitrogenase. To date, all nitrogenases which have been purified to homogeneity consist of an iron protein (component 2) and a molybdoprotein (component 1)2. Azotobacter vinelandii, an obligately aerobic diazotrophic bacterium, has two systems for nitrogen fixation: a conventional nitrogenase involving molybdenum and an… 
Genetic evidence for an Azotobacter vinelandii nitrogenase lacking molybdenum and vanadium
TLDR
A strain of Azotobacter vinelandii which has deletions in the genes for both the molybdenum (Mo) and vanadium (V) nitrogenases can synthesize a nitrogenase which lacks both Mo and V, and reduction of dinitrogen by nitrogenase can occur at a center lacking both these metals.
Unique features of the nitrogenase VFe protein from Azotobacter vinelandii
TLDR
A detailed EPR investigation of the origins and interplays of FeV cofactor- and P-cluster-associated signals is presented herein, which lays the foundation for future kinetic and structural analysis of the VFe protein.
Iron-Only Nitrogenase: Exceptional Catalytic, Structural and Spectroscopic Features
The classical, molybdenum-containing nitrogenase system exists in all diazotrophic microorganisms that have been examined. It was a long-held dogma that Mo is absolutely necessary for nitrogen
Three Nitrogen Fixation Systems in Azotobacter vinelandii
TLDR
Evidence indicating that the N2-fixing bacterium, Azotobacter vinelandii, contained at least two nitrogenase systems: the conventional Mo-containing nitrogenase (nitrogenase-1) system; and an alternative nitrogenase system expressed in the absence of Mo.
Metalloclusters of the nitrogenases.
TLDR
It is now clear that Mo is not an essential metal for N2 fixation, and the soil bacterium Azotobacter chroococcum has an Mo-containing nitrogenase with properties very similar to those found in many other diazotrophic organisms, but it also has a vanadium- containing nitrogenase which functions in the presence of V ifMo is not available.
Metal Regulation of Nitrogenases and Molybdenum Transport in Azotobacter Vinelandii
TLDR
The alternative nitrogenases introduced a new subject of study, the mechanism metal regulation of nitrogenase, and it is now possible to describe in outline the regulatory pathways involved.
Molybdenum-Nitrogenase: Structure and Function
TLDR
This paper reviews only studies of the Mo-based nitrogenase system and deduced amino-acid sequences of the subunits of theMoFe, VFe and FeFe proteins show significant identity with one another and conservation of both the domain structures around the eight strictly conserved Cys residues and the spacing between them indicates that all three protein types will have the same general structural features.
Nitrogenases: Distribution, Composition, Structure and Function
TLDR
It is clear now, however, that Mo is not essential for biological nitrogen fixation.
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References

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Nitrogenase activities were determined from maximum acetylene reduction rates for mutant strains of Azotobacter vinelandii which are unable to fix N2 in the presence of molybdenum (Nif-) but undergo
Evidence for an alternative nitrogen fixation system in Azotobacter vinelandii.
TLDR
Under conditions of molybdenum deprivation, Nif- mutant strains of several different phenotypic classes underwent phenotypesic reversal to Nif+, as shown by their ability to incorporate 15N2 and to grow in N-free media.
Nitrogenase from Azotobacter chroococcum. Purification and properties of the component proteins.
1. A large-scale purification of the nitrogenase components from Azotobacter chroococcum yielded two non-haem iron proteins, both of which were necessary for nitrogenase activity and each had a
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    Proceedings of the National Academy of Sciences of the United States of America
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TLDR
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TLDR
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Advances in Nitrogen Fixation Research
TLDR
The lectins identified in Anabaena azollae Newton cells are constitutive as well, and their actual role in the symbiosis with Azolla has not been verified yet, but they may be involved in the regulation and control of the development of anabaena in the leaf cavi ty and, poss ibly, trigger the exchange of me taboli tes between the hos t Azolla and the N2-fixing AnabaENA.
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