The Alternative Nitrogenases

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- 

The vnfH genes of A. vinelandii, A. chroococum, anAAnabaena var-iabilis have been sequenced and show approximately 90% identity with nifH. In particular, the five conserved cysteine residues of NifH, 

The metal and acid labile sulfur contents of these proteins was somewhat lower than expected by comparison with the MoFe proteins. AcP contained 2 0.3 vanadium atoms, 21 1 iron atoms, and 19 0.2 acid-labile sulfur atoms per a2)32 hexamer (172). The a2fi2(S) form of Avl was reported to contain 1.4 0.2 vanadium atoms, 21.4 iron atoms, and 24 acid-labile sulfur ions per molecule, and the afi2 form had lower levels of metal and sulfur (173). 

Mossbauer spectroscopy of AvF clearly demonstrated the presence of P clusters (174). The EPR spectra of dithionite-reduced VFe proteins are complex, indicating the presence of several paramagnetic species. Avl exhibits broad EPR signals, with g values of 5.8 and 5.4 integrating to 0.9 spins per V atom, which have been assigned to transitions from the ground and first excited state of a spin S = system (175). EPR data for AcF are more complex, with g values at 5.6, 4.3, and 3.77 that appear to arise from a mixture of S = species (176). The signals were associated with a midpoint potential of 

The iron K-edge EXAFS measurements on AVI 182) and the extracted FeVaco from AcF 183) show Fe-S and Fe-Fe interactions at 2.32 and 2.64 A, with a longer Fe-Fe distance of 3.7 A very similar again to the EXAFS data on FeMoco. These data emphasize the structural similarities between the cofactor centers of the MoFe and VFe proteins. 

---

In contrast to the situation with the alternative nitrogenases, but with the notable exception of the C. pasteurianum proteins, the component proteins from aU. Mo-based nitrogenases interact as heterologous crosses to form catalyticaHy active enzymes (52). Carbon monoxide, CO, is a potent inhibitor of aU. nitrogenase-cataly2ed substrate reductions, with the exception of reduction (126). Molecular hydrogen has a unique involvement with Mo-nitrogenase... 

As well as donating electrons to the MoFe protein, the Fe protein has at least two and possibly three other functions (see Section IV,C) It is involved in the biosynthesis of the iron molybdenum cofactor, FeMoco it is required for insertion of the FeMoco into the MoFe protein polypeptides and it has been implicated in the regulation of the biosynthesis of the alternative nitrogenases. 

ParGnthGSGs indicate reactions catalyzed by the alternative nitrogenases. Data taken from reviews 13, 123, 124) and Ref. 125). 

The uncertain role of the Mo atom is emphasized by the existence of the alternative nitrogenases, which contain a FeV or a FeFe cofactor. 

Gollan, U., Schneider, K., Muller, A., Schuddekopf, K., and Klipp, W. 1993. Detection of the in vivo incorporation of a metal cluster into a protein. The FeMo cofactor is inserted into the FeFe protein of the alternative nitrogenase in Rhodobacter capsulatus. Eur. J. Biochem. 215, 25-35. 

Most of the current knowledge concerning the mechanism of nitrogenases is based upon extensive studies of the molybdenum nitrogenase hence this enzyme will be the focus of discussion in this article. The general properties of the molybdenum nitrogenase are described in Section 2, and the catalytic and assembly mechanism of this enzyme system will be dealt with in Sections 3 and 4. The alternative nitrogenase systems will also be briefly discussed in Section 5. 

M. Hawkins and J.R. Postgate, The alternative nitrogenase of AcotobactZK chKOOCOCCum is a vanadium enzyme. 

---