Nitrogenases, alternative

Paramagnetic metal ions are often an intrinsic part of biological materials (e.g. Fe in globins. Mo in nitrogenase). Alternatively, diamagnetic metal... 

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). 

For the sake of completeness, it is worth noting the role of vanadium in the synthesis of alternative nitrogenase systems by several aerobic bacteria, particularly those within the genus Azotobacter (Fallik et al. 1991) low yields of hydrazine are produced, and this suggests a different affinity of the nitrogenase for the N2 substrate (Dilworth and Eady 1991). 

Fallik E, Y-K Chan, RL Robson (1991) Detection of alternative nitrogenases in aerobic gram-negative nitrogen-fixing bacteria. J Bacteriol 173 365-371. 

T. Thiel (1993) Characterization of genes for an alternative nitrogenase in the cyanobacterium Anabaena variabilis. J. Bacteriol., 175 6276-6286... 

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

At the same time, this redox lability makes Mo well suited as a cofactor in enzymes that catalyze redox reactions. An example is the prominence of Mo in nitrogen fixation. This prokaryotic metabolism, the dominant pathway for conversion of atmospheric Nj to biologically-useful NH, utilizes Mo (along with Fe) in the active site of the nitrogenase enzyme that catalyzes Nj reduction. Alternative nitrogenases that do not incorporate Mo have been identified, but are markedly less efficient (Miller and Eady 1988 Eady 1996). 

Eady RR (1996) Structure-function relationships of alternative nitrogenases. Chem Rev 96 3013-3030 Emerson SR, Huested SS (1991) Ocean anoxia and the concentrations of molybdenum and vanadium in seawater. Mar Chem 34 177-196... 

Kentemich, T., Danneberg, G., Hundeshagen, B. and Bothe, H. (1988) Evidence for the occurrence of the alternative, vanadium-containing nitrogenase in the cyanohacterium Anabaena variabilis. PEMS Microbiol. Lett., 51, 19-24. 

Recently, a second or alternative nitrogenase has been isolated from Azotobacter vinelandii (21) and Azotobacter chroococcum (22) that contains vanadium as opposed to molybdenum. The MoFe and VFe nitrogenase proteins from A. vinelandii (called Av and. 4vl , respectively) are known to have different polypeptide structures and it obviously of interest to know to what extent the cluster composition is conserved. Variable temperature MCD studies of the as isolated and thionine oxidized proteins provided a convenient means of addressing this question. 

Of the alternative substrates of nitrogenase, many contain triple bonds In at least one of their resonance forms. As discussed In the next section, the reactivity of model molecules such as acetylene could give insights into the manner in which such unsaturated molecules bind to transition metal sulfur systems. 

The molybdenum-containing enzymes considered in Section F also contain Fe-S clusters. Nitrogenases (Chapter 24) contain a more complex Fe-S-Mo cluster. Carbon monoxide dehydrogenase (Section C) contains 2 Ni, 11 Fe, and 14 S2 as well as Zn in a dimeric structure. In these enzymes the Fe-S clusters appear to participate in catalysis by undergoing alternate reduction and oxidation. 

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