Hoffmann, R. A wiki for the life sciences where authorship matters. Nature Genetics (2008)

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Lowery, T.J. et al.

Nitrogen Fixation Special Feature: Flavodoxin hydroquinone reduces Azotobacter vinelandii Fe protein to the all-ferrous redox state with a S = 0 spin state.

Azotobacter vinelandii flavodoxin hydroquinone (FldHQ) is a physiological reductant to nitrogenase supporting catalysis that is twice as energy efficient (ATP/2e(-) = 2) as dithionite (ATP/2e(-) = 4). This catalytic efficiency results from reduction of Fe protein from A. vinelandii (Av2) to the all-ferrous oxidation state ([Fe(4)S(4)](0)), in contrast to dithionite, which only reduces Av2 to the [Fe(4)S(4)](1+) state. Like FldHQ, Ti(III) citrate yields ATP/2e(-) = 2, and Ti(III)-reduced [Fe(4)S(4)](0) Av2 has a S = 4 spin state and characteristic Mossbauer spectrum, a parallel mode g = 16.4 EPR signal, and a shoulder at 520 nm in its UV-vis spectrum, each of which distinguish the S = 4 [Fe(4)S(4)](0) Av2 from other states. In this study, we demonstrate that FldHQ makes [Fe(4)S(4)](0) Av2, which is sufficiently characterized to demonstrate unique physical properties that distinguish it from the previously characterized Ti(III)-reduced [Fe(4)S(4)](0) Av2. In particular, Evans NMR magnetic susceptibility and EPR measurements indicate that FldHQ-reduced [Fe(4)S(4)](0) Av2 has an S = 0 spin state (like [Fe(4)S(4)](2+) Av2). There is no g = 16.4 EPR signal and no shoulder at 520 nm in its absorbance spectrum, which resembles that of [Fe(4)S(4)](1+) Av2. That the physiological reductant to Av2 is capable of forming [Fe(4)S(4)](0) Av2 has important implications for in vivo nitrogenase activity.[1]

References

Nitrogen Fixation Special Feature: Flavodoxin hydroquinone reduces Azotobacter vinelandii Fe protein to the all-ferrous redox state with a S = 0 spin state. Lowery, T.J., Wilson, P.E., Zhang, B., Bunker, J., Harrison, R.G., Nyborg, A.C., Thiriot, D., Watt, G.D. Proc. Natl. Acad. Sci. U.S.A. (2006) [Pubmed]

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