Flavodoxin hydroquinone reduces Azotobacter vinelandii Fe protein to the all-ferrous redox state with a S=0 spin state

Azotobacter vinelandii flavodoxin hydroquinone (FIdHQ) 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 ([Fe4S4](0)), in contrast to dithionite, which only reduces Av2. to the [Fe4S4](1+) state. Like FIdHQ, Ti(III) citrate yields ATIP/2e(-) = 2, and Ti(Ill)reduced [Fe4S4](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 [Fe4S4](0) Av2 from other states. In this study, we demonstrate that FIdHQ makes [Fe4S4](0) Av2, which is sufficiently characterized to demonstrate unique physical properties that distinguish it from the previously characterized Ti(III)-reduced [Fe4S4](0) Av2. In particular, Evans NMR magnetic susceptibility and EPR measurements indicate that FldHQ-reduced [Fe4S4](0) Av2 has an S = 0 spin state (like [Fe4S4](2+) Av2). There is no g = 16.4 EPR signal and no shoulder at 520 nm in its absorbance spectrum, which resembles that of [Fe4S4](1+) Av2. That the physiological reductant to Av2 is capable of forming [Fe4S4](0) Av2 has important implications or in vivo nitrogenase activity.