title: Flavodoxin hydroquinone reduces Azotobacter vinelandii Fe protein to the all-ferrous redox state with a S = 0 spin state.
abstract: 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 ([Fe4S4]0), in contrast to dithionite, which only reduces Av2 to the [Fe4S4]1+ state. Like FldHQ, Ti(III) citrate yields ATP/2e- = 2, and Ti(III)-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 FldHQ 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 for in vivo nitrogenase activity.
authors.affiliation: Undergraduate Research Program and Department of Chemistry and Biochemistry, Brigham Young University, Provo, UT 84602, USA.
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: Lowery
authors.forename: Phillip E
authors.initials: PE
authors.lastname: Wilson
authors.forename: Bo
authors.initials: B
authors.lastname: Zhang
authors.forename: Jared
authors.initials: J
authors.lastname: Bunker
authors.forename: Roger G
authors.initials: RG
authors.lastname: Harrison
authors.forename: Andrew C
authors.initials: AC
authors.lastname: Nyborg
authors.forename: David
authors.initials: D
authors.lastname: Thiriot
authors.forename: Gerald D
authors.initials: GD
authors.lastname: Watt
mesh_terms: D016948:Azotobacter vinelandii / Q000201:enzymology*; D005418:Flavodoxin / Q000378:metabolism*; D006873:Hydroquinones / Q000737:chemistry / Q000378:metabolism*; D007501:Iron / Q000737:chemistry* / Q000378:metabolism; D009682:Magnetic Resonance Spectroscopy; D009711:Nucleotides / Q000737:chemistry / Q000378:metabolism; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000378:metabolism*; D010100:Oxygen / Q000737:chemistry*; D013455:Sulfur / Q000737:chemistry*
chemical_list: D005418:Flavodoxin; D006873:Hydroquinones; D009711:Nucleotides; D013455:Sulfur; D007501:Iron; D010088:Oxidoreductases; C029130:nitrogenase reductase; D010100:Oxygen; C031927:hydroquinone