title: Oxidative titration of the nitrogenase VFe protein from Azotobacter vinelandii: an example of redox-gated electron flow.
abstract: The nitrogenase VFe protein of Azotobacter vinelandii (Av1') has been shown to exist in two forms called Av1'A, which has a primary alpha beta 2 trimeric structure, and Av1'B, which has an alpha 2 beta 2 tetrameric structure [Blanchard, C. Z., & Hales, B. J. (1996) Biochemistry 35, 472-478]. Both forms exhibit S = 5/2 EPR signals in the as-isolated state that may be assigned to 1-equiv-oxidized P clusters (P+). These signals are abolished by enzymatic reduction with the component 2 protein (Av2'). Stepwise oxidative titrations of enzymatically reduced Av1'B result in the restoration of the S = 5/2 P+ signals and the concurrent decrease of the S = 3/2 vanadium cofactor signal. Further oxidation results in the appearance of an integer spin signal assigned to the 2-equiv-oxidized P cluster (P2+). Unlike the analogous signal previously observed in Mo nitrogenase component 1 (Av1), which arises from an excited state, the integer spin P2+ signal in Av1'B originates from a ground-state doublet. Similar oxidative titrations of enzymatically reduced Av1'A show redox behavior dramatically different from that of Av1'B, as monitored by EPR spectroscopy. We observed spectral evidence for a redox-induced intramolecular electron transfer between the reduced P cluster and the oxidized FeV cofactor cluster during the titrations.
authors.affiliation: Department of Chemistry, Louisiana State University, Baton Rouge 70803-1804, USA.
authors.forename: R C
authors.initials: RC
authors.lastname: Tittsworth
authors.forename: B J
authors.initials: BJ
authors.lastname: Hales
mesh_terms: D016948:Azotobacter vinelandii / Q000201:enzymology*; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D007501:Iron / Q000737:chemistry; D015394:Molecular Structure; D008982:Molybdenum / Q000737:chemistry; D009591:Nitrogenase / Q000737:chemistry* / Q000378:metabolism; D010084:Oxidation-Reduction; D011487:Protein Conformation; D013455:Sulfur / Q000737:chemistry; D014639:Vanadium / Q000737:chemistry
chemical_list: D014639:Vanadium; D013455:Sulfur; D008982:Molybdenum; D007501:Iron; D009591:Nitrogenase