title: The vanadium- and molybdenum-containing nitrogenases of Azotobacter chroococcum. Comparison of mid-point potentials and kinetics of reduction by sodium dithionite of the iron proteins with bound magnesium adenosine 5'-diphosphate.
abstract: The mid-point potentials of the Fe protein components (Ac2 and Ac2* respectively) of the Mo nitrogenase and V nitrogenase from Azotobacter chroococcum were determined in the presence of MgADP to be -450 mV (NHE) [Ac2(MgADP)2-Ac2*ox.(MgADP)2 couple] and -463 mV (NHE) [Ac2* (MgADP)2-Ac2*ox.(ADP)2 couple] at 23 degrees C at pH 7.2. These values are consistent with a flavodoxin characterized by Deistung & Thorneley [(1986) Biochem. J. 239, 69-75] with Em = -522 mV (NHE) being an effective electron donor to both the Mo nitrogenase and the V nitrogenase in vivo. Ac2*ox.(MgADP)2 and Ac2*ox.(MgADP)2 were reduced by SO2.- (formed by the predissociation of dithionite ion, S2O4(2-)) at similar rates, k = 4.7 X 10(6) +/- 0.5 X 10(6) M-1.s-1 and 3.2 X 10(6) +/- 0.2 X 10(6) M-1.s-1 respectively, indicating structural homology at the electron-transfer site associated with the [4Fe-4S] centre in these proteins.
authors.affiliation: Department of Biochemistry and Biophysics, University of Göteborg, Sweden.
authors.forename: J
authors.initials: J
authors.lastname: Bergström
authors.forename: R R
authors.initials: RR
authors.lastname: Eady
authors.forename: R N
authors.initials: RN
authors.lastname: Thorneley
mesh_terms: D000244:Adenosine Diphosphate / Q000378:metabolism; D001395:Azotobacter / Q000201:enzymology*; D004227:Dithionite / Q000494:pharmacology; D007527:Isoenzymes / Q000378:metabolism*; D007700:Kinetics; D008274:Magnesium / Q000378:metabolism; D008667:Metalloproteins / Q000378:metabolism; D008982:Molybdenum; D009591:Nitrogenase / Q000378:metabolism*; D009994:Osmolar Concentration; D010084:Oxidation-Reduction; D014639:Vanadium
chemical_list: D007527:Isoenzymes; D008667:Metalloproteins; D014639:Vanadium; D004227:Dithionite; D000244:Adenosine Diphosphate; D008982:Molybdenum; D009591:Nitrogenase; D008274:Magnesium