title: Nitrogenase of Azotobacter chroococcum. Kinetics of the reduction of oxidized iron-protein by sodium dithionite.
abstract: The kinetics of the reduction of oxidized Fe-protein of nitrogenase from Azotobacter chroococcum by sodium dithionite were studied by stopped-flow and rapid-freezing e.p.r. (electron-paramagnetic-resonance) spectroscopy. The appearance of the gav. = 1.94 e.p.r. signal (0.24 electron integrated intensity/mol) was associated with a one-electron reduction by SO2--with k greater than 10(8)M-1-S-1 at 23 degrees C. A value of k = 1.75s-1 was obtained for the rate of dissociation of S2O42- into 2SO2-- at 23 degrees C. Further reductions by SO2-- occurred in three slower phases with rate constants in the range 10(4) -10(6)M-1-S-1. These latter phases have no corresponding e.p.r. signal changes and are probably associated with enzymically inactive protein. The high rate of reduction by SO2-- of the Fe-protein alone (k greater than 10(8)M-1-S-1) relative to the rate of oxidation of the Fe-protein in the catalytically active Fe:Mo-Fe protein complex (k = 2.2 X 1O(2)s-1) and the observation that in the steady state the Fe-protein is substantially oxidized means that at normal assay concentrations another reaction must limit the rate of reduction of Fe-protein during turnover.
mesh_terms: D000255:Adenosine Triphosphate / Q000494:pharmacology; D001395:Azotobacter / Q000201:enzymology*; D001665:Binding Sites; D004227:Dithionite / Q000494:pharmacology*; D004578:Electron Spin Resonance Spectroscopy; D007501:Iron / Q000378:metabolism; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D008667:Metalloproteins / Q000378:metabolism; D009591:Nitrogenase / Q000378:metabolism*; D009991:Oscillometry; D010084:Oxidation-Reduction; D010100:Oxygen / Q000494:pharmacology; D013447:Sulfites / Q000494:pharmacology*
chemical_list: D008667:Metalloproteins; D013447:Sulfites; D004227:Dithionite; D000255:Adenosine Triphosphate; D007501:Iron; D009591:Nitrogenase; D008274:Magnesium; D010100:Oxygen