title: Oxidation of nitrogenase iron protein by dioxygen without inactivation could contribute to high respiration rates of Azotobacter species and facilitate nitrogen fixation in other aerobic environments.
abstract: The kinetics of oxidation of the Fe proteins of nitrogenases from Klebsiella pneumoniae (Kp2) and Azotobacter chroococcum (Ac2) by O2 and H2O2 have been studied by stopped-flow spectrophotometry at 23 degrees C, pH 7.4. With excess O2, one-electron oxidation of Kp2 and Ac2 and their 2 MgATP or 2 MgADP bound forms occurs with rate constants (k) in the range 5.3 x 10(3) M-1.S-1 to 1.6 x 10(5) M-1.S-1. A linear correlation between log k and the mid-point potentials (Em) of these protein species indicates that the higher rates of electron transfer from the Ac2 species are due to the differences in Em of the 4Fe-4S cluster. The reaction of Ac2(MgADP)2 with O2 is sufficiently rapid for it to contribute significantly to the high respiration rate of Azotobacter under N2-fixing conditions and may represent a new respiratory pathway. Excess O2 rapidly inactivates Ac2(MgADP)2 and Kp2(MgADP)2; however, when these protein species are in greater than 4-fold molar excess over the concentration of O2, 4 equivalents of protein are oxidized with no loss of activity. The kinetics of this reaction suggest that H2O2 is an intermediate in the reduction of O2 to 2 H2O by nitrogenase Fe proteins and imply a role for catalase or peroxidase in the mechanism of protection of nitrogenase from O2-induced inactivation.
authors.affiliation: AFRC Institute of Plant Science Research, University of Sussex, Brighton, U.K.
authors.forename: R N
authors.initials: RN
authors.lastname: Thorneley
authors.forename: G A
authors.initials: GA
authors.lastname: Ashby
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D001395:Azotobacter / Q000201:enzymology; D007700:Kinetics; D007711:Klebsiella pneumoniae / Q000201:enzymology; D009586:Nitrogen Fixation; D009591:Nitrogenase / Q000378:metabolism*; D010084:Oxidation-Reduction; D010088:Oxidoreductases; D010100:Oxygen / Q000378:metabolism*; D010101:Oxygen Consumption
chemical_list: D000255:Adenosine Triphosphate; D010088:Oxidoreductases; D009591:Nitrogenase; C029130:nitrogenase reductase; D010100:Oxygen