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外文期刊>The biochemical journal
>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
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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
pThe 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./p
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机译:通过在23摄氏度,pH 7.4下的定流分光光度法研究了O2和H2O2对肺炎克雷伯菌(Kp2)和蓝细菌(Ac2)的固氮酶Fe蛋白氧化的动力学。随着过量的O2发生,Kp2和Ac2及其2 MgATP或2 MgADP结合形式的单电子氧化发生,速率常数(k)在5.3 x 10(3)M-1.S-1至1.6 x 10(5)范围内)M-1.S-1。 log k和这些蛋白质物种的中点电位(Em)之间的线性相关性表明,从Ac2物种转移电子的速率较高是由于4Fe-4S簇的Em差异所致。 Ac2(MgADP)2与O2的反应足够快,足以在固氮条件下显着提高固氮菌的高呼吸速率,并且可能代表了新的呼吸途径。过量的O2会迅速使Ac2(MgADP)2和Kp2(MgADP)2失活。但是,当这些蛋白质种类超过O2浓度的4倍摩尔过量时,4当量的蛋白质被氧化而没有活性损失。该反应的动力学表明,H2O2是固氮酶Fe蛋白将O2还原为2 H2O的中间体,暗示过氧化氢酶或过氧化物酶在保护固氮酶免受O2诱导的失活的机制中起作用。 p>
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