Anderson W M, Trgovcich-Zacok D
Indiana University School of Medicine, Northwest Center for Medical Education, Gary 46408, USA.
Biochim Biophys Acta. 1995 Jun 30;1230(3):186-93. doi: 10.1016/0005-2728(95)94411-q.
The mammalian mitochondrial electron transport chain catalyzes the oxidation of NADH at pH 8.0 and pH 6.5, and the oxidation of NADPH at pH 6.5. The pH-dependencies of the rate of steady-state oxidation of NADPH and NADH by Complex I as well as by its flavoprotein fraction have been extensively studied by the laboratory of Hatefi. One model to explain these pH-dependent oxidations was proposed by Bakker and Albracht (Biochim. Biophys. Acta 850 (1986) 413-422 and 423-428, modified by Van Belzen and Albracht (Biochim. Biophys Acta 974 (1989) 311-320), which predicts that Complex I is a heterodimer with promoter B, containing FMN and Fe-S clusters 1-4 in stiochiometric amounts, catalyzing NADH oxidation at pH 8, and Protomer A, containing FMN and Fe-S clusters 2, 4, catalyzing NAD(P)H oxidation at pH 6.5. A pH-dependent transfer of electrons from protomer A Fe-S clusters 2, 4 to protomer B Fe-S clusters 2, 4 is an obligate step in the oxidation of NAD(P)H at low pH. Strict interpretation of this model allows for only three types of inhibitor: one which inhibits all three oxidase activities (type 1); one which inhibits NADH oxidase, pH 8.0 (type 4) and a third which inhibits NAD(P)H oxidase, pH 6.5 (type 5). Another possibility is that there are three separate pathways of oxidation of NAD(P)H, which would allow for a total of seven different types of inhibitor, e.g., the three types above plus type 2 inhibiting NADH oxidase pH 8.0 and pH 6.5; type 3 inhibiting NADH oxidase pH 8.0, and NADPH oxidase pH 6.5; type 6 inhibiting NADH oxidase pH 6.5; and type 7 inhibiting NADPH oxidase pH 6.5. Using a series of thirteen inhibitors of Complex I activity and the chemical modification reagent ethoxyformic anhydride (EFA), four different inhibitor types were found: seven inhibitors of type 1, four inhibitors of type 2, one inhibitor of type 3 and one inhibitor of type 4. Treatment of submitochondrial particles (SMP) with EFA abolished NADH-dependent reduction of coenzyme Q at both pH 8.0 and 6.5, while inhibiting NADPH-dependent reduction of coenzyme Q at pH 6.5 by only 30%. These results do not support the heterodimer model of Complex I electron transport of Bakker and Albracht, but do support three separate electron flow pathways through complex 1 from reduced pyridine nucleotides to coenzyme Q. A new model of electron flow through Complex I based on these finding is proposed.
哺乳动物线粒体电子传递链在pH 8.0和pH 6.5时催化NADH的氧化，在pH 6.5时催化NADPH的氧化。哈泰菲实验室对复合物I及其黄素蛋白组分催化NADPH和NADH稳态氧化速率的pH依赖性进行了广泛研究。巴克尔和阿尔布拉赫特提出了一个解释这些pH依赖性氧化的模型（《生物化学与生物物理学报》850 (1986) 413 - 422和423 - 428，范·贝尔岑和阿尔布拉赫特修改，《生物化学与生物物理学报》974 (1989) 311 - 320），该模型预测复合物I是一种异二聚体，其中启动子B含有化学计量的FMN和铁硫簇1 - 4，在pH 8时催化NADH氧化，原聚体A含有FMN和铁硫簇2、4，在pH 6.5时催化NAD(P)H氧化。在低pH下，电子从原聚体A的铁硫簇2、4向原聚体B的铁硫簇2、4的pH依赖性转移是NAD(P)H氧化的一个必要步骤。对该模型的严格解释只允许三种类型的抑制剂：一种抑制所有三种氧化酶活性（1型）；一种抑制pH 8.0的NADH氧化酶（4型），第三种抑制pH 6.5的NAD(P)H氧化酶（5型）。另一种可能性是存在三种独立的NAD(P)H氧化途径，这将允许总共七种不同类型的抑制剂，例如上述三种类型加上2型抑制pH 8.0和pH 6.5的NADH氧化酶；3型抑制pH 8.0的NADH氧化酶和pH 6.5的NADPH氧化酶；6型抑制pH 6.5的NADH氧化酶；7型抑制pH 6.5的NADPH氧化酶。使用一系列13种复合物I活性抑制剂和化学修饰试剂乙氧基甲酸酐（EFA），发现了四种不同类型的抑制剂：7种1型抑制剂、4种2型抑制剂、1种3型抑制剂和1种4型抑制剂。用EFA处理亚线粒体颗粒（SMP）消除了在pH 8.0和6.5时NADH依赖的辅酶Q还原，而在pH 6.5时仅抑制NADPH依赖的辅酶Q还原30%。这些结果不支持巴克尔和阿尔布拉赫特提出的复合物I电子传递的异二聚体模型，但支持从还原的吡啶核苷酸到辅酶Q通过复合物I的三种独立电子流途径。基于这些发现提出了一种通过复合物I的新电子流模型。
