Proton pumping NADH:ubiquinone oxidoreductase (complex I) is the most complicated and least understood enzyme of the respiratory chain. All redox prosthetic groups reside in the peripheral arm of the L-shaped structure. The NADH oxidation domain harbouring the FMN cofactor is connected via a chain of iron-sulfur clusters to the ubiquinone reduction site that is located in a large pocket formed by the PSST- and 49-kDa subunits of complex I. An access path for ubiquinone and different partially overlapping inhibitor binding regions were defined within this pocket by site directed mutagenesis. A combination of biochemical and single particle analysis studies suggests that the ubiquinone reduction site is located well above the membrane domain. Therefore, direct coupling mechanisms seem unlikely and the redox energy must be converted into a conformational change that drives proton pumping across the membrane arm. It is not known which of the subunits and how many are involved in proton translocation. Complex I is a major source of reactive oxygen species (ROS) that are predominantly formed by electron transfer from FMNH(2). Mitochondrial complex I can cycle between active and deactive forms that can be distinguished by the reactivity towards divalent cations and thiol-reactive agents. The physiological role of this phenomenon is yet unclear but it could contribute to the regulation of complex I activity in-vivo.

中文翻译：

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阐明质子泵 NADH 的结构和机制的挑战：泛醌氧化还原酶（复合物 I）。

质子泵 NADH：泛醌氧化还原酶（复合物 I）是呼吸链中最复杂和最不了解的酶。所有氧化还原基团都位于 L 形结构的外围臂中。含有 FMN 辅因子的 NADH 氧化结构域通过铁硫簇链连接到泛醌还原位点，该位点位于由复合物 I 的 PSST 和 49-kDa 亚基形成的大口袋中。泛醌和通过定点诱变在这个口袋内定义了不同的部分重叠的抑制剂结合区域。生化和单粒子分析研究的结合表明泛醌还原位点位于膜域上方。所以，直接耦合机制似乎不太可能，氧化还原能量必须转化为构象变化，驱动质子泵送穿过膜臂。目前尚不清楚哪些亚基以及有多少参与质子易位。复合物 I 是主要由 FMNH(2) 的电子转移形成的活性氧 (ROS) 的主要来源。线粒体复合物 I 可以在活性和非活性形式之间循环，这可以通过对二价阳离子和硫醇反应剂的反应性来区分。这种现象的生理作用尚不清楚，但它可能有助于调节体内复合物 I 的活性。复合物 I 是主要由 FMNH(2) 的电子转移形成的活性氧 (ROS) 的主要来源。线粒体复合物 I 可以在活性和非活性形式之间循环，这可以通过对二价阳离子和硫醇反应剂的反应性来区分。这种现象的生理作用尚不清楚，但它可能有助于调节体内复合物 I 的活性。复合物 I 是主要由 FMNH(2) 的电子转移形成的活性氧 (ROS) 的主要来源。线粒体复合物 I 可以在活性和非活性形式之间循环，这可以通过对二价阳离子和硫醇反应剂的反应性来区分。这种现象的生理作用尚不清楚，但它可能有助于调节体内复合物 I 的活性。