Lorence R M, Carter K, Green G N, Gennis R B
J Biol Chem. 1987 Aug 5;262(22):10532-6.
The aerobic respiratory chain of Escherichia coli contains two terminal oxidases, the cytochrome o complex and the cytochrome d complex. These both function as ubiquinol-8 oxidases and reduce molecular oxygen to water. Electron flux is funneled from a variety of dehydrogenases, such as succinate dehydrogenase, through ubiquinone-8, to either of the terminal oxidases. A strain was examined which lacks the intact cytochrome d complex, but which overproduces one of the two subunits of this complex, cytochrome b558. This cytochrome, in the absence of the other subunit of the oxidase complex, does not possess catalytic activity. It is shown that the extent of reduction of cytochrome b558 in the E. coli membrane monitors the extent of reduction of the quinone pool in the membrane. The activity of each purified oxidase was examined in phospholipid vesicles as a function of the amount of ubiquinone-8 incorporated in the bilayer. A ratio of ubiquinol-8:phospholipid as low as 1:200 is sufficient to saturate each oxidase. The maximal turnover of the oxidases in the reconstituted system is considerably faster than observed in E. coli membranes, demonstrating that the rate-limiting step in the E. coli respiratory chain is at the dehydrogenases which feed electrons into the system.
大肠杆菌的好氧呼吸链包含两种末端氧化酶，即细胞色素o复合体和细胞色素d复合体。它们均作为泛醇-8氧化酶发挥作用，将分子氧还原为水。电子流从多种脱氢酶，如琥珀酸脱氢酶，通过泛醌-8，汇集到两种末端氧化酶中的任一种。研究了一种菌株，该菌株缺乏完整的细胞色素d复合体，但过量产生该复合体的两个亚基之一，即细胞色素b558。在缺乏氧化酶复合体另一个亚基的情况下，这种细胞色素不具有催化活性。结果表明，大肠杆菌膜中细胞色素b558的还原程度可监测膜中醌池的还原程度。在磷脂囊泡中检测了每种纯化氧化酶的活性，作为双层中泛醌-8掺入量的函数。泛醇-8与磷脂的比例低至1:200就足以使每种氧化酶饱和。重组系统中氧化酶的最大周转速度比在大肠杆菌膜中观察到的要快得多，这表明大肠杆菌呼吸链中的限速步骤在于将电子输入系统的脱氢酶。
