Department of Molecular Microbiology and Bioenergetics, Institute of Molecular Biosciences, Johann Wolfgang Goethe University, Frankfurt, Germany.
Microbiol Spectr. 2022 Aug 31;10(4):e0138522. doi: 10.1128/spectrum.01385-22. Epub 2022 Jun 14.
Acetogenic bacteria are a group of strictly anaerobic bacteria that make a living from acetate formation from two molecules of CO via the Wood-Ljungdahl pathway (WLP). The free energy change of this reaction is very small and allows the synthesis of only a fraction of an ATP. How this pathway is coupled to energy conservation has been an enigma since its discovery ~90 years ago. Here, we describe an electron transport chain in the cytochrome- and quinone-containing acetogen Sporomusa ovata that leads from molecular hydrogen as an electron donor to an intermediate of the WLP, methylenetetrahydrofolate (methylene-tetrahydrofolate [THF]), as an electron acceptor. The catalytic site of the hydrogenase is periplasmic and likely linked cytochrome to the membrane. We provide evidence that the MetVF-type methylenetetrahydrofolate reductase is linked proteins MvhD and HdrCBA to the cytoplasmic membrane. Membrane preparations catalyzed the H-dependent reduction of methylene-THF to methyl-THF. In our model, a transmembrane electrochemical H gradient is established by both scalar and vectorial protons that leads to the synthesis of 0.5 mol ATP/mol methylene-THF by a H-FF ATP synthase. This H- and methylene-THF-dependent electron transport chain may be present in other cytochrome-containing acetogens as well and represents a third way of chemiosmotic energy conservation in acetogens, but only in addition to the well-established respiratory enzymes Rnf and Ech. Acetogenic bacteria grow by making acetate from CO and are considered the first life forms on Earth since they couple CO reduction to the conservation of energy. How this is achieved has been an enigma ever since. Recently, two respiratory enzymes, a ferredoxin:NAD oxidoreductase (Rnf) and a ferredoxin:H oxidoreductase (Ech), have been found in cytochrome-free acetogenic model bacteria. However, some acetogens contain cytochromes in addition, and there has been a long-standing assumption of a cytochrome-containing electron transport chain in those acetogens. Here, we provide evidence for a respiratory chain in Sporomusa ovata that has a cytochrome-containing hydrogenase as the electron donor and a methylenetetrahydrofolate reductase as the terminal electron acceptor. This is the third way of chemiosmotic energy conservation found in acetogens.
产乙酸菌是一组严格厌氧的细菌，通过 Wood-Ljungdahl 途径 (WLP) 将两个分子的 CO 转化为乙酸来生存。该反应的自由能变化非常小，只能合成少量的 ATP。自 90 年前发现以来，该途径如何与能量守恒相关一直是个谜。在这里，我们描述了在含有细胞色素和醌的产乙酸菌 Sporomusa ovata 中的电子传递链，该传递链从分子氢作为电子供体到 WLP 的中间产物，即亚甲基四氢叶酸（亚甲基-四氢叶酸[THF]）作为电子受体。氢化酶的催化位点位于周质，可能将细胞色素与膜连接起来。我们提供的证据表明，MetVF 型亚甲基四氢叶酸还原酶与膜相连的蛋白 MvhD 和 HdrCBA 相连。膜制剂催化亚甲基-THF 依赖于 H 的还原为甲基-THF。在我们的模型中，通过标量和矢量质子建立跨膜电化学 H 梯度，导致通过 H-FF ATP 合酶合成 0.5 mol ATP/mol 亚甲基-THF。这种依赖于 H 和亚甲基-THF 的电子传递链可能也存在于其他含有细胞色素的产乙酸菌中，并且代表了产乙酸菌中第三种化学渗透能量守恒的方式，但仅作为已建立的呼吸酶 Rnf 和 Ech 的补充。产乙酸菌通过将 CO 转化为乙酸而生长，被认为是地球上最早的生命形式，因为它们将 CO 还原与能量的保存联系起来。自那以后，这是如何实现的一直是个谜。最近，在不含细胞色素的产乙酸模式细菌中发现了两种呼吸酶，即铁氧还蛋白：NAD 氧化还原酶 (Rnf) 和铁氧还蛋白：H 氧化还原酶 (Ech)。然而，一些产乙酸菌除了细胞色素之外还含有细胞色素，并且长期以来一直假设这些产乙酸菌中存在含有细胞色素的电子传递链。在这里，我们提供了 Sporomusa ovata 中呼吸链的证据，该呼吸链的电子供体是含有细胞色素的氢化酶，末端电子受体是亚甲基四氢叶酸还原酶。这是在产乙酸菌中发现的第三种化学渗透能量守恒方式。
