Role of cytochromes and quinones in acetogenic bacteria

产乙酸菌是在缺氧、非光营养的条件下，通过将两个摩尔的二氧化碳还原为乙酰辅酶A，进而通过Wood-Ljugdahl途径进一步转化为乙酸酯，从而固定二氧化碳的多种多样的生物群。作为电子供体的H2可以还原CO2，从而允许化学自养生长，而H2+CO2的乙酸化被认为是地球上最古老的代谢途径之一，因为它将CO2还原与ATP的净合成结合在一起。然而，每形成一摩尔醋酸酯，三磷酸腺苷的增量只是一小部分，这使得乙酸菌成为研究热力学平衡下微生物生命基础的主要候选者。此外，作为来自温室气体二氧化碳的增值化合物的生产平台，乙酸根的增长令人印象深刻，但由于ATP增益较低，只能产生低滴度和有限数量的产品，因此它们的工业应用仍然受到限制。自从这种营养石质的生活方式被发现以来，在乙酸化过程中如何从H2+CO2合成ATP一直是一个谜。在过去的十年里，人们发现了两条铁氧还蛋白依赖的呼吸链。这些呼吸链由铁氧还蛋白依赖的呼吸酶复合体组成，它是铁氧还蛋白：NAD氧化还原酶(RNF)或铁氧还蛋白：H+氧化还原酶(Ech)，它在细胞膜上产生电化学离子梯度，然后通过膜结合的ATP合成酶驱动ATP合成。这些多亚基、膜结合的呼吸酶以铁硫中心和黄素为电子载体，但没有细胞色素。这是令人惊讶的，因为在32年前就已经在产乙酸型模式生物热醋酸莫雷拉中发现了细胞色素46和苯二酚。后来，它们也在其他产乙酸剂中被发现，但它们的作用只是一个模糊的存在。然而，人们经常推测，除了RNF和Ech之外，在产乙酸菌中还有第三种类型的能量守恒，即基于细胞色素和/或苯二酚依赖的电子传递磷酸化。我们将讨论细胞色素/对苯二酚的作用，并假设细胞色素/对苯二酚是待识别的电子传输链(ETC)的一部分。这些ETC被推测为从电子供体如H2、CO、还原铁还蛋白或NAD(P)H到受体如亚甲基四氢叶酸或替代电子受体如硝酸盐和硫化合物。这些ETCs将被识别，它们的组成将被描述，节能的机制将被揭示。最终目标是找到产乙酸菌中能量节约的最后一块拼图，并揭开这些细菌中是否存在第三种节能方法。该项目的成果还将影响到了解乙酸菌的生态适宜性，并改进其在可持续的二氧化碳生物技术中的使用。

Acetogenic bacteria are a polyphyletic group of organisms that fix carbon dioxide under anoxic, non-phototrophic conditions by reduction of two mol of CO2 to acetyl-CoA and further to acetate via the Wood-Ljungdahl pathway. CO2 can be reduced with H2 as electron donor allowing for chemolithoautotrophic growth and acetogenesis from H2 + CO2 is considered to be one of the oldest metabolic pathways on Earth, since it couples CO2 reduction to the net synthesis of ATP. However, the ATP gain is only a fraction of an ATP per mol of acetate formed, making acetogens prime candidates to study the basis of microbial life at thermodynamic equilibrium. Moreover, acetogens are on an impressive rise as production plattforms for value-added compounds from the greenhouse gas CO2, but their industrial application is still limited due to the low ATP gain which yields only low titers and a limited number of products that can be produced. How ATP is synthesized during acetogenesis from H2 + CO2 has been an enigma since the discovery of this lithothrophic life style. In the last decade two ferredoxin-dependent respiratory chains were discovered. Those respiratory chains comprise of a ferredoxin-dependent respiratory enzyme complex, which is either a ferredoxin:NAD oxidoreductase (Rnf) or a ferredoxin:H+ oxidoreductase (Ech) that generate an electrochemical ion gradient across the cytoplasmic membrane that then drives ATP synthesis via a membrane-bound ATP synthase. These multisubunit, membrane-bound respiratory enzymes have iron-sulfur centers and flavins as electron carriers, but no cytochromes. This is astonishing since cytochromes were already discovered 46 and quinones 32 years ago in the acetogenic model organism Moorella thermoacetica. Later on, they were also found in other acetogens but their role had only a shadowy existence. Nevertheless, it is often speculated that there is a third type of energy conservation in acetogens, besides Rnf and Ech, that is based on cytochrome- and/or quinone-dependent electron transport phosphorylation. We will address the role of cytochromes/quinones and hypothesize that cytochromes-/quinones are part of to be identified electron transport chains (ETCs). These ETCs are speculated to lead from electron donors such as H2, CO, reduced ferredoxin or NAD(P)H to acceptors such as methylene-tetrahydrofolate or alternative electron acceptors such as, for example, nitrate and sulfur compounds. These ETCs will be identified, their constituents will be described and the mechanism of energy conservation will be unravelled. The final goal is to find the last piece of the puzzle of energy conservation in acetogenic bacteria and unravel whether there is a third way of energy conservation in these bacteria. The outcome of the project will also reach as far as to understand the ecological fitness of acetogens and to improve their use in a sustainable, carbon dioxide-based biotechnology.