Role of cytochromes and quinones in acetogenic bacteria

产乙酰细菌是一种多物种生物，在缺氧、非光养条件下，通过Wood-Ljungdahl途径将2 mol CO2还原为乙酰辅酶a并进一步转化为乙酸，从而固定二氧化碳。CO2可以以H2作为电子供体进行还原，从而实现化能自养生长，H2 + CO2的丙酮生成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，ATP的增益仅为每mol乙酸形成的ATP的一小部分，这使得醋酸成为热力学平衡下研究微生物生命基础的首选候选者。此外，作为从温室气体CO2中提取增值化合物的生产平台，合成酶的应用正在显著增加，但由于ATP增益低，只能产生低滴度和有限数量的产品，它们的工业应用仍然受到限制。自从发现这种岩生生物以来，ATP是如何在丙酮生成过程中由H2 + CO2合成的一直是一个谜。在过去的十年中，发现了两种依赖铁氧化还原蛋白的呼吸链。这些呼吸链由铁氧还蛋白依赖的呼吸酶复合物组成，该复合物要么是铁氧还蛋白：NAD氧化还原酶（Rnf），要么是铁氧还蛋白：H+氧化还原酶（Ech），它在细胞质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，膜结合呼吸酶有铁硫中心和黄素作为电子载体，但没有细胞色素。这是令人惊讶的，因为细胞色素和醌类早在32年前就在产丙酮的模式生物摩尔氏菌中被发现了。后来，它们也被发现在其他的酵素中，但它们的作用只是模糊的存在。然而，人们经常推测，除了Rnf和Ech之外，在丙酮中还存在第三种类型的能量守恒，这是基于细胞色素和/或醌依赖的电子传递磷酸化。我们将讨论细胞色素/醌的作用，并假设细胞色素/醌是有待鉴定的电子传递链（ETCs）的一部分。据推测，这些ETCs从H2、CO、还原铁氧还蛋白或NAD(P)H等电子供体引导至亚甲基-四氢叶酸等受体或其他电子受体，如硝酸盐和硫化合物。我们将对这些碳排放体系进行识别，描述它们的成分，并揭示节能机制。最终的目标是找到产丙酮细菌的能量守恒之谜的最后一块，并揭示这些细菌中是否存在第三种能量守恒方式。该项目的成果还将达到了解氧气的生态适应性，并改善其在可持续的、以二氧化碳为基础的生物技术中的使用。

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.