Role of cytochromes and quinones in acetogenic bacteria

细胞色素和醌在产乙酸菌中的作用

• 批准号: 503149329
• 负责人: Professor Dr. Volker Müller
• 金额: --
• 依托单位: Arbeitskreis Molekulare Mikrobiologie und Bioenergetik
• 依托单位国家: 德国
• 项目类别: Reinhart Koselleck Projects
• 资助国家: 德国
• 项目状态: 未结题
• 来源: https://gepris.dfg.de/gepris/projekt/503149329?language=en
• 关键词: Role; cytochromes; quinones; acetogenic; bacteria

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.

产乙酸细菌是一种多系生物，其在缺氧、非光养条件下通过将两摩尔CO2还原为乙酰辅酶A并进一步经由伍德-永达尔途径还原为乙酸来固定二氧化碳。CO2可以用H2作为电子供体还原，允许化能自养生长，并且从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，每摩尔形成的醋酸盐，ATP的增加只是ATP的一小部分，这使得产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，产乙酸菌作为来自温室气体CO2的增值化合物的生产平台正在令人印象深刻地上升，但由于ATP增益低，仅产生低滴度和可生产的有限数量的产品，它们的工业应用仍然受到限制。自从发现这种石腐生活方式以来，在H2 + CO2的产乙酸过程中如何合成ATP一直是一个谜。在过去的十年中，发现了两个铁氧还蛋白依赖性呼吸链。这些呼吸链包括铁氧还蛋白依赖性呼吸酶复合物，其是铁氧还蛋白：NAD氧化还原酶（Rnf）或铁氧还蛋白：H+氧化还原酶（Ech），其产生跨细胞质膜的电化学离子梯度，然后通过膜结合的ATP合酶驱动ATP合成。这些多亚基的膜结合呼吸酶具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化合物早在32年前就已经在产乙酸模式生物热乙酸穆尔氏菌中发现。后来，在其他产乙酸菌中也发现了它们，但它们的作用只是一个影子。然而，人们通常推测，有第三种类型的能量守恒产乙酸菌，除了RNF和Ech，这是基于细胞色素和/或醌依赖的电子传递磷酸化。我们将解决细胞色素/醌类的作用，并假设细胞色素/醌类是被确定的电子传递链（ETC）的一部分。推测这些ETC从电子供体如H2、CO、还原的铁氧还蛋白或NAD（P）H引导至受体如亚甲基-四氢叶酸或替代的电子受体如硝酸盐和硫化合物。这些ETCs将被识别，他们的成分将被描述和节能的机制将被解开。最终的目标是找到产乙酸细菌能量守恒之谜的最后一块，并解开这些细菌中是否存在第三种能量守恒方式。该项目的成果还将有助于了解产乙酸菌的生态适应性，并改善其在可持续的二氧化碳生物技术中的使用。