Institute for Molecular Microbiology and Biotechnology, University of Münster, Münster, Germany.
Bavarian NMR Center-Structural Membrane Biochemistry, Department of Chemistry, Technische Universität München, Garching, Germany.
Nature. 2021 Apr;592(7856):784-788. doi: 10.1038/s41586-021-03456-9. Epub 2021 Apr 21.
It has recently been shown that in anaerobic microorganisms the tricarboxylic acid (TCA) cycle, including the seemingly irreversible citrate synthase reaction, can be reversed and used for autotrophic fixation of carbon. This reversed oxidative TCA cycle requires ferredoxin-dependent 2-oxoglutarate synthase instead of the NAD-dependent dehydrogenase as well as extremely high levels of citrate synthase (more than 7% of the proteins in the cell). In this pathway, citrate synthase replaces ATP-citrate lyase of the reductive TCA cycle, which leads to the spending of one ATP-equivalent less per one turn of the cycle. Here we show, using the thermophilic sulfur-reducing deltaproteobacterium Hippea maritima, that this route is driven by high partial pressures of CO. These high partial pressures are especially important for the removal of the product acetyl coenzyme A (acetyl-CoA) through reductive carboxylation to pyruvate, which is catalysed by pyruvate synthase. The reversed oxidative TCA cycle may have been functioning in autotrophic CO fixation in a primordial atmosphere that is assumed to have been rich in CO.
最近的研究表明，在厌氧微生物中，三羧酸 (TCA) 循环，包括看似不可逆的柠檬酸合酶反应，可以被逆转并用于自养固定碳。这个反向氧化 TCA 循环需要依赖于铁氧还蛋白的 2-酮戊二酸合酶而不是 NAD 依赖性脱氢酶，以及极高水平的柠檬酸合酶（超过细胞中蛋白质的 7%）。在这个途径中，柠檬酸合酶取代了还原 TCA 循环中的 ATP-柠檬酸裂解酶，这导致每一轮循环消耗的 ATP 等价物减少了一个。在这里，我们使用嗜热硫还原 δ 变形菌 Hippea maritima 表明，这条途径是由高二氧化碳分压驱动的。这些高分压对于通过还原羧化作用将产物乙酰辅酶 A（乙酰辅酶 A）去除到丙酮酸特别重要，丙酮酸合酶催化了这个反应。反向氧化 TCA 循环可能在原始大气中通过自养 CO 固定起作用，原始大气被认为富含 CO。
