小柯机器人

德国明斯特大学Ivan A. Berg、慕尼黑工业大学Wolfgang Eisenreich等研究人员合作发现，高水平的二氧化碳促使TCA循环退回为自养。2021年4月21日，国际知名学术期刊《自然》在线发表了这一成果。

据研究人员介绍，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）可以逆转并用于自养固定碳。这种反向的氧化TCA循环需要铁氧还蛋白依赖性的2-酮戊二酸合酶而不是NAD依赖性的脱氢酶以及极高水平的柠檬酸合酶（细胞中蛋白质的7％以上）。在此途径中，柠檬酸合酶替代了还原性TCA循环的ATP柠檬酸裂合酶，这导致该循环每转一圈消耗的ATP等价物减少一个。

研究人员使用嗜热性硫还原性的细菌Hippea maritima发现，这条路线是由较高的CO₂分压驱动的。这些高的分压对于通过还原羧化为丙酮酸去除产物乙酰辅酶A（乙酰辅酶A）尤为重要，这个过程由丙酮酸合酶催化。逆向氧化的TCA循环可能已经在富含CO₂的原始大气中起到了自养性CO₂固定的作用。

附：英文原文

Title: High CO 2 levels drive the TCA cycle backwards towards autotrophy

Author: Lydia Steffens, Eugenio Pettinato, Thomas M. Steiner, Achim Mall, Simone Knig, Wolfgang Eisenreich, Ivan A. Berg

Issue&Volume: 2021-04-21

Abstract: 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 carbon1,2. 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 CO2. 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 CO2 fixation in a primordial atmosphere that is assumed to have been rich in CO2. In the deltaproteobacterium Hippea maritima, the tricarboxylic acid (TCA) cycle can be reversed by high partial pressures of CO2 for the autotrophic fixation of carbon.

DOI: 10.1038/s41586-021-03456-9

Source: https://www.nature.com/articles/s41586-021-03456-9#Sec22

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