Faculty of Biology-Microbiology, Albert-Ludwigs-Universität Freiburg, Freiburg im Breisgau, Germany.
Spemann Graduate School of Biology and Medicine (SGBM), University of Freiburggrid.5963.9, Freiburg, Germany.
mBio. 2021 Feb 22;13(1):e0374021. doi: 10.1128/mbio.03740-21. Epub 2022 Feb 1.
Syntrophic bacteria play a key role in the anaerobic conversion of biological matter to methane. They convert short-chain fatty acids or alcohols to H, formate, and acetate that serve as substrates for methanogenic archaea. Many syntrophic bacteria can also grow with unsaturated fatty acids such as crotonate without a syntrophic partner, and the reducing equivalents derived from the oxidation of one crotonate to two acetate are regenerated by the reduction of a second crotonate. However, it has remained unresolved how the oxidative and reductive catabolic branches are interconnected and how energy may be conserved in the reductive branch. Here, we provide evidence that during axenic growth of the syntrophic model organism Syntrophus aciditrophicus with crotonate, the NAD-dependent oxidation of 3-hydroxybutyryl-CoA to acetoacetyl-CoA is coupled to the reduction of crotonyl-CoA via formate cycling. In this process, the intracellular formate generated by a NAD-regenerating CO reductase is taken up by a periplasmic, membrane-bound formate dehydrogenase that in concert with a membrane-bound electron-transferring flavoprotein (ETF):methylmenaquinone oxidoreductase, ETF, and an acyl-CoA dehydrogenase reduces intracellular enoyl-CoA to acyl-CoA. This novel type of energy metabolism, referred to as enoyl-CoA respiration, generates a proton motive force via a methylmenaquinone-dependent redox-loop. As a result, the beneficial syntrophic cooperation of fermenting bacteria and methanogenic archaea during growth with saturated fatty acids appears to turn into a competition for formate and/or H during growth with unsaturated fatty acids. The syntrophic interaction of fermenting bacteria and methanogenic archaea is important for the global carbon cycle. As an example, it accomplishes the conversion of biomass-derived saturated fatty acid fermentation intermediates into methane. In contrast, unsaturated fatty acid intermediates such as crotonate may serve as growth substrate for the fermenting partner alone. Thereby, the reducing equivalents generated during the oxidation of one crotonate to two acetate are regenerated by reduction of a second crotonate to butyrate. Here, we show that the oxidative and reductive branches of this pathway are connected via formate cycling involving an energy-conserving redox-loop. We refer to this previously unknown type of energy metabolism as to enoyl-CoA respiration with acyl-CoA dehydrogenases serving as cytoplasmic terminal reductases.
互营共生细菌在生物物质厌氧转化为甲烷的过程中发挥着关键作用。它们将短链脂肪酸或醇转化为 H、甲酸盐和乙酸盐，这些物质作为产甲烷古菌的底物。许多互营共生细菌可以在没有共生伙伴的情况下，利用不饱和脂肪酸（如巴豆酸盐）生长，并且由巴豆酸盐氧化为两个乙酸盐所产生的还原当量可以通过第二个巴豆酸盐的还原而再生。然而，氧化和还原分解代谢分支是如何相互连接的，以及能量如何在还原分支中得到保守，这些问题仍未得到解决。在这里，我们提供的证据表明，在专性共生模型生物 S. aciditrophicus 利用巴豆酸盐进行的无菌生长过程中，NAD 依赖性的 3-羟丁酰辅酶 A 到乙酰乙酰辅酶 A 的氧化与通过甲酸盐循环的巴豆酰辅酶 A 的还原偶联。在这个过程中，由 NAD 再生 CO 还原酶产生的细胞内甲酸盐被摄取到周质结合的膜结合甲酸盐脱氢酶中，该酶与膜结合的电子转移黄素蛋白（ETF）、甲酰辅酶 A 脱氢酶一起，将细胞内的烯酰辅酶 A 还原为酰基辅酶 A。这种新型的能量代谢，称为烯酰辅酶 A 呼吸，通过依赖于甲基甲烯醌的氧化还原环产生质子动力。因此，在利用饱和脂肪酸生长时，发酵细菌和产甲烷古菌的有益共生合作似乎在利用不饱和脂肪酸生长时转变为对甲酸盐和/或 H 的竞争。发酵细菌和产甲烷古菌的互营共生相互作用对全球碳循环很重要。例如，它完成了生物质衍生的饱和脂肪酸发酵中间产物转化为甲烷。相比之下，不饱和脂肪酸中间体，如巴豆酸盐，可能仅作为发酵伙伴的生长基质。由此，在一个巴豆酸盐氧化为两个乙酸盐的过程中产生的还原当量，通过第二个巴豆酸盐还原为丁酸而再生。在这里，我们表明该途径的氧化和还原分支通过涉及能量守恒氧化还原环的甲酸盐循环连接。我们将这种以前未知的能量代谢称为烯酰辅酶 A 呼吸，酰基辅酶 A 脱氢酶作为细胞质末端还原酶。
