Jain Surbhi, Dietrich Helge M, Müller Volker, Basen Mirko
Department of Molecular Microbiology and Bioenergetics, Institute of Molecular Biosciences, Goethe University Frankfurt, Frankfurt am Main, Germany.
Front Microbiol. 2020 Jan 31;11:59. doi: 10.3389/fmicb.2020.00059. eCollection 2020.
The hydrogen-dependent carbon dioxide reductase is a soluble enzyme complex that directly utilizes hydrogen (H) for the reduction of carbon dioxide (CO) to formate in the first step of the acetyl-coenzyme A- or Wood-Ljungdahl pathway (WLP). HDCR consists of 2 catalytic subunits, a hydrogenase and a formate dehydrogenase (FDH) and two small subunits carrying iron-sulfur clusters. The enzyme complex has been purified and characterized from two acetogenic bacteria, from the mesophile and, recently, from the thermophile . Physiological studies toward the importance of the HDCR for growth and formate metabolism in acetogens have not been carried out yet, due to the lack of genetic tools. Here, we deleted the genes encoding HDCR in taking advantage of the recently developed genetic system. As expected, the deletion mutant (strain TKV_MB013) did not grow with formate as single substrate or under autotrophic conditions with H + CO. Surprisingly, the strain did also not grow on any other substrate (sugars, mannitol or pyruvate), except for when formate was added. Concentrated cell suspensions quickly consumed formate in the presence of glucose only. In conclusion, HDCR provides formate which was essential for growth of the mutant. Alternatively, extracellularly added formate served as terminal electron acceptor in addition to CO, complementing the growth deficiency. The results show a tight coupling of multi-carbon substrate oxidation to the WLP. The metabolism in the mutant can be viewed as a coupled formate + CO respiration, which may be an ancient metabolic trait.
依赖氢的二氧化碳还原酶是一种可溶性酶复合物，在乙酰辅酶A或伍德-Ljungdahl途径（WLP）的第一步中，它直接利用氢气（H）将二氧化碳（CO）还原为甲酸。HDCR由2个催化亚基、一个氢化酶和一个甲酸脱氢酶（FDH）以及两个携带铁硫簇的小亚基组成。该酶复合物已从两种产乙酸细菌中纯化并进行了表征，一种来自嗜温菌，最近还从嗜热菌中得到了表征。由于缺乏遗传工具，尚未对HDCR在产乙酸菌生长和甲酸代谢中的重要性进行生理学研究。在此，我们利用最近开发的遗传系统，在[具体菌种]中删除了编码HDCR的基因。正如预期的那样，缺失突变体（菌株TKV_MB013）不能以甲酸作为单一底物生长，也不能在H + CO的自养条件下生长。令人惊讶的是，除了添加甲酸时，该菌株在任何其他底物（糖、甘露醇或丙酮酸）上也不能生长。浓缩的细胞悬液仅在有葡萄糖存在时会迅速消耗甲酸。总之，HDCR提供了对[具体菌种]突变体生长至关重要的甲酸。或者，细胞外添加的甲酸除了作为CO之外，还作为末端电子受体，弥补了生长缺陷。结果表明多碳底物氧化与WLP紧密耦合。突变体中的代谢可被视为一种耦合的甲酸 + CO呼吸，这可能是一种古老的代谢特征。
