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首页> 外文期刊>Applied Microbiology >Reply to “Syntrophic Propionate Oxidation via Butyrate: a Novel Window of Opportunity under Methanogenic Conditions”
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Reply to “Syntrophic Propionate Oxidation via Butyrate: a Novel Window of Opportunity under Methanogenic Conditions”

机译：回复“通过丁酸的辛酸化丙酸氧化：产甲烷条件下的新型机会之窗”

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In a study that evaluated the oxidation of propionate in anoxic microcosms of rice paddy soil, we proposed that both the methylmalonyl-coenzyme A (CoA) pathway and the six-carbon-intermediate (6-C) pathway (see reference 1 for historical work on the 6-C pathway) were involved in the metabolism of propionate (2). Dolfing (3) calculated the Gibbs free energy for these two pathways and concluded that the 6-C pathway (referred to as the Smithella pathway by Dolfing) is thermodynamically more advantageous than the methylmalonyl-CoA pathway (referred to as the classical propionate degradation pathway by Dolfing). He proposed that the 6-C pathway opens a novel window of opportunity for the oxidation of propionate under methanogenic conditions. Smithella species are known to utilize the 6-C pathway and have been detected in various anoxic environments (4, 5), suggesting that this pathway may be widespread.Dolfing (3) indicated that the utilization of the 6-C pathway could explain why the oxidation of propionate occurred under conditions that were thermodynamically unfavorable for the methylmalonyl-CoA pathway in anoxic incubations of rice paddy soil (6). This is certainly an excellent explanation for the observations reported. However, the experiments with chemostat cocultures of Syntrophobacter fumaroxidans and Methanospirillum hungatei indicate that propionate oxidation via the methylmalonyl-CoA pathway can occur when the Gibbs free energy is greater than 10 kJ per mol propionate (7), a value much lower than the theoretical minimum energy quantum (20 kJ per mol) needed for the conservation of energy as determined by the synthesis of ATP (8). Thus, even the syntrophs utilizing the methylmalonyl-CoA pathway have probably evolved unusual mechanisms to surpass the unfavorable thermodynamic conditions. In this regard, both Syntrophobacter fumaroxidans and Pelotomaculum thermopropionicum , the classical syntrophs employing the methylmalonyl-CoA pathway, harbor a rich machinery to use formate instead of H_(2) as the electron shuttle for the syntrophic interaction (9, 10). It was known that interspecies formate transfer might sustain a 100-fold-higher conversion of substrate than can interspecies H_(2) transfer based on Fick's diffusion law (11). Furthermore, it has been shown that P. thermopropionicum synthesizes flagellum-like filaments and extracellular polymers that might stimulate microbial aggregation and enhance interspecies electron transfer with a methanogenic partner (12, 13). These collective observations and the thermodynamic considerations delineated by Dolfing (3) reinforce the likelihood that propionate-utilizing syntrophs have diverse mechanisms to overcome energetic barriers.
机译：在一项评估稻田土壤缺氧微观世界中丙酸氧化的研究中，我们提出了甲基丙二酸辅酶A（CoA）途径和六碳中间（6-C）途径（有关历史工作，请参见参考资料1）。在6-C途径上）参与丙酸酯的代谢（2）。 Dolfing（3）计算了这两个途径的吉布斯自由能，并得出结论，6-C途径（Dolfing称为Smithella途径）在热力学上比甲基丙二酰-CoA途径（称为经典丙酸酯降解途径）更具优势。 （Dolfing）。他提出6-C途径为产甲烷条件下丙酸的氧化打开了新的机会之窗。已知史密斯氏菌利用6-C途径，并已在各种缺氧环境中被检测到（4，5），这表明该途径可能是广泛存在的.Dolfing（3）指出6-C途径的利用可以解释为什么丙酸的氧化发生在对水稻田土壤无氧培养的热力学不利于甲基丙二酰辅酶A途径的条件下（6）。对于所报告的观察结果，这无疑是一个极好的解释。然而，对化脓性腐霉和悬液甲烷螺旋菌进行化学恒量培养的实验表明，当吉布斯自由能大于10 kJ / mol丙酸酯时，可以通过甲基丙二酰-CoA途径发生丙酸酯氧化（7），该值远低于理论最小值由ATP的合成确定的能量守恒所需的能量量子（20 kJ / mol）（8）。因此，即使利用甲基丙二酰-CoA途径的同养菌也可能已经进化出异常的机制，以克服不利的热力学条件。在这方面，采用甲基丙二酰-CoA途径的经典辛弗弗氏氧化菌和热腐霉都具有丰富的机制来利用甲酸盐代替H_（2）作为电子进行梭化相互作用（9，10）。已知物种间的甲酸盐转移可能比基于Fick扩散定律的物种间H_（2）转移可以维持100倍高的底物转化率（11）。此外，已经表明，热丙酸丙酸杆菌合成鞭毛状细丝和细胞外聚合物，它们可能刺激微生物聚集并增强与产甲烷伴侣的种间电子转移（12、13）。这些集体的观察和Dolfing（3）所描述的热力学考虑，进一步增强了利用丙酸的共形体具有克服能量障碍的多种机制的可能性。

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