Kung Johannes W, Meier Anne-Katrin, Mergelsberg Mario, Boll Matthias
Microbiology, Institute of Biology II, University of Freiburg, Freiburg, Germany.
Microbiology, Institute of Biology II, University of Freiburg, Freiburg, Germany
J Bacteriol. 2014 Oct;196(20):3667-74. doi: 10.1128/JB.02071-14. Epub 2014 Aug 11.
The anaerobic degradation of cyclohexane carboxylic acid (CHC) has so far been studied only in Rhodopseudomonas palustris, in which CHC is activated to cyclohexanoyl coenzyme A (cyclohexanoyl-CoA [CHCoA]) and then dehydrogenated to cyclohex-1-ene-1-carboxyl-CoA (CHeneCoA). This intermediate is further degraded by reactions of the R. palustris-specific benzoyl-CoA degradation pathway of aromatic compounds. However, CHeneCoA is not an intermediate in the degradation of aromatic compounds in all other known anaerobic bacteria; consequently, degradation of CHC was mostly unknown in anaerobic bacteria. We identified a previously unknown CHC degradation pathway in the Fe(III)-reducing Geobacter metallireducens by determining the following CHC-induced in vitro activities: (i) the activation of CHC to CHCoA by a succinyl-CoA:CHC CoA transferase, (ii) the 1,2-dehydrogenation of CHCoA to CHeneCoA by CHCoA dehydrogenase, and (iii) the unusual 1,4-dehydrogenation of CHeneCoA to cyclohex-1,5-diene-1-carboxyl-CoA. This last represents a previously unknown joint intermediate of the CHC and aromatic compound degradation pathway in bacteria other than R. palustris. The enzymes catalyzing the three reactions were purified and characterized as specific enzymes after heterologous expression of the encoding genes. Quantitative reverse transcription-PCR revealed that expression of these genes was highly induced during growth with CHC but not with benzoate. The newly identified CHC degradation pathway is suggested to be present in nearly all CHC-degrading anaerobic bacteria, including denitrifying, Fe(III)-reducing, sulfate-reducing, and fermenting bacteria. Remarkably, all three CHC degradation pathways always link CHC catabolism to the catabolic pathways of aromatic compounds. We propose that the capacity to use CHC as a carbon source evolved from already-existing aromatic compound degradation pathways.
到目前为止，仅在沼泽红假单胞菌中研究了环己烷羧酸（CHC）的厌氧降解，其中CHC被激活为环己酰辅酶A（环己酰 - CoA [CHCoA]），然后脱氢生成环己 - 1 - 烯 - 1 - 羧基 - CoA（CHeneCoA）。该中间体通过沼泽红假单胞菌特有的芳香族化合物苯甲酰 - CoA降解途径的反应进一步降解。然而，CHeneCoA并非所有其他已知厌氧细菌中芳香族化合物降解的中间体；因此，厌氧细菌中CHC的降解大多未知。我们通过测定以下CHC诱导的体外活性，在还原铁的地杆菌中鉴定出一条以前未知的CHC降解途径：（i）琥珀酰 - CoA：CHC CoA转移酶将CHC激活为CHCoA，（ii）CHCoA脱氢酶将CHCoA进行1,2 - 脱氢生成CHeneCoA，以及（iii）CHeneCoA进行不寻常的1,4 - 脱氢生成环己 - 1,5 - 二烯 - 1 - 羧基 - CoA。这最后一种产物代表了除沼泽红假单胞菌外其他细菌中CHC和芳香族化合物降解途径以前未知的联合中间体。催化这三个反应的酶经编码基因的异源表达后被纯化并鉴定为特异性酶。定量逆转录 - PCR显示，这些基因的表达在以CHC生长期间高度诱导，但以苯甲酸生长时未诱导。新鉴定的CHC降解途径被认为存在于几乎所有降解CHC的厌氧细菌中，包括反硝化细菌、还原铁细菌、硫酸盐还原细菌和发酵细菌。值得注意的是，所有这三种CHC降解途径总是将CHC分解代谢与芳香族化合物的分解代谢途径联系起来。我们提出，利用CHC作为碳源的能力是从已有的芳香族化合物降解途径进化而来的。
