硝酸盐对牛粪梭菌甲酸脱氢酶缺失型Wood-Ljungdahl途径固碳的影响

Nitrate affects carbon fixation by formate dehydrogenase-lacking Wood-Ljungdahl pathway in Clostridium bovifaecis

摘要:

【背景】异于同型产乙酸菌通常利用Wood-Ljungdahl途径将2分子CO₂还原为1分子乙酰辅酶A，Clostridium bovifaecis缺失Wood-Ljungdahl途径甲基支路第1步将CO₂还原为甲酸的甲酸脱氢酶，需甲酸存在时将1分子甲酸和1分子CO₂还原为乙酰辅酶A发生葡萄糖的同型产乙酸型发酵。已有报道显示，硝酸盐也可作为同型产乙酸菌的电子受体，而且对不同同型产乙酸菌的代谢影响有所不同，然而硝酸盐对这种独特的甲酸脱氢酶缺失型Wood-Ljungdahl途径固碳的影响尚不清楚。【目的】探究硝酸盐对C. bovifaecis甲酸脱氢酶缺失型Wood-Ljungdahl途径固碳的影响。【方法】硝酸盐浓度分别为10 mmol/L和30 mmol/L时，以未添加硝酸盐为对照实验，研究C. bovifaecis在葡萄糖+甲酸+CO₂为基质条件下的细菌生长、底物消耗和产物生成情况。【结果】10 mmol/L和30 mmol/L硝酸盐存在时，主要产物乙醇浓度分别为5.80 mmol/L和1.66 mmol/L，明显低于对照组的乙醇浓度(7.13 mmol/L)，葡萄糖消耗量也明显减少。甲酸盐消耗量随着硝酸盐浓度的升高而降低，硝酸盐浓度为30 mmol/L时甲酸盐无明显消耗。【结论】30 mmol/L的硝酸盐抑制了C. bovifaeciss甲酸脱氢酶缺失型Wood-Ljungdahl途径的固碳作用，10 mmol/L硝酸盐降低了C. bovifaecis葡萄糖的糖酵解作用。

Abstract:

[Background] Unlike general acetogenic bacteria which usually reduce two molecules of CO₂ into one molecule of acetyl-CoA via Wood-Ljungdahl pathway, the formate dehydrogenase-lacking acetogen Clostridium bovifaecis reduces one molecule of formate and one molecule of CO₂ into acetyl-CoA and conducts acetogenic utilization of glucose only in the presence of formate. Nitrate is the preferred electron acceptor for acetogens which vary in their ability to utilize this alternative electron acceptor. However, the effect of nitrate on the carbon fixation by formate dehydrogenase-lacking Wood-Ljungdahl pathway remains unclear. [Objective] To investigate the effect of nitrate on the carbon fixation by formate-dependent acetogenesis in C. bovifaecis. [Methods] We measured the growth, substrate consumption, and product yield of C. bovifaecis cultured in the medium containing 10 mmol/L or 30 mmol/L nitrate and using glucose + formate + CO₂ as substrates. The medium without nitrate was taken as the control. [Results] The main product was ethanol with the concentrations of 5.80 mmol/L and 1.66 mmol/L, respectively, in the media with 10 mmol/L and 30 mmol/L nitrate, which were significantly lower than that (7.13 mmol/L) of the control. In addition, the glucose consumption significantly decreased in the presence of nitrate. Formate consumption decreased with the increase in nitrate concentration and became zero at the nitrate concentration of 30 mmol/L. [Conclusion] Nitrate of 30 mmol/L inhibited carbon fixation by formate-dependent acetogenesis, and that of 10 mmol/L decreased the glycolysis in C. bovifaecis.