Succinate Dehydrogenase is the Regulator of Respiration in Mycobacterium tuberculosis

In chronic infection, Mycobacterium tuberculosis bacilli are thought to enter a metabolic program that provides sufficient energy for maintenance of the protonmotive force, but is insufficient to meet the demands of cellular growth. We sought to understand this metabolic downshift genetically by targeting succinate dehydrogenase, the enzyme which couples the growth processes controlled by the TCA cycle with the energy production resulting from the electron transport chain. M. tuberculosis contains two operons which are predicted to encode succinate dehydrogenase enzymes (sdh-1 and sdh-2); we found that deletion of Sdh1 contributes to an inability to survive long term stationary phase. Stable isotope labeling and mass spectrometry revealed that Sdh1 functions as a succinate dehydrogenase during aerobic growth, and that Sdh2 is dispensable for this catalysis, but partially overlapping activities ensure that the loss of one enzyme can incompletely compensate for loss of the other. Deletion of Sdh1 disturbs the rate of respiration via the mycobacterial electron transport chain, resulting in an increased proportion of reduced electron carrier (menaquinol) which leads to increased oxygen consumption. The loss of respiratory control leads to an inability to recover from stationary phase. We propose a model in which succinate dehydrogenase is a governor of cellular respiration in the adaptation to low oxygen environments.在慢性感染过程中，结核分枝杆菌（Mycobacterium tuberculosis）被认为会进入一种代谢程序，该程序可为质子动力势的维持提供充足能量，但不足以满足细胞生长的需求。本研究旨在通过靶向琥珀酸脱氢酶（succinate dehydrogenase），从遗传学角度解析这一代谢下调现象——该酶可将三羧酸循环（TCA cycle）调控的生长过程与电子传递链（electron transport chain）产生的能量进行偶联。 结核分枝杆菌基因组中存在两个被预测可编码琥珀酸脱氢酶的操纵子（sdh-1与sdh-2）；研究发现，敲除Sdh1会使菌株无法在长期稳定期存活。稳定同位素标记结合质谱分析结果显示，Sdh1在需氧生长过程中发挥琥珀酸脱氢酶的功能，而Sdh2对此催化过程并非必需；但二者存在部分重叠的活性，因此单酶缺失无法完全代偿另一酶的功能缺失。 敲除Sdh1会扰乱分枝杆菌电子传递链介导的呼吸速率，导致还原型电子载体甲基萘醌醇（menaquinol）的比例升高，进而引发氧消耗量增加。呼吸控制能力的丧失会使菌株无法从稳定期恢复生长。本研究提出了一个模型：琥珀酸脱氢酶是细胞呼吸的主控因子，参与菌株适应低氧环境的过程。