Lu Zheng, Imlay James A
Department of Microbiology, University of Illinois, Urbana, Illinois, USA.
Department of Microbiology, University of Illinois, Urbana, Illinois, USA
mBio. 2017 Jan 3;8(1):e01873-16. doi: 10.1128/mBio.01873-16.
The impact of oxidative stress upon organismal fitness is most apparent in the phenomenon of obligate anaerobiosis. The root cause may be multifaceted, but the intracellular generation of reactive oxygen species (ROS) likely plays a key role. ROS are formed when redox enzymes accidentally transfer electrons to oxygen rather than to their physiological substrates. In this study, we confirm that the predominant intestinal anaerobe Bacteroides thetaiotaomicron generates intracellular ROS at a very high rate when it is aerated. Fumarate reductase (Frd) is a prominent enzyme in the anaerobic metabolism of many bacteria, including B. thetaiotaomicron, and prior studies of Escherichia coli Frd showed that the enzyme is unusually prone to ROS generation. Surprisingly, in this study biochemical analysis demonstrated that the B. thetaiotaomicron Frd does not react with oxygen at all: neither superoxide nor hydrogen peroxide is formed. Subunit-swapping experiments indicated that this difference does not derive from the flavoprotein subunit at which ROS normally arise. Experiments with the related enzyme succinate dehydrogenase discouraged the hypothesis that heme moieties are responsible. Thus, resistance to oxidation may reflect a shift of electron density away from the flavin moiety toward the iron-sulfur clusters. This study shows that the autoxidizability of a redox enzyme can be suppressed by subtle modifications that do not compromise its physiological function. One implication is that selective pressures might enhance the oxygen tolerance of an organism by manipulating the electronic properties of its redox enzymes so they do not generate ROS.
Whether in sediments or pathogenic biofilms, the structures of microbial communities are configured around the sensitivities of their members to oxygen. Oxygen triggers the intracellular formation of reactive oxygen species (ROS), and the sensitivity of a microbe to oxygen likely depends upon the rates at which ROS are formed inside it. This study supports that idea, as an obligate anaerobe was confirmed to generate ROS very rapidly upon aeration. However, the suspected source of the ROS was disproven, as the fumarate reductase of the anaerobe did not display the high oxidation rate of its E. coli homologue. Evidently, adjustments in its electronic structure can suppress the tendency of an enzyme to generate ROS. Importantly, this outcome suggests that evolutionary pressure may succeed in modifying redox enzymes and thereby diminishing the stress that an organism experiences in oxic environments. The actual source of ROS in the anaerobe remains to be discovered.
氧化应激对生物体健康的影响在专性厌氧现象中最为明显。其根本原因可能是多方面的，但细胞内活性氧（ROS）的产生可能起着关键作用。当氧化还原酶意外地将电子转移给氧气而不是其生理底物时，就会形成ROS。在本研究中，我们证实，主要的肠道厌氧菌多形拟杆菌在通气时会以非常高的速率产生细胞内ROS。延胡索酸还原酶（Frd）是包括多形拟杆菌在内的许多细菌厌氧代谢中的一种重要酶，先前对大肠杆菌Frd的研究表明，该酶异常容易产生ROS。令人惊讶的是，在本研究中，生化分析表明，多形拟杆菌Frd根本不与氧气反应：既不形成超氧化物也不形成过氧化氢。亚基交换实验表明，这种差异并非源于通常产生ROS的黄素蛋白亚基。对相关酶琥珀酸脱氢酶的实验排除了血红素部分起作用的假设。因此，抗氧化性可能反映了电子密度从黄素部分向铁硫簇的转移。本研究表明，氧化还原酶的自氧化性可以通过不损害其生理功能的细微修饰来抑制。一个含义是，选择压力可能通过操纵其氧化还原酶的电子特性来增强生物体的耐氧性，使其不产生ROS。
无论是在沉积物中还是在致病生物膜中，微生物群落的结构都是围绕其成员对氧气的敏感性构建的。氧气会触发细胞内活性氧（ROS）的形成，微生物对氧气的敏感性可能取决于其内部ROS的形成速率。本研究支持了这一观点，因为一种专性厌氧菌在通气时被证实能非常迅速地产生ROS。然而，ROS的疑似来源被否定了，因为该厌氧菌的延胡索酸还原酶并没有表现出与其大肠杆菌同源物相同的高氧化速率。显然，其电子结构的调整可以抑制酶产生ROS的倾向。重要的是，这一结果表明，进化压力可能成功地修饰氧化还原酶，从而减轻生物体在有氧环境中所经历的压力。该厌氧菌中ROS的实际来源仍有待发现。
