Dioxygen Toxicity
Oxidative Stress and Anaerobes
The aerobic lifestyle implies the formation of ROS either through an enzymatic or a non-enzymatic dependent way. Aerobes have to deal with the direct reduction of dioxygen to superoxide and the subsequent formation of hydrogen peroxide, during their normal metabolism. In fact they possess several ways to keep both superoxide and hydrogen peroxide below hazardous concentrations by maintaining a controlled balance between ROS formation and elimination. An imbalance between the production and disposal of ROS is called oxidative stress. In contrast, in anaerobes this definition may vary, since these organisms live in oxygen free habitats. In this case, the term oxidative stress may be used whenever the organism is exposed to any amount of dioxygen and/or any of the ROS. Upon exposure to dioxygen, the highly reducing cytosolic environment of the cell will induce the reduction of dioxygen by powerful reducing organic compounds such as reduced flavins. This reaction will lead to the formation of superoxide, and once formed and if not eliminated, superoxide will quickly lead to the formation of more hazardous ROS, that will damage cells. In this sense, it is conceivable that anaerobes also possess defences against ROS, particularly against superoxide. In fact, some of them possess canonical SODs, typical superoxide detoxifying enzymes in aerobes. Apart from superoxide elimination enzymes, anaerobes also possess catalases and/or peroxidases and alkyl hydroperoxidases, showing their necessity to cope with ROS.
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Bibliography
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