Production of Superoxide
Recent investigations suggest that Complex I is a potent source of reactive oxygen species. Complex I can produce superoxide (as well as hydrogen peroxide), through at least two different pathways. During forward electron transfer, only very small amounts of superoxide are produced (probably less than 0.1% of the overall electron flow).
During reverse electron transfer, Complex I might be the most important site of superoxide production within mitochondria, with up to 5% of electrons being diverted to superoxide formation. Reverse electron transfer, the process by which electrons from the reduced ubiquinol pool (supplied by succinate dehydrogenase, glycerol-3-phosphate dehydrogenase, or dihydro-oorotate dehydrogenase in mammalian mitochondria) pass through Complex I to reduce NAD+ to NADH, driven by the inner mitochondrial membrane potential electric potential. Although it is not precisely known under what pathological conditions reverse-electron transfer would occur in vivo, in vitro experiments indicate that it can be a very potent source of superoxide when succinate concentrations are high and oxaloacetate or malate concentrations are low.
Superoxide is a reactive oxygen species that contributes to cellular oxidative stress and is linked to neuromuscular diseases and aging. NADH dehdyrogenase produces superoxide by transferring one electron from FMNH2 to oxygen (O2). The radical flavin leftover is unstable, and transfers the remaining electron to the iron-sulfur centers. Interestingly, it is the ratio of NADH to NAD+ that determines the rate of superoxide formation.
Read more about this topic: NADH Dehydrogenase
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