Superoxide dismutase (SOD) catalyses the dismutation of superoxide to hydrogen peroxide. The type of SOD found in mitochondria (Mn-SOD) is particularly important because:

• A Mitochondria generate superoxide primarily at Complex I and III of the ETC where O2 can accept single electrons from ubisemiquinone ✓
• B Mitochondria lack catalase, so Mn-SOD is the only defense against reactive oxygen species
• C Mn-SOD is encoded by the nuclear genome and imported into mitochondria, making it susceptible to nuclear DNA mutations
• D Mn-SOD dismutates H2O2 directly to water without needing catalase or glutathione peroxidase

Explanation

The mitochondrial electron transport chain generates superoxide primarily as a byproduct at Complex I (NADH dehydrogenase, via the FMN site and CoQ radical) and Complex III (ubiquinol-cytochrome c reductase, at the Qo site where ubisemiquinone donates a single electron to O2). Mn-SOD in the mitochondrial matrix rapidly converts superoxide to H2O2 (which is then detoxified by glutathione peroxidase using GSH). Mitochondria do contain small amounts of catalase in some organisms but this is not the primary defense; GPx/thioredoxin reductase system is. SOD only converts O2- to H2O2, not H2O2 to water.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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Written and medically reviewed by the StethoPrep medical team.