Mitochondrial Electron Transport Chain Complexes I and II (NADH:ubiquinone oxidoreductase and Succinate dehydrogenase) (Complex I & II (CI & CII, NADH:CoQ reductase, SDH))

The **mitochondrial electron transport chain complexes I and II** are large multi-subunit enzyme complexes embedded in the inner mitochondrial membrane. **Complex I (NADH:ubiquinone oxidoreductase)** accepts electrons from NADH, generated in the citric acid cycle, and passes them to ubiquinone (coenzyme Q), concurrently pumping protons into the intermembrane space to generate a proton gradient necessary for ATP synthesis[1][2][3]. **Complex II (succinate dehydrogenase)** is unique as it participates both in the citric acid cycle (oxidizing succinate to fumarate) and the electron transport chain (transferring electrons via FADH2 to coenzyme Q) but does not contribute to proton pumping[1][3][4][7]. Both complexes are essential for oxidative phosphorylation and ATP generation. Dysfunction or inhibition is linked to neurodegenerative diseases, inherited mitochondrial disorders, cancer, and aging[5][7]. Several inhibitors—natural and synthetic—target these complexes for study or as experimental drugs. Their targeting as therapies is limited by safety concerns due to the essential bioenergetic function of these complexes and the risk of off-target cytotoxicity[1][3][5].

Other names

NADH dehydrogenaseNADH:CoQ oxidoreductasemitochondrial complex ISuccinate-CoQ reductaseSDHsuccinate dehydrogenase complex

Inhibition of electron transfer (most inhibitors) Reduction in ATP synthesis due to disruption of proton gradient Increased oxidative stress (inhibitors promote superoxide/leakage) Induction of apoptosis (particularly with Complex I inhibitors)