Succinate dehydrogenase complex (SDH or Complex II)

Succinate dehydrogenase complex—also known as electron transport chain Complex II—is a multi-subunit enzyme embedded within the inner mitochondrial membrane. It uniquely participates both as an enzyme catalyzing oxidation of succinate to fumarate within the citric acid cycle and as an integral component transferring electrons from FADH₂ produced by this reaction directly into the electron transport chain via reduction of ubiquinone. The human enzyme consists of four subunits (SDHA, SDHB, SDHC, SDHD), with two hydrophilic catalytic subunits facing the matrix and two hydrophobic membrane anchor subunits. Unlike other complexes I/III/IV that pump protons across the membrane contributing directly to proton gradient formation for ATP synthesis, Complex II does not translocate protons but still contributes indirectly by feeding electrons into downstream complexes that do pump protons. Mutations affecting any subunit can result in inherited cancer predisposition syndromes or severe neurometabolic disease due to impaired oxidative phosphorylation capacity.

Other names

Electron transport chain complex IISuccinate ubiquinone oxidoreductaseRespiratory complex IISDH (succinate dehydrogenase)Mitochondrial complex II

For inhibitors: - Competitive inhibition at the succinate binding site or quinone binding site blocks electron transfer from succinate to ubiquinone. For potential therapeutics: - Modulation may alter mitochondrial respiration, ATP production, or reactive oxygen species generation.