complex II
Summary
Complex II (succinate dehydrogenase) is the only electron transport chain complex directly involved in the citric acid cycle, oxidizing succinate to fumarate while reducing FAD to FADH2 and transferring electrons to ubiquinone (CoQ). Unlike Complex I, it does not pump protons across the inner mitochondrial membrane, so electrons entering via Complex II yield fewer ATP than those entering via Complex I.
Detail
Complex II, also known as succinate dehydrogenase (SDH), is a unique enzyme that serves dual roles: it is both an enzyme of the Krebs (TCA) cycle and a component of the electron transport chain (ETC) embedded in the inner mitochondrial membrane. It catalyzes the oxidation of succinate to fumarate, transferring electrons to FAD (forming FADH2 within the enzyme), which then passes electrons through iron-sulfur clusters to ubiquinone (coenzyme Q), reducing it to ubiquinol (CoQH2). Because Complex II does not translocate protons (H+) across the inner mitochondrial membrane, entry of electrons at this point (via FADH2) generates less proton-motive force and thus less ATP compared to electrons entering via Complex I (via NADH), which does pump protons. This is why FADH2-linked substrates yield theoretically ~1.5 ATP versus ~2.5 ATP for NADH-linked substrates in oxidative phosphorylation calculations.
Clinically, Complex II mutations are associated with hereditary paraganglioma and pheochromocytoma syndromes due to mutations in SDH subunit genes (SDHB, SDHC, SDHD), which act as tumor suppressor genes. Loss of SDH function leads to pseudohypoxia via accumulation of succinate, which inhibits prolyl hydroxylases, stabilizing HIF-1α and promoting angiogenesis and tumorigenesis (similar to VHL-associated pathways). SDHB mutations are particularly associated with malignant, extra-adrenal paragangliomas.
Complex II is also a target of certain toxins and drugs: malonate is a classic competitive inhibitor of succinate dehydrogenase, mimicking succinate structurally. Additionally, atpenin A5 inhibits Complex II. Understanding Complex II is essential for questions relating to the electron transport chain order (Complex I → III → IV or Complex II → III → IV), ATP yield calculations, and genetics of paraganglioma/pheochromocytoma syndromes on USMLE Step 1.
Sources
- First Aid for the USMLE Step 1
- Lehninger Principles of Biochemistry
- Robbins and Cotran Pathologic Basis of Disease
- Harper's Illustrated Biochemistry
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