Biochemistry · Enzyme
Succinate dehydrogenase (complex II) !
Succinate dehydrogenase is the Krebs cycle enzyme that is also part of the respiratory chain; if it fails, stored succinate favours tumours.
Updated 2026-10-31
Loading the 3D model…
Key facts
- Succinate + FAD → fumarate + FADH₂; the electrons go on to ubiquinone.
- Four subunits: SDHA (FAD), SDHB (Fe–S clusters), SDHC and SDHD (membrane anchor, heme b, ubiquinone site).
- It is the only cycle enzyme bound to the inner membrane and the only respiratory complex that does not pump protons.
Regulation
- Oxaloacetate inhibits it; succinate and ATP activate it.
Inhibitors and toxins
- Malonate (competitive, looks like succinate)
- Carboxin and some fungicides (ubiquinone site)
Clinical notes
- SDHB, SDHC, SDHD, SDHAF2 and SDHA variants cause hereditary paragangliomas and pheochromocytomas; SDHB carries the most risk of spread.
- Stored succinate blocks the enzymes that break down HIF: the cell behaves as if short of oxygen (pseudohypoxia).
- Defects in both SDHA copies cause Leigh syndrome.
Pathways
Related conditions
Related
Test yourself
Think of the answer, then tap the question to see it.
Which Krebs cycle enzyme is also complex II?
Succinate dehydrogenase.
Which tumours do germline SDH variants cause?
Hereditary paragangliomas and pheochromocytomas (and others, such as SDH-deficient GIST).
Which classic competitive inhibitor looks like succinate?
Malonate.
Educational information, not medical advice. For any symptom, see a health professional.
Sources: SDHA: Succinate dehydrogenase flavoprotein subunit (human) (UniProt, 2026); SDHB gene: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); Hereditary paraganglioma-pheochromocytoma: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); 1ZOY: Mitochondrial respiratory complex II from porcine heart (RCSB Protein Data Bank, 2026).
Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake
Representative model, for educational purposes only.