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[Biochemistry](https://durgical.com/en/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

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## 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.

**EC number**
[1.3.5.1](https://www.enzyme-database.org/query.php?ec=1.3.5.1)

**Genes**
SDHA, SDHB, SDHC, SDHD

**Structure (PDB)**
[1ZOY](https://www.rcsb.org/structure/1ZOY)

## 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

- [Krebs cycle (citric acid cycle)](https://durgical.com/en/biochemistry/pathways/krebs-cycle/)

## Related conditions

- [Hereditary paraganglioma and pheochromocytoma (SDH)](https://durgical.com/en/conditions/hereditary-paraganglioma-and-pheochromocytoma/)

## Related

- [Citrate synthase](https://durgical.com/en/biochemistry/enzymes/citrate-synthase/)
- [α-Ketoglutarate dehydrogenase](https://durgical.com/en/biochemistry/enzymes/ketoglutarate-dehydrogenase/)

## 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.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: [SDHA: Succinate dehydrogenase flavoprotein subunit (human)](https://www.uniprot.org/uniprotkb/P31040/entry) (UniProt, 2026); [SDHB gene: MedlinePlus Genetics](https://medlineplus.gov/genetics/gene/sdhb/) (MedlinePlus, US National Library of Medicine, 2026); [Hereditary paraganglioma-pheochromocytoma: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/hereditary-paraganglioma-pheochromocytoma/) (MedlinePlus, US National Library of Medicine, 2026); [1ZOY: Mitochondrial respiratory complex II from porcine heart](https://www.rcsb.org/structure/1ZOY) (RCSB Protein Data Bank, 2026).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/biochemistry/enzymes/succinate-dehydrogenase/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/biochemistry/enzymes/succinate-dehydrogenase/