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[Biochemistry](https://durgical.com/en/biochemistry/) · Enzyme

# Complex III (cytochrome bc₁)

Complex III passes electrons from ubiquinol to cytochrome c and, through the Q cycle, pumps protons.

Updated 2026-11-01

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## Key facts

- QH₂ + 2 cytochrome c (Fe³⁺) → Q + 2 cytochrome c (Fe²⁺) + 4 H⁺ into the intermembrane space.
- Q cycle: ubiquinol is oxidised at the Qo site; one electron goes to cytochrome c (via Rieske and c₁) and the other returns through the b hemes to a ubiquinone at the Qi site.
- It is a dimer; cytochrome b is the only subunit encoded by mitochondrial DNA.
- Antimycin A blocks the Qi site; myxothiazol and stigmatellin block the Qo site.

**Also called**
Ubiquinol:cytochrome c oxidoreductase

**EC number**
[7.1.1.8](https://www.enzyme-database.org/query.php?ec=7.1.1.8)

**Genes**
MT-CYB, CYC1, UQCRFS1…

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

## Regulation

- Ubiquinol supply and the size of the gradient govern it.
- When the Qi site is blocked, the Qo site leaks electrons to O₂ and forms superoxide.

## Inhibitors and toxins

- Antimycin A (Qi)
- Myxothiazol and stigmatellin (Qo)
- Atovaquone (Plasmodium bc₁)

## Clinical notes

- Atovaquone, an antimalarial, exploits differences between the parasite's bc₁ and the human one.
- MT-CYB variants mainly cause myopathy with exercise intolerance; BCS1L variants cause GRACILE syndrome or Leigh syndrome.

## Pathways

- [Electron transport chain and oxidative phosphorylation](https://durgical.com/en/biochemistry/pathways/electron-transport-chain-and-oxidative-phosphorylation/)

## Related conditions

- [Leigh syndrome](https://durgical.com/en/conditions/leigh-syndrome/)
- [Lactic acidosis](https://durgical.com/en/conditions/lactic-acidosis/)

## Related

- [Ubiquinone (coenzyme Q₁₀)](https://durgical.com/en/biochemistry/molecules/ubiquinone/)
- [Cytochrome c](https://durgical.com/en/biochemistry/enzymes/cytochrome-c/)
- [Complex I (NADH dehydrogenase)](https://durgical.com/en/biochemistry/enzymes/complex-i/)
- [Cytochrome c oxidase (complex IV)](https://durgical.com/en/biochemistry/enzymes/cytochrome-c-oxidase/)
- [Reactive oxygen species and antioxidants](https://durgical.com/en/biochemistry/topics/reactive-oxygen-species-and-antioxidants/)

## Test yourself

Think of the answer, then tap the question to see it.

## What is the mechanism by which complex III pumps protons called?

The Q cycle: from each ubiquinol, one electron goes to cytochrome c and the other is recycled through the b hemes.

## Which antibiotic inhibits the Qi site of complex III?

Antimycin A.

## Which antimalarial acts on the parasite's cytochrome bc₁?

Atovaquone, which binds the Qo site.

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

Sources: [MT-CYB: Cytochrome b (human)](https://www.uniprot.org/uniprotkb/P00156/entry) (UniProt, 2026); [UQCRFS1: Cytochrome b-c1 complex subunit Rieske, mitochondrial (human)](https://www.uniprot.org/uniprotkb/P47985/entry) (UniProt, 2026); [1PPJ: Bovine cytochrome bc1 complex with stigmatellin and antimycin](https://www.rcsb.org/structure/1PPJ) (RCSB Protein Data Bank, 2026); [Respiratory electron transport](https://reactome.org/PathwayBrowser/#/R-HSA-611105) (Reactome, 2026).

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Representative model, for educational purposes only.

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