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
Loading the 3D model…
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
- Genes
- MT-CYB, CYC1, UQCRFS1…
- Structure (PDB)
- 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
Related conditions
Related
- Ubiquinone (coenzyme Q₁₀)
- Cytochrome c
- Complex I (NADH dehydrogenase)
- Cytochrome c oxidase (complex IV)
- 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.
Educational information, not medical advice. For any symptom, see a health professional.
Sources: MT-CYB: Cytochrome b (human) (UniProt, 2026); UQCRFS1: Cytochrome b-c1 complex subunit Rieske, mitochondrial (human) (UniProt, 2026); 1PPJ: Bovine cytochrome bc1 complex with stigmatellin and antimycin (RCSB Protein Data Bank, 2026); Respiratory electron transport (Reactome, 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.