Biochemistry · Protein
Cytochrome c !
Cytochrome c is a small heme protein that carries one electron at a time from complex III to complex IV; out of place, it triggers apoptosis.
Updated 2026-11-01
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Key facts
- A soluble protein of about 104 amino acids in the intermembrane space, loosely bound to the outer face of the inner membrane.
- Its heme c is covalently bound to two cysteines; the iron alternates between Fe³⁺ and Fe²⁺.
- It carries a single electron per trip: four are needed to reduce one O₂.
- It is one of the most conserved proteins in evolution.
- Genes
- CYCS
- Structure (PDB)
- 3ZCF
Structure
- Heme c attached to the protein by two thioether bonds (Cys14 and Cys17); histidine and methionine hold the iron.
Clinical notes
- In intrinsic apoptosis, cytochrome c leaves the mitochondrion (controlled by the BCL-2 family) and forms the apoptosome with APAF-1, activating caspase 9.
- Cardiolipin in the inner membrane holds it; its oxidation favours release.
Pathways
Related
Test yourself
Think of the answer, then tap the question to see it.
Between which complexes does cytochrome c carry electrons?
From complex III to complex IV, one electron at a time.
What role does cytochrome c play in apoptosis?
Once in the cytosol it binds APAF-1, forms the apoptosome and activates caspase 9.
How is the heme attached to cytochrome c?
By covalent (thioether) bonds to two cysteines, unlike the heme of hemoglobin.
Educational information, not medical advice. For any symptom, see a health professional.
Sources: CYCS: Cytochrome c (human) (UniProt, 2026); 3ZCF: Structure of recombinant human cytochrome c (RCSB Protein Data Bank, 2026); Respiratory electron transport (Reactome, 2026).
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