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[Biochemistry](https://durgical.com/en/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](https://www.rcsb.org/structure/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

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

## Related

- [Complex III (cytochrome bc₁)](https://durgical.com/en/biochemistry/enzymes/complex-iii/)
- [Cytochrome c oxidase (complex IV)](https://durgical.com/en/biochemistry/enzymes/cytochrome-c-oxidase/)
- [Ubiquinone (coenzyme Q₁₀)](https://durgical.com/en/biochemistry/molecules/ubiquinone/)

## 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.

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

Sources: [CYCS: Cytochrome c (human)](https://www.uniprot.org/uniprotkb/P99999/entry) (UniProt, 2026); [3ZCF: Structure of recombinant human cytochrome c](https://www.rcsb.org/structure/3ZCF) (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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Source: https://durgical.com/en/biochemistry/enzymes/cytochrome-c/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/biochemistry/enzymes/cytochrome-c/