respiratory chain complex III

GO:0045275 · Energy complex · Multiprotein complex · Proposed

Respiratory chain complex III is a membrane-bound cytochrome bc1 oxidoreductase complex that uses a cytochrome b, Rieske iron-sulfur protein, and cytochrome c1 catalytic core to transfer electrons from quinol to cytochrome c while translocating protons. [GO:0045275]

Synonyms: complex III; cytochrome bc1 complex; cytochrome bc(1) complex; ubiquinol-cytochrome c oxidoreductase complex; ubiquinol-cytochrome-c reductase complex; coenzyme Q-cytochrome c reductase complex; electron transport complex III

UniProt location names: UniProt Consortium, CC BY 4.0. Reference label and route sources.

Components

Component	Type	Part of / acts on	Grounding	Genes	Essential	Role
cytochrome b	Protein	part of	`InterPro:IPR030689`	fbcB, petB	ESSENTIAL	Central membrane cytochrome that binds the quinol redox sites and carries low- and high-potential b-type hemes. `GO:0045275` GO:0045275 defines cytochrome b as one of the three catalytic core subunits of respiratory complex III. `InterPro:IPR030689` InterPro IPR030689 is the source-neutral family for cytochrome b proteins in cytochrome bc1 complexes. `PDB:2QJP` RCSB maps PDB:2QJP polymer entity 1 to Cereibacter sphaeroides cytochrome b, UniProtKB:Q02761.
Rieske iron-sulfur subunit	Protein	part of	`InterPro:IPR006317`	fbcF, petA	ESSENTIAL	2Fe-2S protein that accepts electrons from the quinol-oxidation site and transfers them toward cytochrome c1. `GO:0045275` GO:0045275 defines the Rieske iron-sulfur protein as one of the three catalytic core subunits. `InterPro:IPR006317` InterPro IPR006317 names the ubiquinol-cytochrome c reductase iron-sulphur subunit family and separates it from the cyanobacterial and chloroplast b6f Rieske protein. `PDB:2QJP` RCSB maps PDB:2QJP polymer entity 3 to Cereibacter sphaeroides ubiquinol-cytochrome c reductase iron-sulfur subunit, UniProtKB:Q02762.
cytochrome c1	Protein	part of	`InterPro:IPR002326`	fbcC	ESSENTIAL	c-type cytochrome that receives electrons from the Rieske iron-sulfur subunit and passes them to soluble cytochrome c. `GO:0045275` GO:0045275 defines cytochrome c1 as one of the three catalytic core subunits. `InterPro:IPR002326` InterPro IPR002326 is the source-neutral family for cytochrome c1 subunits of cytochrome bc1 complexes. `PDB:2QJP` RCSB maps PDB:2QJP polymer entity 2 to Cereibacter sphaeroides cytochrome c1, UniProtKB:Q02760.

Canonical examples

NCBITaxon:1063 Cereibacter sphaeroides — Current NCBI Taxonomy name for the Rhodobacter sphaeroides source organism used in the PDB:2QJP cytochrome bc1 structure. [PDB:2QJP]

Functions

quinol-cytochrome-c reductase activity GO:0008121 — Complex III oxidizes quinol, reduces cytochrome c, and couples that electron transfer to proton translocation across the respiratory membrane.
- GO:0008121 GO:0008121 defines the quinol plus ferricytochrome c oxidoreductase reaction coupled to proton translocation.
- DOI:10.1074/jbc.M708608200 Esser et al. 2008 solved inhibitor-bound structures of the Cereibacter sphaeroides cytochrome bc1 complex.

Mechanism graphs

Complex III links quinol oxidation to cytochrome c reduction (FUNCTION)

Respiratory complex III uses a cytochrome b, Rieske iron-sulfur protein, and cytochrome c1 core to move electrons from quinol to cytochrome c and contribute to a proton motive force.

Subject	Predicate	Object	Evidence
respiratory chain complex III	enables	quinol-cytochrome-c reductase activity	`GO:0008121` GO:0008121 defines the complex III molecular function as quinol-dependent cytochrome c reduction coupled to proton translocation.
quinol	donates electrons through	quinol-cytochrome-c reductase activity	`GO:0008121` GO:0008121 defines the quinol-cytochrome-c reductase reaction with quinol as the electron donor.
cytochrome b	contributes to	quinol-cytochrome-c reductase activity	`InterPro:IPR030689` InterPro IPR030689 describes cytochrome b as the central redox catalytic subunit involved in quinone-substrate binding.
Rieske iron-sulfur subunit	contributes to	quinol-cytochrome-c reductase activity	`GO:0045275` GO:0045275 names the Rieske iron-sulfur protein as one of the three catalytic core subunits.
cytochrome c1	contributes to	quinol-cytochrome-c reductase activity	`InterPro:IPR002326` InterPro IPR002326 describes the cytochrome bc1 complex function as electron transfer from ubiquinol to cytochrome c.
quinol-cytochrome-c reductase activity	reduces	cytochrome c	`GO:0008121` GO:0008121 defines ferricytochrome c as the electron acceptor reduced by quinol-cytochrome-c reductase activity.
quinol-cytochrome-c reductase activity	contributes to	transmembrane proton motive force	`GO:0008121` GO:0008121 defines the electron-transfer reaction together with coupled proton translocation.

Evidence

GO:0045275 Gene Ontology cellular-component term for respiratory chain complex III.
GO:0098803 Gene Ontology cellular-component term for respiratory-chain protein complexes.
GO:0008121 Gene Ontology molecular-function term for quinol-cytochrome-c reductase activity.
InterPro:IPR030689 InterPro family for cytochrome b.
InterPro:IPR006317 InterPro entry for the ubiquinol-cytochrome c reductase iron-sulphur subunit.
InterPro:IPR002326 InterPro family for cytochrome c1.
DOI:10.1074/jbc.M708608200 Esser et al. 2008, Inhibitor-complexed structures of the cytochrome bc1 from the photosynthetic bacterium Rhodobacter sphaeroides, Journal of Biological Chemistry.
PMID:18039651 PubMed identifier for Esser et al. 2008 as reported by the RCSB PDB:2QJP primary citation.
PDB:2QJP RCSB Protein Data Bank entry for the Cereibacter sphaeroides cytochrome bc1 structure from Esser et al. 2008.

Curation history

2026-09-24T05:50:28Z codex · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-09-24T05:51:44Z codex · EXPANDED_RECORD — Added GO respiratory-chain parentage, GO synonymy, InterPro-grounded cytochrome b/Rieske/cytochrome c1 components, a Cereibacter sphaeroides PDB:2QJP canonical example, quinol:cytochrome-c reductase activity, literature/PDB evidence, and a compact complex III function graph.
2026-09-24T05:52:23Z codex · REVISED_GRAPH — Replaced a too-directional cytochrome b-to-Rieske edge with component contribution edges around the GO-level quinol-cytochrome-c reductase activity.
2026-09-24T06:03:53Z codex · ADDRESS_REVIEW — #1455/#1457: added the PDB:2QJP petB and petA gene-symbol aliases and the GO exact cytochrome bc(1) complex synonym.

Source: data/structures/energy_complex/respiratory_chain_complex_iii.yaml