respiratory chain complex IV

GO:0045277 · Energy complex · Multiprotein complex · Proposed

Respiratory chain complex IV is a membrane-bound cytochrome c oxidase complex that transfers electrons from reduced cytochrome c to dioxygen and couples oxygen reduction to proton translocation across a respiratory membrane. [GO:0045277]

Synonyms: cytochrome c oxidase complex; electron transport complex IV

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
bacterial cytochrome c oxidase subunit I	Protein	part of	`InterPro:IPR014241`	ctaD, ctaDII, coxA, caaA	ESSENTIAL	Catalytic core subunit that binds the heme-copper binuclear center where dioxygen is reduced. `InterPro:IPR014241` InterPro IPR014241 is the source-neutral family for bacterial cytochrome c oxidase subunit I proteins. `PDB:1QLE` RCSB maps PDB:1QLE polymer entity 1 to Paracoccus denitrificans cytochrome c oxidase subunit 1-beta, UniProtKB:P98002.
cytochrome c oxidase subunit II	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	ctaC, ctaB, coiI, coxB	ESSENTIAL	CuA-bearing subunit on the cytochrome-c acceptor side of the terminal oxidase. `InterPro:IPR045187` InterPro IPR045187 describes the broader cytochrome c/quinol oxidase subunit II family and its electron transfer role. `InterPro:IPR014222` InterPro IPR014222 is the cytochrome c oxidase subunit-II domain entry, supporting the cytochrome-c-specific acceptor-side role without serving as an exact whole-protein grounding. `PDB:1QLE` RCSB maps PDB:1QLE polymer entity 2 to Paracoccus denitrificans cytochrome c oxidase subunit 2, UniProtKB:P08306.
cytochrome c oxidase subunit III	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	ctaE, coiII, coxC	UNKNOWN	Membrane core subunit of the aa3 cytochrome c oxidase complex. `InterPro:IPR024791` InterPro IPR024791 covers cytochrome c oxidase subunit III and related heme-copper oxidase subunit-III proteins. `PDB:1QLE` RCSB maps PDB:1QLE polymer entity 3 to Paracoccus denitrificans cytochrome c oxidase subunit 3, UniProtKB:P06030.
bacterial aa3-type cytochrome c oxidase subunit IV	Protein	part of	`InterPro:IPR012422`	ctaH	UNKNOWN	Small single-pass membrane subunit present in bacterial aa3 cytochrome c oxidases such as the Paracoccus complex. `InterPro:IPR012422` InterPro IPR012422 is the source-neutral entry for bacterial aa3-type cytochrome c oxidase subunit IV. `PDB:1QLE` RCSB maps PDB:1QLE polymer entity 4 to Paracoccus denitrificans cytochrome c oxidase subunit 4, UniProtKB:P77921.

Canonical examples

NCBITaxon:266 Paracoccus denitrificans — Bacterial source of the four-subunit aa3 cytochrome c oxidase structure deposited as PDB:1QLE. [PDB:1QLE]

Functions

cytochrome-c oxidase activity GO:0004129 — Complex IV oxidizes reduced cytochrome c, reduces molecular oxygen to water, and moves protons across the respiratory membrane.
- GO:0004129 GO:0004129 defines the cytochrome-c oxidase reaction as cytochrome c oxidation and oxygen reduction coupled to proton translocation.
- DOI:10.1074/jbc.274.47.33296 Harrenga and Michel 1999 solved the four-subunit Paracoccus denitrificans cytochrome c oxidase structure deposited as PDB:1QLE.

Mechanism graphs

Complex IV reduces oxygen with cytochrome c electrons (FUNCTION)

Respiratory complex IV oxidizes reduced cytochrome c, reduces oxygen to water, and contributes to a proton motive force across the respiratory membrane.

Subject	Predicate	Object	Evidence
respiratory chain complex IV	enables	cytochrome-c oxidase activity	`GO:0045277` GO:0045277 defines complex IV as the respiratory chain cytochrome c oxidase complex.
reduced cytochrome c	donates electrons to	cytochrome-c oxidase activity	`GO:0004129` GO:0004129 defines the reaction with reduced cytochrome c as the electron donor.
bacterial cytochrome c oxidase subunit I	contributes to	cytochrome-c oxidase activity	`InterPro:IPR014241` InterPro IPR014241 describes bacterial cytochrome c oxidase subunit I as the catalytic subunit containing the heme-copper binuclear center.
cytochrome c oxidase subunit II	contributes to	cytochrome-c oxidase activity	`InterPro:IPR014222` InterPro IPR014222 describes the cytochrome c oxidase subunit-II domain that forms the cytochrome-c electron-entry side of complex IV.
molecular oxygen	accepts electrons in	cytochrome-c oxidase activity	`GO:0004129` GO:0004129 records oxygen as the acceptor in the cytochrome-c oxidase reaction.
cytochrome-c oxidase activity	produces	water	`GO:0004129` GO:0004129 records water as the product of oxygen reduction by cytochrome-c oxidase.
cytochrome-c oxidase activity	contributes to	transmembrane proton motive force	`GO:0004129` GO:0004129 defines cytochrome-c oxidase activity as oxygen reduction coupled to proton movement.

Discussions

CURATION_TODO (OPEN): Resolve exact source-neutral families for cytochrome c oxidase subunits II and III, and add lineage-specific mitochondrial, cbb3, ba3, and caa3 accessory subunits without collapsing them into the Paracoccus aa3 four-subunit model. GO exactly grounds respiratory chain complex IV across mitochondrial and bacterial cytochrome c oxidases, but the first component model is anchored to the PDB:1QLE Paracoccus aa3 complex and should not invent exact family CURIEs for subunits whose InterPro entries are broad or domain-only.

Evidence

GO:0045277 Gene Ontology cellular-component term for respiratory chain complex IV.
GO:0098803 Gene Ontology cellular-component term for respiratory-chain protein complexes.
GO:0004129 Gene Ontology molecular-function term for cytochrome-c oxidase activity.
InterPro:IPR014241 InterPro family for bacterial cytochrome c oxidase subunit I.
InterPro:IPR045187 InterPro family for cytochrome c/quinol oxidase subunit II.
InterPro:IPR014222 InterPro domain entry for cytochrome c oxidase subunit II.
InterPro:IPR024791 InterPro family for cytochrome c oxidase subunit III.
InterPro:IPR012422 InterPro entry for bacterial aa3-type cytochrome c oxidase subunit IV.
DOI:10.1074/jbc.274.47.33296 Harrenga and Michel 1999, The Cytochrome C Oxidase from Paracoccus Denitrificans Does not Change the Metal Center Ligation Upon Reduction, Journal of Biological Chemistry.
PMID:10559205 PubMed identifier for Harrenga and Michel 1999 as reported by the RCSB PDB:1QLE primary citation.
PDB:1QLE RCSB Protein Data Bank entry for the Paracoccus denitrificans four-subunit cytochrome c oxidase structure from Harrenga and Michel 1999.

Curation history

2026-09-24T06:25:40Z codex · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-09-24T06:26:59Z codex · EXPANDED_RECORD — Added GO respiratory-chain parentage and synonymy, PDB:1QLE Paracoccus denitrificans cytochrome c oxidase components, InterPro-grounded subunit I and IV entries, reviewed-label-only subunit II and III entries, cytochrome-c oxidase activity, literature/PDB evidence, a compact complex IV function graph, and a component-grounding TODO.
2026-09-24T06:27:58Z codex · REVISED_COMPONENTS — Added Paracoccus denitrificans cytochrome c oxidase gene-symbol aliases ctaDII, ctaB, coiI, and coiII verified against the PDB:1QLE UniProtKB component accessions.
2026-09-24T06:39:50Z codex · ADDRESS_REVIEW — #1459/#1460: qualified the InterPro-grounded subunit I component as bacterial and replaced broad subunit-II graph evidence with the cytochrome-c-specific InterPro subunit-II domain entry.

Source: data/structures/energy_complex/respiratory_chain_complex_iv.yaml