respiratory chain complex I

GO:0045271 · Energy complex · Multiprotein complex · Proposed

Respiratory chain complex I is an L-shaped enzyme of the respiratory chain that oxidizes NADH, routes electrons through a hydrophilic peripheral arm to quinone, and couples quinone reduction to proton translocation through a membrane arm. [GO:0045271]

Synonyms: NADH dehydrogenase (ubiquinone) complex; NADH-Q oxidoreductase complex

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

Components

Component	Type	Part of / acts on	Grounding	Genes	Stoichiometry	Essential	Role
peripheral electron-transfer arm	Protein complex	part of	Reviewed label only (reviewed name; no accession asserted)			ESSENTIAL	Hydrophilic arm that binds the NADH-oxidizing module and an iron-sulfur relay, accepting electrons for delivery toward the quinone site. `GO:0045271` GO:0045271 defines complex I as an L-shaped respiratory-chain enzyme with an arm that projects from the membrane and receives NADH-derived electrons. `PMID:23417064` Baradaran et al. 2013 resolved the Thermus thermophilus complex I structure, including the hydrophilic electron-transfer domain and its iron-sulfur clusters.
membrane proton-pumping arm	Protein complex	part of	Reviewed label only (reviewed name; no accession asserted)			ESSENTIAL	Membrane-embedded antiporter-like arm that converts quinone-site redox chemistry into proton translocation across the membrane. `GO:0045271` GO:0045271 defines respiratory complex I as L-shaped, with one arm lying in the membrane. `PMID:23417064` Baradaran et al. 2013 resolved the T. thermophilus membrane arm and described antiporter-like domains linked to proton translocation.

Canonical examples

NCBITaxon:274 Thermus thermophilus — Thermophilic bacterium used by Baradaran et al. to solve the 3.3 A crystal structure of the entire respiratory complex I. [PMID:23417064]

Functions

NADH dehydrogenase (ubiquinone) activity GO:0008137 — Complex I oxidizes NADH, passes electrons through its peripheral arm to the quinone site, reduces quinone, and couples that redox reaction to proton translocation by the membrane arm.
- GO:0008137 GO:0008137 defines the complex I molecular function as NADH-dependent ubiquinone reduction coupled to proton translocation.
- PMID:23417064 Baradaran et al. 2013 connected the intact bacterial complex I structure to electron transfer, quinone reduction, and coupled proton translocation.

Mechanism graphs

Complex I couples NADH oxidation to proton pumping (FUNCTION)

Respiratory complex I uses a peripheral electron-transfer arm to route NADH-derived electrons to quinone and a membrane arm to couple quinone reduction to proton translocation.

Subject	Predicate	Object	Evidence
NADH	donates electrons to	peripheral electron-transfer arm	`PMID:23417064` Baradaran et al. 2013 resolved the hydrophilic electron-transfer domain and iron-sulfur relay in the intact Thermus thermophilus complex I structure.
peripheral electron-transfer arm	feeds electrons toward	quinone pool	`PMID:23417064` Baradaran et al. 2013 resolved the complete bacterial electron-transfer route leading toward the enclosed quinone-binding chamber.
NADH dehydrogenase (ubiquinone) activity	couples quinone reduction to	membrane proton-pumping arm	`PMID:23417064` Baradaran et al. 2013 proposed that redox energy from quinone reduction drives long-range conformational changes in antiporter-like membrane domains.
membrane proton-pumping arm	contributes to	transmembrane proton motive force	`PMID:23417064` Baradaran et al. 2013 resolved the antiporter-like membrane-arm domains and proposed that long-range conformational changes in those domains drive proton translocation.
membrane proton-pumping arm	contributes to	NADH dehydrogenase (ubiquinone) activity	`PMID:23417064` Baradaran et al. 2013 resolved the intact enzyme that couples NADH:ubiquinone electron transfer to proton translocation.

Discussions

CURATION_TODO (OPEN): Resolve exact source-neutral groundings for the conserved Nuo/Nqo NADH-dehydrogenase, iron-sulfur, quinone-site and antiporter-like complex I subfamilies before replacing arm-level reviewed-label-only components. GO exactly grounds the complete respiratory chain complex I, but a first generic record should not collapse bacterial Nuo/Nqo subunits, core mitochondrial homologs, iron-sulfur proteins, and accessory eukaryotic subunits into unsupported protein-family rows.

Evidence

GO:0045271 Gene Ontology cellular-component term for respiratory chain complex I.
GO:0098803 Gene Ontology cellular-component term for respiratory-chain protein complexes.
GO:0008137 Gene Ontology molecular-function term for NADH dehydrogenase (ubiquinone) activity.
PMID:23417064 Baradaran et al. 2013, Crystal structure of the entire respiratory complex I, Nature.
PDB:4HEA RCSB Protein Data Bank entry for the complete Thermus thermophilus respiratory complex I structure reported by Baradaran et al.

Curation history

2026-09-24T02:49:13Z codex · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-09-24T02:49:58Z codex · EXPANDED_RECORD — Added GO respiratory-chain parentage, GO synonymy, reviewed-label-only complex I arm components, a Thermus structural example, NADH:ubiquinone activity evidence, a compact complex I coupling graph, and a component-grounding TODO.
2026-09-24T02:52:35Z codex · REVISED_GRAPH — Retyped the NADH dehydrogenase ubiquinone activity graph node from BIOLOGICAL_PROCESS to MOLECULAR_FUNCTION to match GO:0008137.
2026-09-24T03:02:34Z codex · ADDRESS_REVIEW — #1444: narrowed the NADH-to-peripheral-arm edge evidence to Baradaran et al. 2013 and rewrote quinone coupling so the NADH:ubiquinone molecular function, not free quinone, drives the membrane-arm edge.
2026-09-24T03:20:19Z codex · ADDRESS_REVIEW — #1445/#1446: generalized the definition to remove mitochondrial matrix wording and cited Baradaran et al. 2013 on the membrane-arm proton-pumping edge.

Source: data/structures/energy_complex/respiratory_chain_complex_i.yaml