respiratory chain complex II (succinate dehydrogenase)

GO:0045273 · Energy complex · Multiprotein complex · Proposed

Respiratory chain complex II (succinate dehydrogenase) is a respiratory-chain enzyme complex that oxidizes succinate, passes electrons through flavin and iron-sulfur centers to membrane quinone, and thereby connects tricarboxylic-acid-cycle electron flow with the respiratory quinone pool. [GO:0045273]

Synonyms: electron transport complex II

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
succinate dehydrogenase flavoprotein subunit	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	sdhA	ESSENTIAL	Catalytic flavoprotein subunit that oxidizes succinate at FAD and begins electron transfer through the complex II soluble head. `GO:0045273` GO defines respiratory chain complex II as a succinate dehydrogenase complex that contains flavin-adenine dinucleotide and iron-sulfur. `PDB:1NEK` RCSB maps PDB:1NEK polymer entity 1 to the E. coli SdhA flavoprotein subunit.
succinate dehydrogenase iron-sulfur subunit	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	sdhB	ESSENTIAL	Iron-sulfur subunit that relays electrons from the FAD site of SdhA toward the membrane quinone-reduction site. `DOI:10.1126/science.1079605` Yankovskaya et al. 2003 resolved the four-subunit E. coli complex II architecture with an iron-sulfur protein linking the flavoprotein to the membrane anchor. `PDB:1NEK` RCSB maps PDB:1NEK polymer entity 2 to the E. coli SdhB iron-sulfur protein.
succinate dehydrogenase SdhCD membrane anchor	Protein complex	part of	Reviewed label only (reviewed name; no accession asserted)	sdhC, sdhD	ESSENTIAL	Membrane-anchor module that binds quinone-site cofactors and connects the soluble succinate-oxidizing subunits to the respiratory membrane. `DOI:10.1126/science.1079605` Yankovskaya et al. 2003 resolved the E. coli SdhC and SdhD transmembrane subunits as the membrane domain of respiratory complex II. `PDB:1NEK` RCSB maps PDB:1NEK polymer entities 3 and 4 to E. coli SdhC and SdhD, respectively.

Canonical examples

NCBITaxon:562 Escherichia coli — Bacterial source of the four-protein succinate dehydrogenase structure deposited as PDB:1NEK. [PDB:1NEK]

Functions

succinate dehydrogenase (quinone) activity GO:0008177 — Complex II oxidizes succinate to fumarate while reducing a membrane quinone to quinol.
- GO:0008177 GO:0008177 defines the overall succinate plus quinone to fumarate plus quinol reaction of the entire succinate dehydrogenase complex.
- DOI:10.1126/science.1079605 Yankovskaya et al. 2003 structurally tied the E. coli complex II subunit layout to succinate oxidation and quinone reduction.

Mechanism graphs

Complex II links succinate oxidation to quinone reduction (FUNCTION)

Respiratory complex II accepts electrons from succinate at its flavoprotein, relays them through iron-sulfur centers, and reduces a membrane quinone at its anchor.

Subject	Predicate	Object	Evidence
succinate	feeds electrons to	succinate dehydrogenase flavoprotein subunit	`DOI:10.1126/science.1079605` Yankovskaya et al. 2003 resolved the E. coli complex II flavoprotein subunit on the succinate-oxidizing side of the enzyme.
succinate dehydrogenase flavoprotein subunit	passes electrons to	succinate dehydrogenase iron-sulfur subunit	`DOI:10.1126/science.1079605` Yankovskaya et al. 2003 described the FAD-to-iron-sulfur architecture of E. coli complex II.
succinate dehydrogenase iron-sulfur subunit	relays electrons toward	succinate dehydrogenase SdhCD membrane anchor	`DOI:10.1126/science.1079605` Yankovskaya et al. 2003 placed the iron-sulfur subunit between the flavoprotein and SdhC/SdhD membrane subunits in the E. coli structure.
succinate dehydrogenase SdhCD membrane anchor	binds	ubiquinone Q2	`PDB:1NEK` The RCSB PDB:1NEK deposition resolves the E. coli SdhC/SdhD transmembrane subunits with bound ubiquinone Q2. `DOI:10.1126/science.1079605` Yankovskaya et al. 2003 resolved the E. coli membrane anchor that carries bound ubiquinone on the quinone side of the complex.
respiratory chain complex II (succinate dehydrogenase)	enables	succinate dehydrogenase (quinone) activity	`GO:0045273` GO:0045273 defines respiratory chain complex II as the succinate dehydrogenase complex that catalyzes succinate oxidation by ubiquinone. `PDB:1NEK` PDB:1NEK resolves the E. coli complex II SdhA, SdhB, SdhC, and SdhD proteins in one four-polymer structure.

Discussions

CURATION_TODO (OPEN): Resolve exact source-neutral InterPro, Pfam, NCBIfam, or Complex Portal groundings for respiratory complex II subunits before replacing reviewed-label-only rows. The first record deliberately models the conserved four-polypeptide architecture without asserting that domain-level Pfam matches exactly ground full taxon-agnostic protein families.

Evidence

GO:0045273 Gene Ontology cellular-component term for respiratory chain complex II, succinate dehydrogenase.
GO:0098803 Gene Ontology cellular-component term for respiratory-chain protein complexes.
GO:0008177 Gene Ontology molecular-function term for the overall succinate dehydrogenase quinone oxidoreductase activity.
DOI:10.1126/science.1079605 Yankovskaya et al. 2003, Architecture of succinate dehydrogenase and reactive oxygen species generation, Science.
PMID:12560550 PubMed identifier for Yankovskaya et al. 2003 as reported by the RCSB PDB:1NEK primary citation.
PDB:1NEK RCSB Protein Data Bank entry for the E. coli respiratory complex II structure from Yankovskaya et al. 2003.

Curation history

2026-09-24T03:46:49Z codex · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-09-24T03:48:33Z codex · EXPANDED_RECORD — Added GO respiratory-chain parentage, GO synonymy and molecular function, three reviewed-label-only Sdh component rows, an E. coli PDB:1NEK canonical example, literature/PDB evidence, a compact complex II function graph, and a component-grounding TODO.
2026-09-24T03:49:07Z codex · REVISED_GRAPH — Retyped the terminal function edge so complete respiratory complex II, not the SdhCD membrane anchor alone, enables GO:0008177.
2026-09-24T03:58:25Z codex · NARROWED_GRAPH_EDGE — Weakened the SdhCD-to-quinone graph edge from quinone-pool reduction to quinone-site binding and cited both PDB:1NEK and Yankovskaya et al. 2003 for the membrane-anchor quinone site.
2026-09-24T04:00:33Z codex · NARROWED_GRAPH_EDGE — Retyped the quinone-side graph node from a generic pool to bound ubiquinone Q2 and narrowed the SdhCD edge predicate to ligand binding.

Source: data/structures/energy_complex/respiratory_chain_complex_ii_succinate_dehydrogenase.yaml