fumarate reductase complex

GO:0045283 · Energy complex · Multiprotein complex · Proposed

A membrane-bound four-subunit respiratory quinol:fumarate reductase complex whose extrinsic catalytic module and membrane-anchor module couple fumarate reduction to quinol oxidation. [GO:0045283]

Synonyms: quinol-fumarate reductase

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
Fumarate reductase, flavoprotein subunit	Protein	part of	`InterPro:IPR005884`	frdA	ESSENTIAL	FAD-binding flavoprotein subunit in the membrane-extrinsic catalytic domain of the fumarate reductase complex. `InterPro:IPR005884` InterPro IPR005884 is the source-neutral family for the fumarate reductase flavoprotein subunit. `PDB:1KF6` RCSB maps PDB:1KF6 polymer entity 1 to the E. coli FrdA flavoprotein subunit, UniProtKB:P00363.
fumarate reductase iron-sulfur subunit	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	frdB	ESSENTIAL	Iron-sulfur subunit in the membrane-extrinsic catalytic domain that links the flavoprotein to the FrdC/FrdD membrane anchor. `GO:0045283` GO:0045283 defines subunits A and B as the fumarate reductase membrane-extrinsic catalytic domain. `PDB:1KF6` RCSB maps PDB:1KF6 polymer entity 2 to the E. coli FrdB iron-sulfur subunit, UniProtKB:P0AC47.
Fumarate reductase, subunit C	Protein	part of	`InterPro:IPR003510`	frdC	ESSENTIAL	Fifteen-kilodalton hydrophobic membrane-anchor subunit that helps link the catalytic centers of fumarate reductase to the respiratory electron-transfer chain. `InterPro:IPR003510` InterPro IPR003510 is the source-neutral family for Fumarate reductase, subunit C. `PDB:1KF6` RCSB maps PDB:1KF6 polymer entity 3 to the E. coli FrdC hydrophobic subunit, UniProtKB:P0A8Q0.
Fumarate reductase, subunit D	Protein	part of	`InterPro:IPR003418`	frdD	ESSENTIAL	Thirteen-kilodalton hydrophobic membrane-anchor subunit that helps anchor the fumarate reductase catalytic module to the cytoplasmic membrane. `InterPro:IPR003418` InterPro IPR003418 is the source-neutral family for Fumarate reductase, subunit D. `PDB:1KF6` RCSB maps PDB:1KF6 polymer entity 4 to the E. coli FrdD hydrophobic subunit, UniProtKB:P0A8Q3.

Canonical examples

NCBITaxon:562 Escherichia coli — Bacterial source of the four-subunit quinol:fumarate reductase structure deposited as PDB:1KF6. [PDB:1KF6]

Functions

quinol:fumarate reductase activity — Fumarate reductase oxidizes quinol to quinone while reducing fumarate to succinate during anaerobic respiratory electron transport.
- GO:0045283 GO:0045283 defines fumarate reductase by the quinol-to-quinone and fumarate-to-succinate reaction opposite to respiratory complex II.
- DOI:10.1126/science.284.5422.1961 Iverson et al. 1999 solved the structure of the E. coli fumarate reductase respiratory complex.

Mechanism graphs

Fumarate reductase couples quinol oxidation to fumarate reduction (FUNCTION)

The fumarate reductase FrdAB catalytic domain and FrdCD membrane anchor form a respiratory complex that oxidizes quinol and reduces fumarate during anaerobic electron transport.

Subject	Predicate	Object	Evidence
fumarate reductase complex	enables	quinol:fumarate reductase activity	`GO:0045283` GO:0045283 defines the four-subunit fumarate reductase complex by quinol oxidation coupled to fumarate reduction.
Fumarate reductase, flavoprotein subunit	assembles with	fumarate reductase iron-sulfur subunit	`GO:0045283` GO:0045283 states that subunits A and B comprise the fumarate reductase membrane-extrinsic catalytic domain.
Fumarate reductase, subunit C	contributes to	fumarate reductase complex	`InterPro:IPR003510` InterPro IPR003510 defines Fumarate reductase, subunit C as a 15 kDa hydrophobic subunit of the four-subunit complex.
Fumarate reductase, subunit D	contributes to	fumarate reductase complex	`InterPro:IPR003418` InterPro IPR003418 defines fumarate reductase subunit D as the 13 kDa hydrophobic subunit.
quinol	donates electrons to	quinol:fumarate reductase activity	`GO:0045283` GO:0045283 records quinol oxidation as coupled to fumarate reduction by the complex.
fumarate	accepts electrons in	quinol:fumarate reductase activity	`GO:0045283` GO:0045283 records fumarate reduction to succinate as part of the complex's reaction.
quinol:fumarate reductase activity	produces	quinone	`GO:0045283` GO:0045283 records quinone production as the oxidized side of the quinol:fumarate reaction.
quinol:fumarate reductase activity	produces	succinate	`GO:0045283` GO:0045283 records succinate production as the reduced side of the quinol:fumarate reaction.

Discussions

CURATION_TODO (OPEN): Resolve an exact source-neutral protein-family identifier for fumarate reductase subunit B before replacing its reviewed-label-only component row. InterPro IPR004489 covers succinate dehydrogenase and fumarate reductase iron-sulfur proteins, so this record grounds exact FrdA, FrdC, and FrdD families while leaving FrdB ungrounded until a FrdB-specific family is verified.

Evidence

GO:0045283 Gene Ontology cellular-component term for the fumarate reductase complex.
GO:0098803 Gene Ontology cellular-component term for respiratory-chain protein complexes.
InterPro:IPR005884 InterPro family for the fumarate reductase FrdA flavoprotein subunit.
InterPro:IPR003510 InterPro family for fumarate reductase subunit C.
InterPro:IPR003418 InterPro family for fumarate reductase subunit D.
DOI:10.1126/science.284.5422.1961 Iverson et al. 1999, Structure of the Escherichia coli Fumarate reductase respiratory complex, Science.
PMID:10373108 PubMed identifier for the Iverson et al. 1999 E. coli fumarate reductase respiratory-complex structure paper as reported by InterPro.
DOI:10.1074/jbc.M200815200 Iverson et al. 2002, Crystallographic studies of the Escherichia coli quinol-fumarate reductase with inhibitors bound to the quinol-binding site, Journal of Biological Chemistry.
PMID:11850430 PubMed identifier for Iverson et al. 2002 as reported by the RCSB PDB:1KF6 primary citation.
PDB:1KF6 RCSB Protein Data Bank entry for the E. coli quinol:fumarate reductase structure from Iverson et al. 2002.

Curation history

2026-09-24T05:14:44Z codex · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-09-24T05:15:37Z codex · EXPANDED_RECORD — Added respiratory-chain parentage, FrdA/FrdB/FrdC/FrdD component rows, an E. coli PDB:1KF6 canonical example, quinol:fumarate reductase activity, GO/InterPro/DOI/PMID/PDB evidence, a compact quinol-coupling function graph, and an FrdB family-grounding TODO.
2026-09-24T05:16:23Z codex · NARROWED_GRAPH_EDGE — Narrowed FrdC and FrdD graph edges from anchoring FrdA specifically to evidence-backed constituent contributions to the complete fumarate reductase complex.
2026-09-24T05:27:24Z codex · RESOLVED_REVIEW_ISSUE — Resolved issue #1452 by adding source-attested quinol-fumarate reductase synonymy from the PDB:1KF6 primary citation title.

Source: data/structures/energy_complex/fumarate_reductase_complex.yaml