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reductive tricarboxylic acid cycle

traitmech:000021 · CLASS · REVIEWED

An autotrophic carbon-fixation pathway (reductive citric acid / Arnon-Buchanan cycle) that runs the tricarboxylic acid cycle in reverse to fix CO2. It operates in anaerobic and microaerophilic bacteria such as green sulfur bacteria (Chlorobium) and Aquificales.

Reductive TCA cycle runs the TCA cycle in reverse to fix CO2

Evidence-backed causal sketch linking reverse TCA cycle operation in anaerobic/microaerophilic autotrophs to CO2 fixation.

Reductive TCA cycle runs the TCA cycle in reverse to fix CO2 Interactive directed graph showing evidence-backed causal relationships for reductive tricarboxylic acid cycle.

Edge evidence

  • carbon dioxide fixed by reductive tricarboxylic acid cycle METPO:2007404

    CO2 is fixed by the reverse TCA cycle in anaerobic autotrophs.

    • DOI:10.1128/AEM.02473-10 Berg describes the reductive citric acid cycle as functional in anaerobic/microaerophilic autotrophs.
  • reductive tricarboxylic acid cycle produces citrate METPO:2000202

    The reverse cycle assembles citrate from CO2.

    • DOI:10.1146/annurev-marine-120709-142712 Hügler & Sievert document the rTCA cycle in chemolithoautotrophs and green sulfur bacteria.
  • ATP citrate lyase (aclAB) enables reductive tricarboxylic acid cycle RO:0002327

    ATP citrate lyase cleaves citrate into oxaloacetate and acetyl-CoA, enabling the rTCA cycle.

    • DOI:10.1101/2022.10.25.513756 Citrate cleavage into oxaloacetate and acetyl-CoA can be performed by ATP-citrate lyase (ACL).
  • citryl-CoA synthetase/lyase route alternative to ATP citrate lyase (aclAB)

    The two-enzyme CCS/CCL system is an alternative route to ACL for citrate cleavage in rTCA.

    • DOI:10.1101/2022.10.25.513756 Citrate cleavage can be performed by either a two-enzyme system (CCS/CCL) or a single enzyme (ACL).
  • pyruvate:ferredoxin oxidoreductase (PFOR) part of reductive tricarboxylic acid cycle biolink:part_of

    Pyruvate:ferredoxin oxidoreductase is a canonical key enzyme of the rTCA cycle.

    • DOI:10.3390/life13030627 Key enzymes of this pathway are pyruvate:ferredoxin oxidoreductase (PFOR) and 2-oxoglutarate:ferredoxin oxidoreductase (OGOR).
  • 2-oxoglutarate:ferredoxin oxidoreductase (OGOR/KOR) part of reductive tricarboxylic acid cycle biolink:part_of

    2-oxoglutarate:ferredoxin oxidoreductase is a canonical key enzyme of the rTCA cycle.

    • DOI:10.3390/life13030627 Key enzymes of this pathway are PFOR and 2-oxoglutarate:ferredoxin oxidoreductase (OGOR).
  • reduced ferredoxin electron donor for pyruvate:ferredoxin oxidoreductase (PFOR)

    Reduced low-potential ferredoxin provides electrons to PFOR for reductive carboxylation.

    • DOI:10.3390/life13030627 PFOR and OGOR require a strong reduction potential; Fd6/Fd7 could be the physiological electron donors.
  • reduced ferredoxin electron donor for 2-oxoglutarate:ferredoxin oxidoreductase (OGOR/KOR)

    Reduced low-potential ferredoxin provides electrons to OGOR for reductive carboxylation.

    • DOI:10.3390/life13030627 Fd6 and Fd7 can physically interact and exchange electrons with both PFOR and OGOR.

Provenance

Source
METPO (2025-11-25)
Definition source
DOI:10.1128/AEM.02473-10

Parent traits (1)

Synonyms (3)

  • reductive citric acid cycle EXACT_SYNONYM · DOI:10.1128/AEM.02473-10
  • rTCA cycle RELATED_SYNONYM · DOI:10.1128/AEM.02473-10
  • Arnon-Buchanan cycle RELATED_SYNONYM · DOI:10.1146/annurev-marine-120709-142712

kg-microbe context

Matched 1 kg-microbe node via parent_proxy.

  • METPO:1000060 [-1.052, -1.766, -1.194, +0.291, …]

512-dim DeepWalkSkipGramEnsmallen embedding from kg-microbe (2026-04-25).

Nearest neighbors in embedding space

Top-8 cosine-similar METPO traits from the 2026-04-25 deepwalk (512-D).

Curation history

  1. · PROPOSED_FROM_RESEARCH · claude

    Proposed candidate METABOLISM trait (reductive TCA carbon-fixation cycle); sub-variant of carbon fixation.

  2. · CURATED_CAUSAL_GRAPH · claude

    Added evidence-backed causal graph (rTCA reverse TCA / CO2 fixation) with CHEBI node groundings and METPO predicate groundings; promoted PROPOSED to REVIEWED.

  3. · ENRICH_CAUSAL_GRAPH · claude

    Added 6 evidence-backed generic edges (5 new nodes) from the deep-research report.

  4. · GROUND_CAUSAL_PREDICATES · claude

    Grounded 3 causal-edge predicate_id field(s) via mappings/predicate_grounding.tsv (biolink:part_of×2, RO:0002327×1).

  5. · GROUND_CAUSAL_NODES · claude

    Grounded 1 causal-node grounding field(s) via mappings/node_grounding.tsv (CHEBI:17513×1).