reductive tricarboxylic acid cycle

Deep research

Generated by just research-trait; source: research/traits/metabolism/reductive_tca_cycle-deep-research-falcon.md

# Curation report: reductive tricarboxylic acid cycle

## Trait record and scope

**Trait:** reductive tricarboxylic acid cycle  
**Identifier:** `traitmech:000021`  
**Category/kind/status:** METABOLISM / CLASS / REVIEWED  
**Parent:** `traitmech:000019`  
**Synonyms:** reductive citric acid cycle; rTCA cycle; Arnon–Buchanan cycle.

### Recommended operational definition

This trait is the physiological capacity for **net autotrophic inorganic-carbon assimilation through cyclic reverse flux around the tricarboxylic-acid network**, yielding acetyl-CoA and central biosynthetic precursors. The pathway reverses most oxidative-TCA reactions but replaces three conventionally irreversible steps with (i) ATP-dependent citrate cleavage, (ii) fumarate reduction, and (iii) ferredoxin-dependent reductive carboxylation of succinyl-CoA to 2-oxoglutarate. In the canonical Chlorobium-type pathway, two CO2 molecules yield acetyl-CoA; downstream pyruvate synthase and PEP carboxylase reactions distribute fixed carbon into pyruvate/PEP and oxaloacetate. Berg estimated at least two ATP equivalents to form pyruvate in Chlorobium, plus three additional ATP equivalents to reach triose phosphates. The pathway uses reduced ferredoxin and NAD(P)H. (berg2011ecologicalaspectsof pages 5-6, berg2011ecologicalaspectsof media 8a2cffc6)

The trait is most securely associated with anaerobic or microaerophilic autotrophic bacteria, including green sulfur bacteria, Aquificota, Campylobacterota, Nitrospirota, and some Proteobacteria. Aquificota include thermophiles growing optimally at ≥70°C; *Aquifex aeolicus* can grow up to approximately 95°C. Nevertheless, oxygen tolerance is not an absolute exclusion: aerobic *Hydrogenobacter thermophilus* operates rTCA, and some sulfur-oxidizing symbionts encode both oxygen-sensitive rTCA and oxygen-tolerant CBB pathways. (berg2011ecologicalaspectsof pages 5-6, rubinblum2019geneticevidencefor pages 1-2)

### Inclusion criteria

Curate the trait as **experimentally supported** when there is evidence for net reverse-cycle flux, preferably autotrophic growth plus ^13CO2 incorporation, enzyme activity, metabolomics, or a combination of expression and physiological evidence. A complete diagnostic gene set is useful but establishes **genomic potential**, not the phenotype by itself.

A practical diagnostic core is:

1. citrate cleavage by `aclAB`, or the alternative `ccs` + `ccl` system;
2. ferredoxin-dependent 2-oxoglutarate synthase/oxidoreductase (`kor`/`oor`/`for` family, nomenclature taxon-dependent);
3. fumarate reductase;
4. the remaining reversible TCA reactions arranged so that oxaloacetate is regenerated.

### Boundary cases and exclusions

- **Oxidative TCA is not rTCA.** Shared enzymes do not establish reverse-cycle carbon fixation.
- **Reversed oxidative TCA (roTCA)** using ordinary citrate synthase under unusual thermodynamic conditions is a nearby but mechanistically distinct pathway and should not automatically be merged with canonical ACL/CCS–CCL rTCA. Ordinary citrate lyase can also support unusual variants, but bioinformatic discrimination is difficult. (garritano2022carbonfixationpathways pages 2-3)
- **Anaplerotic CO2 fixation is insufficient.** PEP carboxylase, pyruvate synthase, or reversible isocitrate-dehydrogenase activity alone does not demonstrate a closed autocatalytic rTCA cycle.
- **Incomplete rTCA segments** used biosynthetically or in heterotrophy should not receive the full trait.
- **Citrate cleavage alone is insufficient.** Eukaryotes commonly use ATP-citrate lyase for cytosolic acetyl-CoA production without operating rTCA. (sokolskyi2023roleofhorizontal pages 1-6)
- **Citrate-cleavage variants belong within the trait:** Chlorobium-type ACL and Aquificaceae-type CCS/CCL implement the same pathway function by one-step versus two-step chemistry. (sokolskyi2023roleofhorizontal pages 1-6, garritano2022carbonfixationpathways pages 2-3)
- **Aquificota low-CO2 variant:** biotin-dependent 2-oxoglutarate carboxylase can first produce oxalosuccinate, followed by nondecarboxylating isocitrate dehydrogenase. This is a taxon-specific implementation, not a universal required edge. (berg2011ecologicalaspectsof pages 4-5, scott2024widespreaddissolvedinorganic pages 13-15)

## Candidate nodes

### Pathway/process nodes

| Candidate | Suggested grounding | Curation note |
|---|---|---|
| reductive tricarboxylic acid cycle | `traitmech:000021`; KEGG module `M00173` | Primary trait node. |
| autotrophic CO2 fixation | GO label candidate; verify current GO CURIE before import | Parent biological capacity. |
| citrate-cleavage module | Label-only | Represent ACL and CCS/CCL alternatives beneath this node. |
| acetyl-CoA assimilation to pyruvate/PEP | Label-only | Downstream biosynthetic branch, not necessarily part of a minimal cycle definition. |
| dissolved inorganic carbon acquisition | Label-only | Modifier module involving transporters and carbonic anhydrases. |
| oxidative TCA cycle | KEGG pathway/module candidate | Explicit contrast node. |
| reversed oxidative TCA cycle | Label-only | Boundary-case pathway. |

### Enzymes, proteins, and genes

| Entity | Suggested identifier | Gene labels/notes |
|---|---|---|
| ATP-citrate lyase | `EC:2.3.3.8` | Usually `aclA`, `aclB` in bacteria; diagnostic citrate cleavage. |
| citryl-CoA synthetase | `EC:6.2.1.18` | Large/small subunits; nomenclature varies. |
| citryl-CoA lyase | `EC:4.1.3.34` | Completes two-step citrate cleavage. |
| fumarate reductase | `EC:1.3.5.4` for quinol:fumarate reductase where applicable | `frdABCD` or taxon-specific `tfrAB`; electron donor varies. |
| succinyl-CoA synthetase | `EC:6.2.1.5` | `sucCD`; reversible CoA activation step. |

Showing the first 60 of 227 lines of findings; the linked file also carries the run's front matter and the prompt it was given — read the full report.