Wood-Ljungdahl pathway

Deep research

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

# Curation report: Wood–Ljungdahl pathway

## Trait record and scope

- **Trait label:** Wood–Ljungdahl pathway
- **Trait identifier:** **`traitmech:000022`**
- **Category / kind / status:** METABOLISM / CLASS / REVIEWED
- **Parent:** `traitmech:000019`
- **Synonym:** reductive acetyl-CoA pathway

### Recommended operational definition

For TraitMech, this trait should mean the **physiological capacity to operate the complete reductive Wood–Ljungdahl pathway (WLP)**: one C1 unit is reduced through a methyl branch, a second CO₂ is reduced to enzyme-bound CO in a carbonyl branch, and CODH/ACS combines the methyl group, CO, and CoA to produce acetyl-CoA. It is a linear, strictly anaerobic carbon-assimilation mechanism that may support both catabolism and anabolism. In acetogens, acetyl-CoA is commonly converted to acetate; acetate production itself, however, should not define pathway identity because methanogenic archaea and some sulfate-reducing bacteria use homologous modules in different physiological contexts. A 2023 experimental paper describes the pathway in strictly anaerobic acetogens, methanogens, and sulfate reducers and explicitly states that it forms acetyl-CoA from two CO₂ molecules. (moon2023anewmetabolic pages 1-2)

The defining graph endpoint should therefore be **acetyl-CoA formation**, not acetate, ethanol, growth, or autotrophy alone. Acetate formation and energy conservation are important downstream consequences in acetogens. Recent reviews describe more than 100 acetogenic species distributed across roughly 23–28 genera, with most isolates in Firmicutes/Bacillota but representatives also reported among Spirochaetes, Desulfobacterota, and Acidobacteria. The discrepancy in genus counts reflects source scope and taxonomy rather than a mechanistic difference. (frolov2023obligateautotrophyat pages 1-2, zhang2024engineeredacetogenicbacteria pages 1-2)

### Boundary cases

1. **Acetogenesis is not identical to WLP presence.** Acetogenesis is an organismal physiology in which acetate is a major reduced product; the WLP is the underlying carbon-fixation/acetyl-CoA module. Curate `produces acetate` as an acetogen-context consequence, not a universal defining edge. (zhang2024engineeredacetogenicbacteria pages 2-3, moon2023anewmetabolic pages 1-2)
2. **Methanogenesis is a nearby but distinct trait.** Methanogens can use the methyl branch or related WLP machinery, but carbon flow and terminal energy metabolism may lead to methane rather than acetate.
3. **Reverse/oxidative WLP is directionally distinct.** Some organisms oxidize acetyl-CoA through pathway homologues. Gene presence alone does not establish the reductive trait; directionality requires physiological, transcriptomic, flux, or thermodynamic evidence.
4. **Partial pathways are insufficient.** FDH, FolD, CODH, ACS-like proteins, or an incomplete methyl branch can participate in other C1 processes. A genome lacking branch convergence at functional CODH/ACS should be annotated as `partial WLP module`, not as the complete trait.
5. **Hydrogenogenic CO oxidation is distinct.** CODH-mediated CO oxidation coupled to H₂ evolution—the water–gas shift reaction—can occur without acetyl-CoA synthesis. The reported standard free-energy change is approximately −20 kJ mol⁻¹ CO. (bahrle2023currentstatusof pages 8-9)
6. **Reductive glycine metabolism is distinct.** It shares formate/THF chemistry but uses glycine-cleavage-system reactions rather than the canonical CFeSP–CODH/ACS convergence.
7. **Autotrophy alone is nonspecific.** Six other recognized CO₂-fixation routes exist, and heterotrophic or mixotrophic organisms can use the WLP as an electron sink.

## Current mechanistic understanding

The WLP has two converging branches. In the **methyl branch**, CO₂ is reduced to formate by FDH; in organisms such as *Acetobacterium woodii* and *Thermoanaerobacter kivui*, a hydrogen-dependent CO₂ reductase, HDCR, performs this reaction. Formate is ATP-dependently attached to tetrahydrofolate and then converted through methenyl-, methylene-, and methyl-THF. The methyl group is transferred to a corrinoid iron–sulfur protein (CFeSP). In the **carbonyl branch**, Ni,Fe-CODH reduces a second CO₂ to CO. Acetyl-CoA synthase receives the CFeSP methyl group and combines it with CO and CoA to form acetyl-CoA. (zhang2024engineeredacetogenicbacteria pages 2-3, moon2023anewmetabolic pages 1-2, bahrle2023currentstatusof pages 8-9, davin2024clostridiumautoethanogenumalters pages 1-2)

The pathway operates close to the energetic limit of life. Reported standard free energies for H₂/CO₂ acetogenesis are −95 to −104 kJ mol⁻¹ acetate, depending on reaction convention. The CO₂-to-CO step is a major thermodynamic barrier, with an approximate standard reduction potential of −520 mV and a requirement for low-potential electrons, commonly supplied by reduced ferredoxin. (moon2023anewmetabolic pages 1-2, frolov2023obligateautotrophyat pages 1-2)

Net energy conservation cannot be represented solely as substrate-level phosphorylation. Conversion of acetyl-CoA through phosphotransacetylase and acetate kinase generates ATP, but upstream formyl-THF synthesis consumes ATP. Acetogens consequently depend on chemiosmotic coupling. Rnf-type acetogens couple reduced-ferredoxin oxidation to NAD⁺ reduction and Na⁺ or H⁺ translocation; Ech-type acetogens use an energy-converting hydrogenase. The resulting ion gradient drives ATP synthase. Rnf and Ech are **alternative, taxon-dependent modules**, not universal components of the WLP. (zhang2024engineeredacetogenicbacteria pages 2-3, bahrle2023currentstatusof pages 8-9, frolov2023obligateautotrophyat pages 1-2)

## Candidate nodes and ontology grounding

Identifiers below are deliberately conservative. Where an exact stable identifier was not verified, a label-only node is preferable to an invented CURIE.

### Trait, pathway, and process nodes

| Candidate node | Suggested grounding | Curation note |
|---|---|---|
| Wood–Ljungdahl pathway | `traitmech:000022` | Root trait node; quote CURIE verbatim in YAML. |
| Methyl branch of WLP | Label only | Pathway module, not independently sufficient for trait assertion. |
| Carbonyl branch of WLP | Label only | Pathway module centered on CODH. |
| Anaerobic carbon fixation | GO label candidate | Verify exact GO term before import. |
| Acetogenesis | GO or METPO label candidate | Downstream physiology, not synonymous with WLP. |
| Chemiosmotic ATP synthesis | GO label candidate | Consequence of Rnf/Ech-generated gradient. |
| Autotrophic growth on H₂ + CO₂ | METPO label candidate | Assay phenotype supporting pathway operation. |
| Autotrophic growth on CO | METPO label candidate | Carboxydotrophic assay phenotype; not specific by itself. |

### Chemicals and cofactors

| Node | Suggested CURIE |
|---|---|
| carbon dioxide | `CHEBI:16526` |
| carbon monoxide | `CHEBI:17245` |
| formate | `CHEBI:15740` |
| hydrogen | `CHEBI:18276` |
| coenzyme A | `CHEBI:15346` |

Showing the first 60 of 218 lines of findings; the linked file also carries the run's front matter and the prompt it was given — read the full report.