3-hydroxypropionate/4-hydroxybutyrate cycle

Deep research

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

# Curation report: 3-hydroxypropionate/4-hydroxybutyrate cycle

## Trait record

- **Trait label:** 3-hydroxypropionate/4-hydroxybutyrate cycle
- **Trait identifier:** `traitmech:000024`
- **Category / kind / status:** METABOLISM / CLASS / REVIEWED
- **Parent:** `traitmech:000019`
- **Synonym:** 3HP/4HB cycle

## 1. Scope and current definition

This trait should represent the **physiological capacity for autotrophic inorganic-carbon assimilation through the complete 3HP/4HB cycle**, rather than possession of an isolated enzyme or production of 3-hydroxypropionate. Its defining topology is:

**acetyl-CoA → malonyl-CoA → malonate semialdehyde → 3-hydroxypropionate → 3-hydroxypropionyl-CoA → acryloyl-CoA → propionyl-CoA → methylmalonyl-CoA → succinyl-CoA → succinate semialdehyde → 4-hydroxybutyrate → 4-hydroxybutyryl-CoA → crotonyl-CoA → (S)-3-hydroxybutyryl-CoA → acetoacetyl-CoA → two acetyl-CoA.**

Two bicarbonate/carbon-dioxide equivalents are incorporated as acetyl-CoA is converted to succinyl-CoA. The C4 intermediate is reduced through 4-hydroxybutyrate and ultimately cleaved into two acetyl-CoA molecules: one regenerates the carbon acceptor and the other is available for biosynthesis. The experimentally described *Metallosphaera sedula* cycle comprises 13 enzymes catalyzing 16 reactions. (hawkins2014conversionof4hydroxybutyrate pages 1-2, hawkins2013roleof4hydroxybutyratecoa pages 1-2, liu2021(s)3hydroxybutyrylcoadehydrogenasefrom pages 1-2)

### Validated biological scope

Two independently evolved variants should be represented under this trait:

1. **Crenarchaeal/Sulfolobales variant:** associated with thermoacidophilic, hydrogen-oxidizing, aerobic or microaerophilic organisms such as *Metallosphaera* and *Sulfolobus*.
2. **Thaumarchaeal/Nitrososphaeria variant:** associated with mesophilic, aerobic ammonia-oxidizing archaea such as *Nitrosopumilus maritimus*.

The variants are homologous in overall pathway topology but differ in several enzyme families and energetic costs. In particular, thaumarchaeal 3-hydroxypropionyl-CoA and 4-hydroxybutyryl-CoA synthetases are ADP-forming, whereas corresponding Sulfolobales enzymes are AMP-forming. This saves two ATP equivalents per cycle turn and is interpreted as adaptation to the low-energy, oligotrophic niche of ammonia-oxidizing archaea. (liu2021convergentevolutionof pages 1-2, johnson2024crystalstructureof pages 1-2)

### Boundary cases

- **Not the bacterial 3-hydroxypropionate bicycle.** That pathway shares acetyl-CoA carboxylation and 3HP chemistry but has a different cycle architecture and lacks the defining 4HB-to-two-acetyl-CoA regeneration arm.
- **Not the dicarboxylate/4-hydroxybutyrate cycle.** DC/4HB shares the succinyl-CoA→4HB→two-acetyl-CoA arm, but fixes carbon through ferredoxin-dependent pyruvate synthase and phosphoenolpyruvate carboxylase. The 3HP/4HB cycle instead uses acetyl-CoA/propionyl-CoA carboxylase twice and converts malonyl-CoA through 3HP and propionyl-CoA. DC/4HB is classically associated with anaerobic Thermoproteales and Desulfurococcales. (liu2021convergentevolutionof pages 1-2, liu2021(s)3hydroxybutyrylcoadehydrogenasefrom pages 1-2)
- **Partial 3HP production is insufficient.** Heterologous expression of the acetyl-CoA→3HP module does not establish the complete trait.
- **A single marker gene is insufficient.** Acetyl-CoA/propionyl-CoA carboxylases, enoyl-CoA hydratases, dehydrogenases, and thiolases have functions outside this cycle.
- **Genomic potential is not demonstrated physiology.** Bacterial MAGs predicted to encode the pathway and newly inferred geothermal carriers should remain provisional until pathway completeness, directionality, expression, and carbon-fixation flux are established. (garritano2022carbonfixationpathways pages 1-2, qi2024analysisofnearly pages 7-8)

## 2. Candidate nodes

### A. Pathway and process nodes

- `traitmech:000024` — 3-hydroxypropionate/4-hydroxybutyrate cycle
- `traitmech:000019` — supplied parent trait
- Autotrophic inorganic-carbon fixation — label-only unless the project already specifies its preferred GO/METPO term
- Bicarbonate assimilation
- Acetyl-CoA regeneration
- Biosynthetic acetyl-CoA production
- Crenarchaeal 3HP/4HB variant
- Thaumarchaeal modified 3HP/4HB variant

### B. Chemicals and cofactors

Recommended stable chemical identifiers, where unambiguous:

- Carbon dioxide — `CHEBI:16526`
- Hydrogencarbonate/bicarbonate — `CHEBI:17544`
- Acetyl-CoA — `CHEBI:15351`
- Malonyl-CoA — `CHEBI:15531`
- 3-hydroxypropionic acid/3-hydroxypropionate — `CHEBI:33404`
- Propionyl-CoA — `CHEBI:15539`
- Succinyl-CoA — `CHEBI:15380`
- 4-hydroxybutyrate — use a locally verified ChEBI record; do not assign from name alone because protonation-state records differ

Showing the first 60 of 278 lines of findings; the linked file also carries the run's front matter and the prompt it was given — read the full report.