Calvin-Benson-Bassham cycle

Deep research

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

# Curation report: Calvin–Benson–Bassham cycle

## Trait record and scope

- **Trait:** Calvin–Benson–Bassham (CBB) cycle
- **Identifier:** `traitmech:000020`
- **Category / kind / status:** METABOLISM / CLASS / REVIEWED
- **Parent:** `traitmech:000019`
- **Synonyms:** Calvin cycle; reductive pentose-phosphate cycle

### Recommended operational definition

This trait represents the genetically encoded and physiologically operable **autocatalytic pathway that incorporates inorganic carbon into biomass through Rubisco-dependent carboxylation of ribulose-1,5-bisphosphate (RuBP), ATP/NADPH-dependent reduction of 3-phosphoglycerate (3-PGA), and phosphoribulokinase (PRK)-dependent regeneration of RuBP**. Fixation of three CO2 to one net glyceraldehyde-3-phosphate requires nine ATP and six NADPH. Rubisco and PRK are the most diagnostic enzymes; much of the remaining chemistry overlaps glycolysis, gluconeogenesis, and the pentose-phosphate pathway. (berg2011ecologicalaspectsof pages 3-4, wang2023microbialconversionand pages 2-3, meloni2023ribulose15bisphosphateregenerationin pages 1-2, prywes2023rubiscofunctionevolution pages 10-13)

The CBB cycle is quantitatively the dominant autotrophic pathway and occurs in cyanobacteria and diverse Proteobacteria, among other bacteria. It can be powered by oxygenic photosynthesis or by chemolithotrophic oxidation of compounds such as H2, reduced sulfur, Fe(II), ammonia, or nitrite. A 2024 environmental synthesis estimated that CBB accounts for **>99% of planetary autotrophy**, although that estimate is dominated by oxygenic phototrophs rather than microbes alone. (berg2011ecologicalaspectsof pages 2-3, harrison2024prevalenceofthe pages 1-5)

### Inclusion and exclusion boundaries

**Include:** complete native or engineered CBB operation; Rubisco carboxylation; ATP/NADPH-dependent 3-PGA reduction; RuBP regeneration through PRK; directly supporting carbon-concentrating mechanisms (CCMs), activases, regulators, and phosphoglycolate salvage when represented as modifiers.

**Do not infer the trait from Rubisco alone.** Forms III and some II/III Rubiscos can participate in nucleoside salvage or other pathways, while Form IV Rubisco-like proteins generally do not carboxylate RuBP. Proteobacteria may encode several Rubisco forms, so gene context and PRK evidence are important. (prywes2023rubiscofunctionevolution pages 10-13, harrison2024prevalenceofthe pages 1-5)

**Nearby but distinct traits/processes:**

1. rTCA, Wood–Ljungdahl, 3-hydroxypropionate, 3HP/4HB, DC/4HB, reductive-glycine, and synthetic carbon-fixation cycles.
2. Isolated anaplerotic CO2 incorporation by PEP or pyruvate carboxylase.
3. C4 and CAM carbon-concentrating adaptations, which deliver CO2 to CBB but are not the CBB cycle itself.
4. Carboxysomes and bicarbonate uptake: supporting CCM modules, not universal defining components.
5. Rubisco oxygenation and phosphoglycolate salvage: competing/repair processes, not positive evidence of productive CBB flux.
6. Photoheterotrophic CBB activity as an electron sink: genuine cycle activity, but not necessarily autotrophic growth. (berg2011ecologicalaspectsof pages 3-4, berg2011ecologicalaspectsof pages 2-3)

## Candidate nodes

### Core pathway and processes

- Calvin–Benson–Bassham cycle — `traitmech:000020`; candidate cross-reference **KEGG:M00165**.
- Carbon fixation / carbon assimilation — candidate **GO:0015977**.
- Carboxylation, reduction, and RuBP-regeneration phases.
- Photorespiration/phosphoglycolate salvage — modifier or competing pathway, not part of the positive trait core.
- Carbon-concentrating mechanism — label-only unless a validated ontology term is selected.

### Genes, proteins, enzymes, and complexes

- **Rubisco**; bacterial genes `rbcL/rbcS` or `cbbL/cbbS`, Form-II `cbbM`; **EC:4.1.1.39**, **KEGG:K01601**.
- **Phosphoribulokinase**; `prk`, `prkA`, or `cbbP`; **EC:2.7.1.19**, **KEGG:K00855**.
- Phosphoglycerate kinase, **EC:2.7.2.3**.
- NAD(P)-dependent glyceraldehyde-3-phosphate dehydrogenase; curate the taxon-appropriate isoenzyme only after sequence/context validation.
- Fructose-bisphosphate aldolase, **EC:4.1.2.13**.
- Fructose-1,6-bisphosphatase, **EC:3.1.3.11**.
- Transketolase, **EC:2.2.1.1**.
- Ribose-5-phosphate isomerase; ribulose-phosphate 3-epimerase; sedoheptulose-bisphosphatase or bifunctional FBPase/SBPase where taxonomically appropriate.
- Rubisco activases/chaperones CbbQ, CbbX, and RbcX — accessory and non-universal. Comparative genomics associates CbbQ/CbbX and several regeneration enzymes with CBB-positive genomes. (asplundsamuelsson2021widerangeof pages 12-13, asplundsamuelsson2021widerangeof pages 8-11, asplundsamuelsson2021widerangeof pages 7-8)
- CbbR — bacterial cbb-regulon transcriptional regulator; taxon-specific.
- Cyanobacterial CcmR/NdhR, CmpR, CyAbrB2, and RbcR — CCM/carboxysome regulators, not universal CBB regulators.
- Cyanobacterial phosphoketolase SeXPK — negative flux branch under low ATP in *Synechococcus elongatus* PCC 7942. (lu2023anatpsensitivephosphoketolase pages 1-2)

### Chemicals and metabolites

- Carbon dioxide, bicarbonate, oxygen.
- RuBP, ribulose-5-phosphate, 3-PGA, 1,3-bisphosphoglycerate, glyceraldehyde-3-phosphate.

Showing the first 60 of 234 lines of findings; the linked file also carries the run's front matter and the prompt it was given — read the full report.