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[Biochemistry](https://durgical.com/en/biochemistry/) · Enzyme

# Cystathionine β-synthase (CBS)

Cystathionine β-synthase joins homocysteine and serine on the route to cysteine. Its deficiency causes classic homocystinuria.

Updated 2026-11-01

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## Key facts

- Homocysteine + serine → cystathionine + H₂O; the first step of transsulphuration.
- It uses pyridoxal phosphate (B6) and carries a haem group, rare among PLP enzymes.
- Next, cystathionine γ-lyase (also B6) makes cysteine and α-ketobutyrate.
- SAM activates it: when methionine is plentiful, homocysteine is sent to cysteine.

**EC number**
[4.2.1.22](https://www.enzyme-database.org/query.php?ec=4.2.1.22)

**Genes**
CBS

**Structure (PDB)**
[4COO](https://www.rcsb.org/structure/4COO)

## Structure

- 4COO: nearly full-length human CBS with PLP, haem and the regulatory domain that binds SAM.

## Regulation

- Allosteric activator: S-adenosylmethionine.
- It is expressed mainly in liver, kidney, pancreas and brain.

## Inhibitors and toxins

- Aminooxyacetate (experimental)

## Clinical notes

- Classic homocystinuria: high homocysteine and methionine, downward lens dislocation, marfanoid habitus, intellectual disability and thrombosis.
- About half of patients respond to pyridoxine (B6).
- In these patients cysteine becomes essential.

## Pathways

- [Methionine cycle and transsulfuration](https://durgical.com/en/biochemistry/pathways/methionine-cycle-and-transsulfuration/)

## Related conditions

- [Homocystinuria](https://durgical.com/en/conditions/homocystinuria/)

## Related

- [Homocysteine](https://durgical.com/en/biochemistry/molecules/homocysteine/)
- [S-adenosylmethionine (SAM)](https://durgical.com/en/biochemistry/molecules/s-adenosylmethionine/)
- [Methionine synthase](https://durgical.com/en/biochemistry/enzymes/methionine-synthase/)
- [One-carbon metabolism (folate, B12 and SAM)](https://durgical.com/en/biochemistry/topics/one-carbon-metabolism/)

## Test yourself

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## Which vitamin does cystathionine β-synthase need?

B6 as pyridoxal phosphate; that is why some patients with homocystinuria respond to pyridoxine.

## Which molecule activates CBS?

S-adenosylmethionine, which signals excess methionine.

## How are methionine and homocysteine in its deficiency?

Both are high, because homocysteine cannot leave by transsulphuration and is remethylated to methionine.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: [CBS: Cystathionine beta-synthase (human)](https://www.uniprot.org/uniprotkb/P35520/entry) (UniProt, 2026); [CBS gene: MedlinePlus Genetics](https://medlineplus.gov/genetics/gene/cbs/) (MedlinePlus, US National Library of Medicine, 2026); [4COO: Crystal structure of human cystathionine beta-synthase (delta516-525) at 2.0 angstrom resolution](https://www.rcsb.org/structure/4COO) (RCSB Protein Data Bank, 2026).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/biochemistry/enzymes/cystathionine-synthase/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/biochemistry/enzymes/cystathionine-synthase/