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.
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
Related conditions
Related
- Homocysteine
- S-adenosylmethionine (SAM)
- Methionine synthase
- One-carbon metabolism (folate, B12 and SAM)
Test yourself
Think of the answer, then tap the question to see it.
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.
Educational information, not medical advice. For any symptom, see a health professional.
Sources: CBS: Cystathionine beta-synthase (human) (UniProt, 2026); CBS gene: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); 4COO: Crystal structure of human cystathionine beta-synthase (delta516-525) at 2.0 angstrom resolution (RCSB Protein Data Bank, 2026).
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