Biochemistry · Molecule
Tetrahydrobiopterin (BH4) !
BH4 is the cofactor of the phenylalanine, tyrosine and tryptophan hydroxylases and of nitric oxide synthase.
Updated 2026-11-01
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Key facts
- It is made from GTP (GTP cyclohydrolase I is the rate-limiting step); it is not a vitamin.
- It gives electrons and is left as dihydrobiopterin; dihydropteridine reductase regenerates it with NADH.
- Cofactor of the phenylalanine, tyrosine and tryptophan hydroxylases and of the nitric oxide synthases.
- It is not tetrahydrofolate: DHFR regenerates folate, not biopterin.
- Also called
- Sapropterin (drug name)
- Formula
- C₉H₁₅N₅O₃
- PubChem
- CID 135398654
Structure
- A reduced pterin (pyrimidine and pyrazine rings) with a dihydroxypropyl side chain.
Clinical notes
- BH4 deficiency causes hyperphenylalaninaemia with lack of dopamine and serotonin: neurological symptoms even when phenylalanine is controlled.
- Sapropterin (synthetic BH4) lowers phenylalanine in some responsive patients with phenylketonuria.
- Some L-DOPA-responsive dystonias are due to faults in BH4 synthesis.
Pathways
- Phenylalanine and tyrosine metabolism
- Catecholamine synthesis and breakdown
- Serotonin and melatonin synthesis
Related conditions
Related
- Phenylalanine hydroxylase (PAH)
- Tyrosine hydroxylase
- Phenylalanine
- Tryptophan
- Molecules derived from amino acids
- Dihydrofolate reductase (DHFR)
Test yourself
Think of the answer, then tap the question to see it.
Which four kinds of enzymes use BH4?
The phenylalanine, tyrosine and tryptophan hydroxylases and the nitric oxide synthases.
Why does BH4 deficiency cause symptoms even when phenylalanine is lowered?
Because dopamine and serotonin are also short, as their hydroxylases need BH4.
Which molecule is BH4 made from?
GTP; the first and rate-limiting step is GTP cyclohydrolase I.
Molecules are shown in their neutral form, as the databases deposit them; in the cell the acids and phosphates are ionised.
Educational information, not medical advice. For any symptom, see a health professional.
Sources: Sapropterin (CID 135398654) (PubChem, NCBI, 2026) · Public domain; Tetrahydrobiopterin deficiency: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); Metabolism of folate and pterines (Reactome, 2026).
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