[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Biochemistry](https://durgical.com/en/biochemistry/) › Tetrahydrobiopterin (BH4)

[Biochemistry](https://durgical.com/en/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](https://pubchem.ncbi.nlm.nih.gov/compound/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](https://durgical.com/en/biochemistry/pathways/phenylalanine-and-tyrosine-metabolism/)
- [Catecholamine synthesis and breakdown](https://durgical.com/en/biochemistry/pathways/catecholamine-synthesis-and-breakdown/)
- [Serotonin and melatonin synthesis](https://durgical.com/en/biochemistry/pathways/serotonin-and-melatonin-synthesis/)

## Related conditions

- [Phenylketonuria (PKU)](https://durgical.com/en/conditions/phenylketonuria/)

## Related

- [Phenylalanine hydroxylase (PAH)](https://durgical.com/en/biochemistry/enzymes/phenylalanine-hydroxylase/)
- [Tyrosine hydroxylase](https://durgical.com/en/biochemistry/enzymes/tyrosine-hydroxylase/)
- [Phenylalanine](https://durgical.com/en/biochemistry/molecules/phenylalanine/)
- [Tryptophan](https://durgical.com/en/biochemistry/molecules/tryptophan/)
- [Molecules derived from amino acids](https://durgical.com/en/biochemistry/topics/molecules-derived-from-amino-acids/)
- [Dihydrofolate reductase (DHFR)](https://durgical.com/en/biochemistry/enzymes/dihydrofolate-reductase/)

## 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.

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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)](https://pubchem.ncbi.nlm.nih.gov/compound/135398654) (PubChem, NCBI, 2026) · Public domain; [Tetrahydrobiopterin deficiency: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/tetrahydrobiopterin-deficiency/) (MedlinePlus, US National Library of Medicine, 2026); [Metabolism of folate and pterines](https://reactome.org/PathwayBrowser/#/R-HSA-196757) (Reactome, 2026).

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