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

# Phenylalanine

Phenylalanine is an essential aromatic amino acid. The liver turns it into tyrosine; if it cannot, it builds up and damages the brain.

Updated 2026-11-01

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## Key facts

- Essential: it must come from the diet.
- Phenylalanine hydroxylase turns it into tyrosine with BH4, O₂ and Fe²⁺.
- If that route fails, it is shunted to phenylpyruvate, phenylacetate and phenyllactate, the urinary «phenylketones».
- It is both glucogenic and ketogenic.

**Formula**
C₉H₁₁NO₂

**PubChem**
[CID 6140](https://pubchem.ncbi.nlm.nih.gov/compound/6140)

## Structure

- An amino acid with a benzene ring in its side chain; non-polar and hydrophobic.

## Clinical notes

- Phenylketonuria: high blood phenylalanine, intellectual disability if untreated and a musty or mousy odour.
- The sweetener aspartame contains phenylalanine; foods with it carry a warning for phenylketonuria.
- Poorly controlled maternal phenylketonuria harms the fetus (microcephaly, congenital heart disease).

## Pathways

- [Phenylalanine and tyrosine metabolism](https://durgical.com/en/biochemistry/pathways/phenylalanine-and-tyrosine-metabolism/)

## Related conditions

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

## Related

- [Tyrosine](https://durgical.com/en/biochemistry/molecules/tyrosine/)
- [Phenylalanine hydroxylase (PAH)](https://durgical.com/en/biochemistry/enzymes/phenylalanine-hydroxylase/)
- [Tetrahydrobiopterin (BH4)](https://durgical.com/en/biochemistry/molecules/tetrahydrobiopterin/)
- [Glucogenic and ketogenic amino acids](https://durgical.com/en/biochemistry/topics/glucogenic-and-ketogenic-amino-acids/)

## Test yourself

Think of the answer, then tap the question to see it.

## Which metabolites give phenylketonuria its name?

Phenylpyruvate and its products (phenylacetate, phenyllactate), which appear in urine when phenylalanine cannot become tyrosine.

## Why do products with aspartame carry a warning?

Aspartame releases phenylalanine when digested, which is dangerous for people with phenylketonuria.

## Is phenylalanine glucogenic, ketogenic or both?

Both: its breakdown ends in fumarate and acetoacetate.

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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: [L-Phenylalanine (CID 6140)](https://pubchem.ncbi.nlm.nih.gov/compound/6140) (PubChem, NCBI, 2026) · Public domain; [Phenylalanine metabolism](https://reactome.org/PathwayBrowser/#/R-HSA-8964208) (Reactome, 2026); [Phenylketonuria: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/phenylketonuria/) (MedlinePlus, US National Library of Medicine, 2026).

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Source: https://durgical.com/en/biochemistry/molecules/phenylalanine/
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