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

# Leucine

Leucine is an essential branched-chain amino acid. It is purely ketogenic and a strong signal for muscle protein synthesis.

Updated 2026-11-01

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## Key facts

- One of the three BCAAs (with isoleucine and valine), oxidised mainly in muscle.
- Purely ketogenic: it ends as acetyl-CoA and acetoacetate; lysine is the other purely ketogenic amino acid.
- It activates mTORC1 and so stimulates protein synthesis.
- Its ketoacid is α-ketoisocaproate, oxidised by BCKDH.

**Also called**
L-Leucine, Leu, L

**Formula**
C₆H₁₃NO₂

**PubChem**
[CID 6106](https://pubchem.ncbi.nlm.nih.gov/compound/6106)

## Structure

- An isobutyl side chain (–CH₂–CH(CH₃)₂): non-polar and branched.

## Clinical notes

- In maple syrup urine disease leucine is the most neurotoxic metabolite; its plasma level guides treatment.
- Isovaleric acidaemia and HMG-CoA lyase deficiency are disorders of its later breakdown steps.
- High BCAA levels are seen in obesity and insulin resistance.

## Pathways

- [Branched-chain amino acid breakdown (leucine route)](https://durgical.com/en/biochemistry/pathways/branched-chain-amino-acid-breakdown/)

## Related conditions

- [Maple syrup urine disease](https://durgical.com/en/conditions/maple-syrup-urine-disease/)

## Related

- [Branched-chain α-keto acid dehydrogenase (BCKDH)](https://durgical.com/en/biochemistry/enzymes/branched-chain-keto-acid-dehydrogenase/)
- [Acetoacetate (acetoacetic acid)](https://durgical.com/en/biochemistry/molecules/acetoacetate/)
- [Acetyl-CoA](https://durgical.com/en/biochemistry/molecules/acetyl-coa/)
- [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.

## Why is leucine called purely ketogenic?

Its carbons end as acetyl-CoA and acetoacetate, which cannot make glucose.

## Which enzyme complex is missing in maple syrup urine disease?

The branched-chain α-ketoacid dehydrogenase complex (BCKDH).

## Which signalling hub does leucine activate to promote protein synthesis?

mTORC1, which drives translation in muscle.

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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-Leucine (CID 6106)](https://pubchem.ncbi.nlm.nih.gov/compound/6106) (PubChem, NCBI, 2026) · Public domain; [Maple syrup urine disease: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/maple-syrup-urine-disease/) (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/biochemistry/molecules/leucine/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/biochemistry/molecules/leucine/