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
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
Related conditions
Related
- Branched-chain α-keto acid dehydrogenase (BCKDH)
- Acetoacetate (acetoacetic acid)
- Acetyl-CoA
- 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.
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) (PubChem, NCBI, 2026) · Public domain; Maple syrup urine disease: MedlinePlus Genetics (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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