Biochemistry · Molecule
L-DOPA (levodopa) !
L-DOPA is the first product of catecholamine synthesis. Given as levodopa, it crosses into the brain and replaces dopamine in Parkinson disease.
Updated 2026-11-01
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Key facts
- Made from tyrosine by tyrosine hydroxylase (BH4), the rate-limiting step of catecholamine synthesis.
- Unlike dopamine, it crosses the blood–brain barrier on the large neutral amino acid carrier.
- AADC (B6) decarboxylates it to dopamine; carbidopa blocks this outside the brain.
- In melanocytes tyrosinase also makes L-DOPA on the way to melanin.
- Also called
- 3,4-Dihydroxyphenylalanine
- Formula
- C₉H₁₁NO₄
- PubChem
- CID 6047
Structure
- Tyrosine with a second hydroxyl on the ring: a catechol (3,4-dihydroxyphenyl) amino acid.
Clinical notes
- Levodopa with carbidopa is the most effective treatment for the motor signs of Parkinson disease.
- Protein-rich meals compete with levodopa for absorption and brain entry.
- Dopa-responsive dystonia (tyrosine hydroxylase or GTP cyclohydrolase defects) improves dramatically with levodopa.
Pathways
Related conditions
Related
Test yourself
Think of the answer, then tap the question to see it.
Why is Parkinson disease treated with levodopa rather than dopamine?
Dopamine does not cross the blood–brain barrier; levodopa does and is decarboxylated to dopamine in the brain.
Which enzyme makes L-DOPA, and what cofactor does it need?
Tyrosine hydroxylase, which needs tetrahydrobiopterin (BH4), Fe²⁺ and O₂.
What does carbidopa do?
It blocks DOPA decarboxylase outside the brain, so more levodopa reaches the brain.
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: Levodopa (CID 6047) (PubChem, NCBI, 2026) · Public domain; TH gene: 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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