Biochemistry · Molecule
Methionine !
Methionine is an essential sulfur amino acid. It starts every protein and is the source of SAM, the body's main methyl donor.
Updated 2026-11-01
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Key facts
- Essential: the body cannot make its carbon skeleton; it can only regenerate it from homocysteine.
- AUG, the start codon, codes for methionine, so it is the first amino acid of every new protein.
- MAT turns it into SAM; its sulfur ends up in cysteine through transsulfuration.
- It is glucogenic: its carbons leave as propionyl-CoA, then succinyl-CoA.
- Also called
- L-Methionine, Met, M
- Formula
- C₅H₁₁NO₂S
- PubChem
- CID 6137
Structure
- A thioether side chain (–CH₂–CH₂–S–CH₃); the sulfur carries the methyl group that SAM later donates.
Clinical notes
- Plasma methionine is high in classic homocystinuria (CBS deficiency) and low or normal in remethylation defects; newborn screening uses it.
- A methionine-restricted diet is part of the treatment of CBS deficiency.
- Isolated hypermethioninaemia can come from MAT1A deficiency, liver disease or a very high-protein formula.
Pathways
Related conditions
Related
- S-adenosylmethionine (SAM)
- Homocysteine
- Methionine synthase
- Cystathionine β-synthase (CBS)
- Propionyl-CoA
- One-carbon metabolism (folate, B12 and SAM)
Test yourself
Think of the answer, then tap the question to see it.
Which codon codes for methionine?
AUG, the start codon, so methionine opens every newly made protein.
What does methionine become when it is activated by ATP?
S-adenosylmethionine (SAM), the main methyl donor, made by methionine adenosyltransferase.
Why is plasma methionine high in classic homocystinuria?
CBS is blocked, so homocysteine is pushed back to methionine instead of going on to cystathionine.
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-Methionine (CID 6137) (PubChem, NCBI, 2026) · Public domain; Hypermethioninemia: 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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