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[Biochemistry](https://durgical.com/en/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](https://pubchem.ncbi.nlm.nih.gov/compound/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

- [Methionine cycle and transsulfuration](https://durgical.com/en/biochemistry/pathways/methionine-cycle-and-transsulfuration/)
- [Propionyl-CoA to succinyl-CoA](https://durgical.com/en/biochemistry/pathways/propionyl-coa-to-succinyl-coa/)

## Related conditions

- [Homocystinuria](https://durgical.com/en/conditions/homocystinuria/)

## Related

- [S-adenosylmethionine (SAM)](https://durgical.com/en/biochemistry/molecules/s-adenosylmethionine/)
- [Homocysteine](https://durgical.com/en/biochemistry/molecules/homocysteine/)
- [Methionine synthase](https://durgical.com/en/biochemistry/enzymes/methionine-synthase/)
- [Cystathionine β-synthase (CBS)](https://durgical.com/en/biochemistry/enzymes/cystathionine-synthase/)
- [Propionyl-CoA](https://durgical.com/en/biochemistry/molecules/propionyl-coa/)
- [One-carbon metabolism (folate, B12 and SAM)](https://durgical.com/en/biochemistry/topics/one-carbon-metabolism/)

## 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.

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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-Methionine (CID 6137)](https://pubchem.ncbi.nlm.nih.gov/compound/6137) (PubChem, NCBI, 2026) · Public domain; [Hypermethioninemia: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/hypermethioninemia/) (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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