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

# S-adenosylmethionine (SAM)

SAM is the body's main methyl-group donor: it methylates DNA, neurotransmitters, phospholipids and creatine.

Updated 2026-11-01

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## Key facts

- It is made from methionine + ATP (methionine adenosyltransferase); the ATP loses all three phosphates.
- Its methyl sits on a positively charged sulphur (sulphonium), so it is easily transferred.
- After giving the methyl, S-adenosylhomocysteine remains and is hydrolysed to homocysteine.
- Examples: noradrenaline → adrenaline, guanidinoacetate → creatine, phosphatidylethanolamine → phosphatidylcholine and DNA methylation.
- It activates cystathionine β-synthase and inhibits MTHFR: when methionine is plentiful, homocysteine goes to cysteine.

**Also called**
SAM, AdoMet

**Formula**
C₁₅H₂₂N₆O₅S

**PubChem**
[CID 34755](https://pubchem.ncbi.nlm.nih.gov/compound/34755)

## Structure

- It joins the adenosine of ATP to the sulphur of methionine; the sulphur is left with three bonds and a positive charge.

## Clinical notes

- In B12 deficiency SAM falls and myelin methylation fails: subacute combined degeneration of the spinal cord.
- DNA methylation by SAM regulates genes (epigenetics) and is altered in many tumours.

## Pathways

- [Catecholamine synthesis and breakdown](https://durgical.com/en/biochemistry/pathways/catecholamine-synthesis-and-breakdown/)
- [Methionine cycle and transsulfuration](https://durgical.com/en/biochemistry/pathways/methionine-cycle-and-transsulfuration/)
- [Serotonin and melatonin synthesis](https://durgical.com/en/biochemistry/pathways/serotonin-and-melatonin-synthesis/)

## Related conditions

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

## Related

- [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/)
- [One-carbon metabolism (folate, B12 and SAM)](https://durgical.com/en/biochemistry/topics/one-carbon-metabolism/)
- [ATP (adenosine triphosphate)](https://durgical.com/en/biochemistry/molecules/atp/)
- [Molecules derived from amino acids](https://durgical.com/en/biochemistry/topics/molecules-derived-from-amino-acids/)

## Test yourself

Think of the answer, then tap the question to see it.

## Which two molecules make SAM?

Methionine and ATP; the enzyme moves the adenosine onto the sulphur and releases all three phosphates.

## What is left after SAM gives up its methyl?

S-adenosylhomocysteine, which is hydrolysed to adenosine and homocysteine.

## Which enzyme does SAM activate to send homocysteine to cysteine?

Cystathionine β-synthase; at the same time SAM slows MTHFR, which would supply methyls for remethylation.

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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: [S-Adenosylmethionine (CID 34755)](https://pubchem.ncbi.nlm.nih.gov/compound/34755) (PubChem, NCBI, 2026) · Public domain; [Sulfur amino acid metabolism](https://reactome.org/PathwayBrowser/#/R-HSA-1614635) (Reactome, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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