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
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
- Methionine cycle and transsulfuration
- Serotonin and melatonin synthesis
Related conditions
Related
- Homocysteine
- Methionine synthase
- Cystathionine β-synthase (CBS)
- One-carbon metabolism (folate, B12 and SAM)
- ATP (adenosine triphosphate)
- 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.
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) (PubChem, NCBI, 2026) · Public domain; Sulfur amino acid metabolism (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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