Methionine becomes the main methyl donor in the cell, gives up its methyl group and is rebuilt
Route: Methionine → SAM → SAH → Homocysteine → Methionine
S-adenosylmethionine synthase joins methionine to the adenosine of ATP. All three phosphates leave, which pays for the reaction, and the sulfur becomes a sulfonium ion carrying a methyl group that is easy to transfer. Methyltransferases pass that methyl group to targets such as carbon 5 of cytosine in DNA. The enzyme shown, glycine N-methyltransferase, methylates glycine to sarcosine. What remains is S-adenosylhomocysteine (SAH), a potent inhibitor of methyltransferase activity. Adenosylhomocysteinase splits it into adenosine and homocysteine. Methionine synthase then rebuilds methionine with the methyl group of 5-methyl-THF.
Practice this pathway All pathways
| Step | Enzyme | EC | In | Out |
|---|---|---|---|---|
| 1 | S-adenosylmethionine synthase | 2.5.1.6 | ATP, H₂O | Pᵢ, PPᵢ |
| 2 | Glycine N-methyltransferase | 2.1.1.20 | Glycine | Sarcosine |
| 3 | Adenosylhomocysteinase | 3.13.2.1 | H₂O | Adenosine |
| 4 | Methionine synthase | 2.1.1.13 | 5-CH₃-THF | THF |
Enzyme names and EC numbers are those used on the interactive board. Checked against sources: see the claim table in the repository (tools/pathway_data/checks).