Acetylserotonin O-methyltransferase

Acetylserotonin O-methyltransferase is an enzyme that transfers a methyl group from S-adenosylmethionine to N-acetylserotonin, producing melatonin and S-adenosylhomocysteine. Its official activity is EC 2.1.1.4. The O in its name identifies methyl transfer to an oxygen-containing group; it is a methyltransferase, rather than the activity that first supplies the substrate's acetyl group.^[ref]

Writing the donor as SAM, the acceptor as NAS and the donor-derived coproduct as SAH, the reported reaction isThose local abbreviations specify chemical participants in this equation. The reaction connects a modification of one functional group to the biosynthesis of melatonin; it does not construct the entire N-acetylserotonin molecule from simpler precursors.

For a constructed balance, begin with 10 nanomoles of S-adenosylmethionine and 6 nanomoles of N-acetylserotonin, with neither product initially present. The stated reaction can form at most 6 nanomoles each of melatonin and S-adenosylhomocysteine. If only this reaction consumes the donor, at least 4 nanomoles of S-adenosylmethionine remain. Observing consumption of eight nanomoles of donor would therefore require an explanation beyond six turnovers of this reaction.

The official account notes that some other hydroxyindole acceptors react more slowly.^[ref] Substrate specificity thus concerns comparative activity, not simply whether any chemically related molecule can ever react. Product identification and the chosen substrate together determine which activity an assay measures.