= 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.<Enzyme Nomenclature by the IUBMB>[EC 2.1.1.4, Reaction and Systematic name]{ref}
Writing the donor as SAM, the acceptor as NAS and the donor-derived coproduct as SAH, the reported reaction is
$$
\mathrm{SAM+NAS\longrightarrow SAH+melatonin}.
$$
Those 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.<Enzyme Nomenclature by the IUBMB>[EC 2.1.1.4, Comments]{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.
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