{c}

<N-acetylserotonin> is <serotonin> whose primary amino group has been converted to an <amide> bearing an <acetyl group>. Its neutral <molecular formula> is $\mathrm{C_{12}H_{14}N_2O_2}$. The N prefix specifies modification at nitrogen, not acetylation of the phenolic oxygen.<N-acetylserotonin in ChEBI>[CHEBI:17697; Definition, neutral formula and structure]{ref}

<Serotonin N-acetyltransferase> uses <acetyl-CoA> to form <N-acetylserotonin> from <serotonin>. A subsequent reaction catalysed by <acetylserotonin O-methyltransferase> uses <S-adenosylmethionine> to methylate oxygen, producing <melatonin>.<Enzyme Nomenclature by the IUBMB>[EC2.3.1.87, substrate scope; EC2.1.1.4, reaction and accepted name]{ref} The two enzymes therefore alter different atoms and transfer different groups.

A structural count makes the second step visible. Replacing an oxygen-bound hydrogen by $\mathrm{CH_3}$ adds a net $\mathrm{CH_2}$, converting $\mathrm{C_{12}H_{14}N_2O_2}$ to $\mathrm{C_{13}H_{16}N_2O_2}$ for neutral <melatonin>. The precursor is chemically distinct from the final product; observing more precursor does not by itself establish greater product formation if the next reaction is limiting.


 N-acetylserotonin (source code)