N-acetylserotonin

N-acetylserotonin is serotonin whose primary amino group has been converted to an amide bearing an acetyl group. Its neutral molecular formula is ${\mathrm{C}}_{12}{\mathrm{H}}_{14}{\mathrm{N}}_2{\mathrm{O}}_2$. The N prefix specifies modification at nitrogen, not acetylation of the phenolic oxygen.^[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.^[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 $\mathrm{C}{\mathrm{H}}_3$ adds a net $\mathrm{C}{\mathrm{H}}_2$, converting ${\mathrm{C}}_{12}{\mathrm{H}}_{14}{\mathrm{N}}_2{\mathrm{O}}_2$ to ${\mathrm{C}}_{13}{\mathrm{H}}_{16}{\mathrm{N}}_2{\mathrm{O}}_2$ 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.