Figure 9.

Tryptophan produces the neurotransmitter serotonin. The initial rate-limiting step in synthesis of serotonin is the conversion of l-tryptophan to 5-hydroxytryptophan by the enzyme l-tryptophan hydroxylase (TPH). Subsequently, aromatic l-amino acid decarboxylase (DDC) generates serotonin from 5-hydroxytryptophan. Serotonin can also generate melatonin. Arylalkylamine N-acetyltransferase (AANAT) adds an acetyl group to serotonin, resulting in the production of N-acetyl serotonin. Acetylserotonin O-methyltransferase (ASMT) adds a methyl group from S-adenosylmethionine (SAM) to N-acetyl serotonin, resulting in the formation of S-adenosylhomocysteine (SAH) and melatonin.