Tryptophan. Serotonin is synthesized from the essential amino acid tryptophan. The rate-limiting step is the conversion of the amino acid to 5-hydroxytrptophan by tryptophan hydroxylase. This is then conveed to serotonin by the aromatic L-amino acid decarboxylase which needs Pyridoxal Phosphate as a cofactor Ref : vasudevan 7th ed