Synthetic and catabolic pathway of serotonin and dopamine. 5-hydroxyindole acetic acid is formed initially from tryptophan in a reaction catalysed by tryptophan hydroxylase○2 to form 5-hydroxytryptophan (5HTP), which requires molecular oxygen and tetrahydrobiopterin (BH4) for its activity. 5-hydroxytryptophan is decarboxylated by pyrimidine dependent aromatic L-amino acid decarboxylase○3 to form the active neurotransmitter serotonin. Serotonin is catabolised by monoamine oxidase○4 to form 5-hydroxyindole acetic acid (5HIAA). Tyrosine is metabolised by tyrosine hydroxylase○1 to L-dopa. Aromatic L-amino acid decarboxylase○3 is also required for the decarboxylation of L-dopa to dopamine, which is then catabolised to homovanillic acid by monoamine oxidase and catechol O-methyl-transferase○5.