an enzyme that catalyzes the first step in the biosynthesis of serotonin. It uses tetrahydrobiopterin as a coenzyme to transform the dietary amino acid L-tryptophan to 5-hydroxytryptophan (5-HTP). This reaction is the rate-limiting step in serotonin synthesis, limited by levels of tryptophan in the brain as well as by levels of activity of neurons that use serotonin as a neurotransmitter.