(6R)-L-erythro-5,6,7,8-tetrahydrobiopterin + L-tryptophan + O2 = (4aS,6R)-4a-hydroxy-L-erythro-5,6,7,8-tetrahydrobiopterin + 5-hydroxy-L-tryptophan Aromatic compound metabolism; serotonin biosynthesis; serotonin from L-tryptophan: step 1/2. Brain specific. Modulates the kinetic properties of both isoforms. Disease susceptibility is associated with variants affecting the gene represented in this entry. Disease susceptibility is associated with variants affecting the gene represented in this entry. Naturally occurring variants of TPH2 with impaired enzyme activity could cause deficiency of serotonin production and result in an increased risk of developing behavioral disorders. Belongs to the biopterin-dependent aromatic amino acid hydroxylase family.