Oxidizes L-tryptophan to 5-hydroxy-l-tryptophan in the rate-determining step of serotonin biosynthesis. (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. Homotetramer. Ubiquitinated, leading to its degradation by the proteasome. Ubiquitinated is triggered by phosphorylation. Phosphorylated; triggering degradation by the proteasome. Belongs to the biopterin-dependent aromatic amino acid hydroxylase family.