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. Mice display impaired hemostasis, resulting in a reduced risk of thrombosis and thromboembolism, although the ultrastructure of the platelets is not affected (PubMed:14697203). Blood platelet counts are normal (PubMed:14697203). Defects are caused by inability to inability to mediate protein serotonylation of small GTPases during activation and aggregation of platelets (PubMed:14697203). Belongs to the biopterin-dependent aromatic amino acid hydroxylase family.