any of three monooxygenase enzymes: phenylalanine 4-monooxygenase (EC 1.14.16.1); tyrosine 3-monooxygenase (EC 1.14.16.2); and tryptophan 5-monooxygenase (EC 1.14.16.4). All require tetrahydrobiopterin as coenzyme (see biopterin), and are homologous in amino acid sequence and gene structure. In each the N-terminal region binds the substrate, while the C-terminal region binds the coenzyme and catalyses the monooxygenation.