Catalyzes the hydroxylation of L-phenylalanine to L-tyrosine. (6R)-L-erythro-5,6,7,8-tetrahydrobiopterin + L-phenylalanine + O2 = (4aS,6R)-4a-hydroxy-L-erythro-5,6,7,8-tetrahydrobiopterin + L-tyrosine N-terminal region of PAH is thought to contain allosteric binding sites for phenylalanine and to constitute an 'inhibitory' domain that regulates the activity of a catalytic domain in the C-terminal portion of the molecule. Amino-acid degradation; L-phenylalanine degradation; acetoacetate and fumarate from L-phenylalanine: step 1/6. Homodimer and homotetramer. Phosphorylation at Ser-16 increases basal activity and facilitates activation by the substrate phenylalanine. Belongs to the biopterin-dependent aromatic amino acid hydroxylase family.