Plays an important role in the physiology of adrenergic neurons. (6R)-L-erythro-5,6,7,8-tetrahydrobiopterin + L-tyrosine + O2 = (4aS,6R)-4a-hydroxy-L-erythro-5,6,7,8-tetrahydrobiopterin + L-dopa Phosphorylation leads to an increase in the catalytic activity. Catecholamine biosynthesis; dopamine biosynthesis; dopamine from L-tyrosine: step 1/2. Expressed in dopaminergic axons and axon terminals. Belongs to the biopterin-dependent aromatic amino acid hydroxylase family.