 Phenylalanine is converted into tyrosine by the enzyme phenylalanine hydroxylase , with molecular oxygen ( O2 ) and tetrahydrobiopterin as cofactors . Tyrosine is converted into L @-@ DOPA by the enzyme tyrosine hydroxylase , with tetrahydrobiopterin , O2 , and probably ferrous iron ( Fe2 + ) as cofactors . L @-@ DOPA is converted into dopamine by the enzyme aromatic L @-@ amino acid decarboxylase ( also known as DOPA decarboxylase ) , with pyridoxal phosphate as cofactor . Dopamine is then converted into norepinephrine by the enzyme dopamine β @-@ monooxygenase ( formerly known as dopamine β @-@ hydroxylase ) , with O2 and ascorbic acid as cofactors . 
