 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 a 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.