High Quality Content by WIKIPEDIA articles! Tyrosine hydroxylase or tyrosine 3-monooxygenase is the enzyme responsible for catalyzing the conversion of the amino acid L-tyrosine to dihydroxyphenylalanine (DOPA). It does so using tetrahydrobiopterin as a coenzyme. DOPA is a precursor for dopamine, which, in turn, is a precursor for norepinephrine (noradrenaline) and epinephrine (adrenaline). In humans, tyrosine hydroxylase is encoded by the TH gene.