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Tyrosine hydroxylase takes part in the first step of the pathway that produces a group of hormones called catecholamines. This enzyme helps convert the protein building block (amino acid) tyrosine to a catecholamine called dopamine. In humans, tyrosine hydroxylase is encoded by the TH gene, and the enzyme is present in the central nervous system (CNS), peripheral sympathetic neurons and the adrenal medulla.<br /><br />Tetrahydrobiopterin (BH4, THB), also known as sapropterin (INN), is a cofactor of the three aromatic amino acid hydroxylase enzymes, used in the degradation of amino acid phenylalanine and in the biosynthesis of the neurotransmitters serotonin (5-hydroxytryptamine, 5-HT), melatonin, dopamine, norepinephrine (noradrenaline), epinephrine (adrenaline), and is a cofactor for the production of nitric oxide (NO) by the nitric oxide synthases. Tetrahydrobiopterin also is an essential cofactor for the neurotransmitter synthesizing enzymes tyrosine hydroxylase (which catalyzes the conversion of tyrosine to l-dopa) and tryptophan hydroxylase (which catalyzes the conversion of tryptophan to 5-hydroxytryptophan [5-HTP]), as well as for phenylalanine hydroxylase (which converts phenylalanine to tyrosine).