Tetrahydrobiopterin, also known as sapropterin, 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, melatonin, dopamine, norepinephrine, epinephrine, and is a cofactor for the production of nitric oxide by the nitric oxide synthases. Wikipedia