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Metabolism pathway of tetrahydrobiopterin (BH4). N (neopterin); B (biopterin); BH2 (dihydrobiopterin); NH2 (dihydroneopterin). GTP (guanosine triphosphate); GTPCH (guanosine triphosphate cyclohydrolase EC 3.5.4.16); PTPS (6-pyruvoyl-tetrahydropterin synthase EC 4.2.3.12); SR (sepiapterin reductase EC 1.1.1.153); PCD (pterin-4α-carbinolamine dehydratase EC 3.5.4.16); DHPR (dihydropteridin reductase EC 1.5.1.34). (Dashed arrows: nonenzymatic.)
