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Tetrahydrobiopterin (BH4) biosynthesis pathway. De novo (red): BH4 is produced from guanosine-5′-triphosphate (GTP) by enzymes GTP cyclohydrolase 1 (GTPCH1), 6-pyruvoyl-tetrahydrobiopterin synthase (PTPS), and sepiapterin reductase (SR). Salvage (purple): Sepiapterin reduction into dihydrobiopterin (BH2) by SR, and BH2 conversion into BH4 by dihydrofolate reductase (DHFR). Recycling (blue): quinoid-BH2 reduction into BH4 by dihydropteridine reductase (DHPR).
