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BH4 synthesis. BH4 is synthesized from GTP by a de novo pathway. The conversion of GTP to 7,8-dihydroneopterin triphosphate by GTPCH is the rate-limiting step of this pathway. The salvage pathway generates BH4 from its oxidized forms. The salvage pathway is also necessary to convert exogenous sepiapterin into BH4. GTPCH: GTP cyclohydrolase I; PTPS: 6-pyruvoyl-tetrahydropterin synthase; SR: sepiapterin reductase; DHFR: dihydrofolate reductase.
