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Pathways for the biosynthesis of BH4. The de novo pathway (left) is synthesized from GTP to BH4 in three steps. GCH1 is the rate-limiting enzyme in BH4 de novo biosynthesis. The salvage pathway (right) produces BH4 from its oxidized form, starting with sepiapterin in two steps, which is essential to convert exogenous sepiapterin into BH4.
