Phenylalanine is converted by the mono-oxygenase, phenylalanine hydroxylase, to tyrosine. This REDOX reaction requires the cofactor tetrahydrobiopterin, which in the course of the reaction is converted to quinonoid dihydrobiopterin. The cofactor is regenerated through the action of the enzyme dihydropteridine reductase, which in turn uses NADPH as a source of reducing power.
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