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a compound related to folic acid that functions as a coenzyme in the reactions hydroxylating phenylalanine, tryptophan, and tyrosine by carrying electrons to oxygen. Defects in its biosynthesis or regeneration affect all three hydroxylation reactions, interfere with production of the corresponding neurotransmitter precursors, and result in malignant hyperphenylalaninemia.
tetrahydrobiopterinMetabolic disease A natural cofactor of aromatic amino acid hydroxylases and nitric oxide synthase, which may be used as an alternative to low-phenylalanine diets in Pts with mild phenylketonuria. See Phenylketonuria.
pterin cofactor in the mixed-function oxygenase, phenylalanine hydroxylase which catalyzes the conversion of phenylalanine to tyrosine. Hereditary deficiency of this enzyme is responsible for phenylketonuria (PKU).