tetrahydrobiopterin /tet′rə·hī′drō·bī·op′tər·in/ , 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.
