PAH and phenylketonuria: Phenylketonuria (PKU; MIM #261600) is an inborn error of metabolism caused by the deficiency of phenylalanine hydroxylase (PAH, EC 1.14.16.1), the hepatic enzyme that converts phenylalanine (Phe) into tyrosine (Tyr), using tetrahydrobiopterin (BH4) as a coenzyme.