Phenylketonuria involves the body's inability to degrade the amino acid phenylalanine (Phe). This can lead to mental retardation and behavioral disorders in children. Diet restriction is oftentimes prescribed for this problem.

The inability to degrade phenylalanine (Phe) can arise from various enzyme deficiencies. The most common enzyme disorder involves phenylalanine hydroxylase (PAH). PAH converts Phe to tyrosine (Tyr) and is the first enzyme in the Phe degradative pathway. Other defective enzymes may include dihydrobiopterin synthetase and dihydrobiopterin reductase. These are involved in the metabolism of biopterin, an essential cofactor for PAH.