PKU is a congenital amino acid metabolic disorder. In one of the following rare variants of PKU Dihydro Biopterin synthesis is affected. The enzyme deficient is

Atypical form of phenylketonuria is due to deficiency of dihydrobiopterin reductase which is required for converion of H2-biopterin intorin. H4-biopterin is required for oxidation of phenylalanine because it is a cofactor for phenylalanine hydroxylase.