QDPR belongs to the short-chain dehydrogenases/reductase（SDR) family of enzymes. Operating as a homodimer， QDPR has an imperative role in the recycling of tetrahydrobiopterin（BH4)， a vital cofactor for the hydroxylation of the aromatic amino acids（tryptophan， tyrosine and phenylalanine). More precisely， QDPR catalyzes the regeneration of BH4 from quinonoid dihydrobiopterin（qBH2)， the product generated from the hydroxylation reactions. Mutations in the QDPR gene may lead to phenylketonuria II.
