H4-biopterin (tetrahydrobiopterin) is required for metabolism of

Conversion of phenylalanine to tyrosine: The reaction involves hydroxylation of phenylalanine at p-position in benzene ring. Enzyme: Phenylalanine hydroxylase. Present in liver and the conversion occurs in Liver. Coenzymes and cofactors: The enzyme requires the following for its activity: * Molecular oxygen * NADPH *F e++ and * Pteridine (folic acid) coenzyme: Tetrahydrobiopterin- FH4 The reaction is complex and takes place in two stages as shown below: I. Reduction of O2 to H2O and conversion of phenylalanine to tyrosine. Reduced form of pteridine, FH4 acts as H-donor to the molecular O2. II. Reduction of dihydrobiopterin, FH2 by NADPH, catalyzed by the enzyme Dihydrobiopterin reductase.Ref: Textbook of medical biochemistry, MN Chatterji, 8th edition, page no: 495