<prompt> Tetrahydrobiopterin is required for the metabolism ofAlanineLysinePhenylalanineSerine  <response>: THB is a cofactor for Aromatic amino acid Tryptophan, Phenylalanine Tyrosine hydroxylases See Fig Nitric acid synthase Serotonin synthesis Non-classical phenylketonuria types are due to defect in the synthesis or recycling of THB. Tetrahydrobiopterin THB is a nucleotide cofactor derived from Guanosine triphosphate. 