cofactor biosynthesis pteridine biosynthesis tetrahydrobiopterin biosynthesis UPa:UPA00849 5,6,7,8-tetrahydrobiopterin biosynthesis KEGG:map01100 BH4 biosynthesis KEGG:map00790 GO:0006729 Biosynthesis of tetrahydrobiopterin (sapropterin, BH4, H4-biopterin). Tetrahydrobiopterin is an essential cofactor of a set of enzymes that are of central metabolic importance, i.e. the hydroxylases of the three aromatic amino acids phenylalanine, tyrosine, and tryptophan, of ether lipid oxidase, and of the three nitric oxide synthase (NOS) isoenzymes. Tetrahydrobiopterin is a naturally occurring essential cofactor of the three aromatic amino acid hydroxylases; phenylalanine-4-hydroxylase, tyrosine-3-hydroxylase and tryptophan-5-hydroxylase. Tetrahydrobiopterin is also essential for the synthesis of nitric oxide by nitric oxide synthase (NOS). sapropterin biosynthesis H4-biopterin biosynthesis PMID:17220358 pathway