Pterin

Pterin is a chemical compound composed of a pyrazine ring and a pyrimidine ring; the pyrimidine ring has a carbonyl oxygen and an amino group. Several tautomers of pterin exist and are shown below. Pterin belongs to the pteridine family of heterocycles.

Pterins were first discovered in the pigments of butterfly wings (hence the origin of their name, from the Greek pteron, wing)  and perform many roles in coloration in the biological world. Pterins also function as cofactors in enzymatic catalysis.

Folates, “conjugated” pterins which contain para-aminobenzoic acid and glutamates in addition to the pterin ring system, are critical compounds in a large number of biological group transfer reactions. These folate-dependent biosynthetic reactions include transfer of methyl groups to homocystine in the S-adenosyl methionine cycle and formyl groups to N-formylmethionine in initiator tRNAs.

Biosynthesis
The biosynthesis of pterins begins with the molecule guanosine triphosphate (GTP); the enzyme which controls the conversion of GTP to pterin, GTP cyclohydrolase I, is found in both prokaryotes and eukaryotes.

Other pterins
Pterin can exist in many different forms in nature depending on its function. Tetrahydrobiopterin, the major unconjugated pterin in vertebrates, is a co-factor in the hydroxylation of aromatic compounds and synthesis of nitric oxide. Molybdopterin is a substituted pterin that binds molybdenum to give redox enzymes involved in biological hydroxylations, reduction of nitrate, and respiratory oxidation. Tetrahydromethanopterin is used in methanogenic organisms. Cyanopterin is a glycosylated version of pterin of unknown function in cyanobacteria.