Pterin is a heterocyclic compoundcomposed of a pteridinering system, with a "keto group" and an amino group on positions 4 and 2 respectively. It is structurally related to the parent bicyclic heterocycle called pteridine. Pterins, as a group, are compounds related to pterin with additional substituents. Pterin itself is of no biological significance. Pterins were first discovered in the pigments of butterfly wings and perform many roles in coloration in the biological world.
Pterin cofactors
One important family of pterin derivatives are Folates. Folates are pterins that contain p-aminobenzoic acid connected to the methyl group at position 6 of the pteridine ring system conjugated with one or more L-glutamates. They participate in numerous biological group transfer reactions. Folate-dependent biosynthetic reactions include the transfer of methyl groups from 5-methyltetrahydrofolate to homocysteine to form L-methionine, and the transfer of formyl groups from 10-formyltetrahydrofolate to L-methionine to form N-formylmethionine in initiator tRNAs. Folates are also essential for the biosynthesis of purines and one pyrimidine.
, the major unconjugated pterin in vertebrates, is involved in three families of enzymes that effect hydroxylation. The aromatic amino acid hydroxylases include phenylalanine hydroxylase, tyrosine hydroxylase, and tryptophan hydroxylases. They are involved in the synthesis of neurotransmitters catecholamine and serotonin. Tetrahydrobiopterin is also required for the functioning of alkylglycerol monooxygenase, whereby monoalkylglycerols are broken down to glycerol and an aldehyde. In the synthesis of nitric oxide the pterin-dependent nitric oxide synthase converts arginine to N-hydroxy derivative, which in turn releases NO.
Biosynthesis
The biosynthesis of pterins begins with guanosine triphosphate, which is the substrate for GTP cyclohydrolase I. The enzyme is found in both prokaryotes and eukaryotes. Molybdopterin biosynthesis occurs in four steps: the radical-mediated cyclization of nucleotide, guanosine 5'-triphosphate, to ‑3 ́,8‐cyclo‑7,8‑dihydroguanosine 5 ́‑triphosphate, the formation of cyclic pyranopterin monophosphate from the 3 ́,8‑cH2GTP, the conversion of cPMP into molybdopterin, the insertion of molybdate into MPT to form Moco. The human enzymes are indicated in parenthesis.
Other pterins
Pterin can exist in many different forms in nature depending on its function. Tetrahydrobiopterin, the major unconjugated pteridine in vertebrates, is a co-factor in the hydroxylation of aromatic compounds and synthesis of nitric oxide. Molybdopterin is a substituted pteridine that binds molybdenum to yield redox cofactors involved in biological hydroxylations, reduction of nitrate, and respiratory oxidation. Tetrahydromethanopterin is used in methanogenic organisms. Cyanopterin is a glycosylated version of pteridine of unknown function in cyanobacteria.
