pterins

Pterins are a family of bicyclic heterocyclic compounds based on a pyrazine ring fused to a dihydropteridine scaffold. The core pterin structure can carry various substituents, giving rise to a range of biologically important molecules such as biopterin, neopterin, and related reduced and oxidized forms. Pterins occur in many organisms and can function as cofactors, pigments, or metabolic intermediates.

A central role for pterins in biology is as cofactors. Tetrahydrobiopterin (BH4) is a key pterin derivative required by several enzymes, including aromatic amino acid hydroxylases (phenylalanine, tyrosine, tryptophan hydroxylases) and nitric oxide synthase, enabling essential redox chemistry and biosynthesis of neurotransmitters and nitric oxide. The BH4 biosynthetic pathway begins with GTP and proceeds through enzymes such as GTP cyclohydrolase I, 6-pyruvoyl-tetrahydropterin synthase, and sepiapterin reductase; recycling of oxidized BH2 back to BH4 is accomplished by dihydropteridine reductase.

In addition to cofactors, pterins contribute to pigmentation and immune signaling. Pterins participate in insect eye pigments and other biological colorants. Neopterin, produced by activated macrophages, serves as a clinical biomarker for cellular immune activation and certain infectious or inflammatory states. Pterins are also found across bacteria, plants, and animals, with related pterin-like compounds such as lumazine playing roles in riboflavin biosynthesis.