Biopterins are pterin derivatives which function as endogenous enzyme cofactors in many species of animals and in some bacteria and fungi.

Biopterins act as cofactors for aromatic amino acid hydroxylases (AAAH), which are involved in synthesizing a number of neurotransmitters including dopamine, norepinephrine, epinepherine, and serotonin, along with several trace amines.

[1] Biopterin compounds found in the animal body include BH4, the free radical[2] BH3•, and the semi-oxidized form BH2.

Single-gene defects affecting the gene GCH1 block the first step in biopterin synthesis, and lead to dopamine-responsive dystonia, also known as Segawa's syndrome.

Mouse gene knockout models that block biopterin synthesis completely die shortly after birth due to their inability to produce catecholamines and neurotransmitters.