Biopterin

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 the synthesis of a number of neurotransmitters including dopamine, norepinephrine, epinepherine, and serotonin, along with several trace amines. Nitric oxide synthesis also uses biopterin derivatives as cofactors. In humans, tetrahydrobiopterin is the endogenous cofactor for AAAH enzymes.
Compounds.
Biopterin compounds found within the body include BH4, the free radical BH3, and BH2 (also a free radical).
Synthesis.
Biopterin synthesis occurs through two principal pathways; the de novo pathway involves three enzymatic steps and proceeds from GTP, while the salvage pathway converts sepiapterin to biopterin. BH4 is the principal active cofactor, and a recycling pathway converts BH2 to BH4.
Biopterin disorders.
A number of disorders of biopterin regulation exist.
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. This is due to the role of BH4 in synthesising neurotransmitters, including Dopamine, and is treated with supplementation with levodopa, which does not require BH4 for conversion to dopamine. GCH1 defects are autosomal dominant, meaning that only one defective gene copy is required for the condition to occur. Mouse gene knockout models that block biopterin synthesis completely die shortly after birth due to their inability to produce catecholamines and neurotransmitters.
Biopterin synthesis disorders are also a cause of hyperphenylalaninemia; phenylalanine metabolism requires BH4 as a cofactor.
External links.
WiseGeek. (2012). What is biopterin?. Retrieved from http://www.wisegeek.com/what-is-biopterin.htm