Dihydrobiopterin is an intermediate in the pathway to the
cofactor, tetrahydrobiopterin, which is used by three
aromatic amino acid hydroxylase enzymes which are important in the metabolism of the amino acid
phenylalanine and to make the
neurotransmitters
serotonin,
melatonin,
dopamine,
noradrenaline, and
adrenaline. In higher organisms, tetrahydrobiopterin is derived from
guanosine triphosphate by the action of a sequence of three enzymes
GTP cyclohydrolase,
6-pyruvoyltetrahydropterin synthase and
sepiapterin reductase. The final enzyme performs a stepwise reduction of L-sepiapterin In some bacteria, for example
Chlorobium tepidum, the
reduction of L-sepiapterin leads to the L-
threo isomer of dihydrobiopterin.
Quinonoid isomer When tetrahydrobiopterin is used in
oxidation reactions as an enzyme cofactor, it can be converted into an alternative
isomer of the L-
erythro-7,8-dihydrobiopterin form of dihydrobiopterin that is involved in the biosynthesis. This compound, called (6
R)-L-erythro-6,7-dihydrobiopterin, is unstable and may spontaneously revert to the more stable isomer. Nevertheless, the enzyme
6,7-dihydropteridine reductase acts directly on it to allow its recycling to tetrahydrobiopterin: == See also ==