Dopamine is a neurotransmitter that is important to vital brain functions like motor control and short term memory. Guanosine triphosphate cyclohydrolase I (GCH1) catalyzes the production of tetrahydrobiopterin (BH4), a required cofactor for tyrosine hydroxylase (TH), which is the rate-limiting enzyme in dopamine biosynthesis. TH is required for the conversion of L-tyrosine to L-DOPA which is then converted to dopamine. Dopamine is released at the synapse of the presynaptic neuron where it binds to its receptor on the postsynaptic neuron.Dopamine receptors are G-protein coupled receptors that can belong to either of two subtypes: D1-like receptor (D1, D5) or D2-like receptor (D2, D3, D4)...