Category: biosynthesis
Stepwise biosynthesis of dopamine, norepinephrine, and epinephrine from L-tyrosine. The rate-limiting step is tyrosine hydroxylation (TH); the BH4 cofactor regenerates via DHPR + DHFR. Loss of nigrostriatal dopamine neurons is the proximate cause of Parkinson disease — levodopa replaces the missing precursor and bypasses TH. DBH and PNMT are restricted to noradrenergic/adrenergic neurons + the adrenal medulla.