Final answer:
The rate-limiting enzyme in the dopamine synthetic pathway is tyrosine hydroxylase, which converts L-tyrosine to L-DOPA, the precursor to dopamine and other catecholamines.
Step-by-step explanation:
The rate-limiting enzyme in the dopamine synthetic pathway is tyrosine hydroxylase. This enzyme catalyzes the conversion of L-tyrosine to 3,4-dihydroxyphenylalanine (L-DOPA), which is a precursor to dopamine, norepinephrine, and epinephrine. Tyrosine hydroxylase's activity is tightly regulated and often limited by the availability of its cofactor, tetrahydrobiopterin (BH4), and substrate, tyrosine. This makes it the rate-limiting step in the synthesis of catecholamines.
The other enzymes listed, such as DOPA decarboxylase, dopamine β-hydroxylase, phenylethanolamine N-methyltransferase (PNMT), and catechol-O-methyltransferase, are also important in the metabolism of dopamine and other catecholamines. However, they are involved in subsequent steps of catecholamine synthesis and metabolism after the action of tyrosine hydroxylase. The metabolism of dopamine in the liver involves catechol-O-Methyl-Transferase (COMT) and Mono-Amine Oxidase (MAO) to form metabolites like vanillylmandelic acid (VMA), which is a major excretory product of catecholamines.