Catecholamine biosynthesis proceeds from tyrosine to dopa, dopamine, norepinephrine, and finally epinephrine. The rate-limiting enzyme of this entire pathway, whose product feeds back to inhibit it, is:

• A Dopa decarboxylase
• B Tyrosine hydroxylase ✓
• C Dopamine beta-hydroxylase
• D Phenylethanolamine N-methyltransferase

Explanation

Tyrosine hydroxylase converts tyrosine to L-dopa using tetrahydrobiopterin as cofactor, and this committed step limits flux through the whole pathway. It is inhibited by its end products dopamine and norepinephrine through feedback, and activated by phosphorylation during sympathetic stimulation. Option A is a nonspecific decarboxylase present in many tissues, and option D is adrenal medulla specific but acts last in the sequence, so neither controls overall rate. Dopamine beta-hydroxylase works inside vesicles, not at the regulatory step.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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