CVS Practice Test 2026 – The All-In-One Guide to Exam Success!

In the synthesis of norepinephrine, the initial step involving tyrosine is tyrosine hydroxylation. This process is essential as it is the first reaction that transforms the amino acid tyrosine into L-DOPA (levodopa), the precursor for dopamine. Tyrosine hydroxylase, the enzyme responsible for this reaction, adds a hydroxyl (-OH) group to the aromatic ring of tyrosine. This step is critical because it sets the stage for the subsequent formation of dopamine, which in turn can be converted into norepinephrine and then epinephrine. Understanding this sequence is vital because tyrosine hydroxylation is the rate-limiting step in the catecholamine biosynthetic pathway; it controls the overall rate of production of norepinephrine and its derivatives. The other choices represent later steps in the biosynthesis or processes involving catecholamines rather than the direct initial transformation of tyrosine.