What is the precursor amino acid for tyrosine?

Tyrosine is synthesized from phenylalanine through a biochemical pathway involving hydroxylation. Specifically, the enzyme phenylalanine hydroxylase catalyzes the conversion of phenylalanine to tyrosine. This step is critical because it introduces a hydroxyl group to the phenylalanine structure, transforming it into tyrosine. In humans, phenylalanine is an essential amino acid that must be obtained through the diet, making its role as a precursor to tyrosine particularly important. Tyrosine itself is later involved in the production of neurotransmitters like dopamine, as well as hormones such as epinephrine and norepinephrine.

The other amino acids listed, valine, leucine, and isoleucine, are branched-chain amino acids and do not serve as precursors for tyrosine synthesis. They have distinct metabolic pathways and functions within the body, unrelated to the production of tyrosine or its derivatives. Therefore, phenylalanine is the only correct precursor amino acid for the synthesis of tyrosine.