Which amino acid is needed for the biosynthesis of norepinephrine and epinephrine?

Which amino acid is needed for the biosynthesis of norepinephrine and epinephrine?

• A. Alanine
• B. Phenylalanine
• C. Isoleucine
• D. Leucine

Answer: (B)

Norepinephrine and epinephrine are made from the amino acid tyrosine through a chain of enzymatic steps: tyrosine is hydroxylated to DOPA, decarboxylated to dopamine, then converted to norepinephrine and finally to epinephrine in the adrenal medulla. Tyrosine is the direct substrate for the first committed step in this pathway, so the availability of tyrosine controls catecholamine synthesis. Phenylalanine fits as the needed precursor because it can be converted into tyrosine by phenylalanine hydroxylase, supplying the tyrosine pool used to make these neurotransmitters. The other amino acids listed do not feed into this pathway. Also, disorders that block phenylalanine to tyrosine conversion (like phenylketonuria) illustrate why phenylalanine is essential for maintaining catecholamine production.

Norepinephrine and epinephrine are made from the amino acid tyrosine through a chain of enzymatic steps: tyrosine is hydroxylated to DOPA, decarboxylated to dopamine, then converted to norepinephrine and finally to epinephrine in the adrenal medulla. Tyrosine is the direct substrate for the first committed step in this pathway, so the availability of tyrosine controls catecholamine synthesis. Phenylalanine fits as the needed precursor because it can be converted into tyrosine by phenylalanine hydroxylase, supplying the tyrosine pool used to make these neurotransmitters. The other amino acids listed do not feed into this pathway. Also, disorders that block phenylalanine to tyrosine conversion (like phenylketonuria) illustrate why phenylalanine is essential for maintaining catecholamine production.