Which amino acid's side chain is most likely to act as a substrate for HRP?

• A. Lys
• B. Leu
• C. Tyr
• D. Gln

Answer: (C)

Tyrosine is the amino acid whose side chain is most likely to act as a substrate for horseradish peroxidase (HRP). This is primarily due to the presence of a phenolic group in the side chain of tyrosine, which can readily undergo oxidation. HRP is an enzyme that catalyzes the oxidation of various substrates, with a particular affinity for phenolic compounds, making tyrosine an ideal substrate.

The other amino acids do not possess the same characteristics that would make them suitable substrates for HRP. For instance, lysine has an amino group in its side chain that is more basic and does not exhibit the reactivity that a phenolic group does. Leucine, being a nonpolar and hydrophobic amino acid, has a side chain that lacks functional groups capable of participating in the oxidation reactions that HRP facilitates. Glutamine's side chain, while containing an amide functional group, does not have the oxidation potential associated with the phenolic hydroxyl of tyrosine.

In summary, the specific structure of tyrosine allows it to fit the reactivity profile needed for HRP's enzymatic activity, making it the most appropriate choice among the amino acids listed.