Which amino acid possesses an imidazole side chain capable of acting as both a proton donor and proton acceptor at physiological pH?
Answer and explanation
Correct answer: A. Histidine
Histidine contains an aromatic imidazole side chain with a pKa near 6.0. Because its pKa is close to physiological pH, histidine easily shifts between protonated (positively charged) and deprotonated (neutral) forms, allowing it to function as a versatile catalytic acid-base residue in many enzyme active sites. Lysine (pKa ~10.5) and arginine (pKa ~12.5) are fully protonated at pH 7.4.
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