Which of the following amino acids has a side-chain pKₐ near neutrality and therefore plays an important role as proton donor and acceptor in many enzyme-catalyzed reactions?

Question

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Histidine has a side-chain pKₐ of approximately 6.0, which is close to physiological pH. This allows it to act as both a proton donor and acceptor in many enzyme-catalyzed reactions. Cysteine has a much higher pKₐ, around 8.5, and is typically involved in redox reactions and disulfide bond formation. Aspartic acid has a much lower pKₐ, around 4.0, and is typically involved in acid-base catalysis. Arginine has a pKₐ around 12.5 and is typically involved in stabilizing negatively charged groups.

Which of the Following Amino Acids Has a Side-Chain PKₐ