In the muscarinic binding site, which amino acid participates in ionic bonding?

• A. Aspartic acid 311
• B. Asparagine 617
• C. Tryptophan
• D. Serine 200

Answer: (A)

Charged electrostatic interactions drive how a ligand anchors in a receptor binding site. Acetylcholine has a quaternary ammonium group that carries a positive charge, so it seeks out a negatively charged partner in the receptor pocket. An aspartic acid residue provides that negative carboxylate side chain, making an ionic bond with the positively charged ammonium. This ionic interaction helps lock acetylcholine into the muscarinic binding site and contributes strongly to binding affinity.

Among the options, only the aspartic acid has a negatively charged side chain capable of forming a true ionic bond at physiological pH. The other residues—asparagine, serine, and tryptophan—are neutral (or nonionic in this context) and would not form the same strong ionic interaction with the ligand.