Aspartic acid 311 participates in which type of interaction in the muscarinic binding site?

• A. Ionic bonding
• B. Hydrogen bonding
• C. Van der Waals
• D. Covalent bonding

Answer: (A)

Charged groups in a binding site often stabilize ligand binding through electrostatic attractions. Aspartic acid has a negatively charged carboxylate side chain at physiological pH, while a muscarinic ligand like acetylcholine carries a positively charged quaternary ammonium group. The opposite charges attract and form an ionic interaction (a salt bridge) that helps anchor the ligand in the site. Hydrogen bonding could occur with nearby donors/acceptors, but the prominent feature here is the strong ion–ion interaction between Asp311’s carboxylate and the ligand’s positive nitrogen. Van der Waals contacts are nonpolar and less specific for a charged pair, and covalent bonding would imply an irreversible bond, which is not how reversible receptor–ligand binding works.