Which statement is NOT a characteristic of receptor-ligand binding?

• A. Receptor alternates between active and inactive conformations
• B. Receptor reversibly binds ligands
• C. Once bound, the ligand is permanently bound and never released
• D. Ligand is unchanged by binding

Answer: (C)

Receptor-ligand binding is a dynamic, reversible interaction driven by non-covalent forces. The receptor can switch between active and inactive shapes, and when a ligand binds it often stabilizes one of these states to propagate a signal. The ligand is typically not chemically altered by binding and can dissociate, allowing the system to reset and signals to be turned off. The idea that binding is permanent and the ligand is never released contradicts the reversible, transient nature of these interactions, so that statement is not a characteristic.