What does the protein ligand dissociation equilibrium constant (Kd) indicate about ligand concentration?

• A. It indicates the ligand concentration at which half of the binding sites are occupied.
• B. It indicates the ligand concentration at which the binding rate is maximized.
• C. It indicates the ligand concentration at which no binding sites are occupied.
• D. It indicates the ligand concentration at which all binding sites are occupied.

Answer: (A)