In protein-ligand interactions, what does the association rate constant (ka) represent?

• A. The rate at which the ligand is converted into a product.
• B. The equilibrium constant for the protein-ligand interaction.
• C. The rate at which the protein-ligand complex dissociates into free protein and ligand.
• D. The rate at which the free protein and ligand associate to form the protein-ligand complex.

Answer: (D)