The association constant Ka describes how much drug-receptor complexes are being formed.

• A. Dissociation constant
• B. Association constant
• C. Receptor density
• D. Ligand stability

Answer: (B)

The main idea is that Ka is the equilibrium constant for forming the drug–receptor complex. For the binding reaction where a free ligand L binds a free receptor R to form LR, Ka = [LR] / ([L][R]). This shows how readily the complex is produced: a larger Ka means more LR at the same concentrations, i.e., stronger binding affinity. Since Ka is the reciprocal of the dissociation constant (Kd = 1/Ka), a high Ka corresponds to a low Kd and tight binding. The amount of complex formed also depends on how much ligand and receptor are available, described by the binding isotherm, but Ka specifically measures the tendency to form the complex rather than how many receptors exist or how stable the ligand is chemically. Receptor density affects the total number of complexes, not the affinity, and ligand stability is about chemical stability, not binding propensity.