Kd = koff / kon = ([R] · [L]) / [RL]; ΔG° = R · T · ln(Kd)
pioneer
Law of mass action applied to enzyme/receptor complexes by Leonor Michaelis, Maud Menten (1913), and Alfred J. Clark
Core Principles & Analysis
Defined by the law of mass action as Kd = ([R][L]) / [RL] and kinetically as the ratio of the unimolecular off-rate constant to the bimolecular on-rate constant (koff / kon), Kd equals the free ligand concentration at which exactly 50% of receptor binding sites are occupied at equilibrium.
A smaller Kd indicates tighter binding affinity: sub-nanomolar drugs (Kd < 10⁻⁹ M) and picomolar biologics (such as the avidin–biotin complex around 10⁻¹⁵ M) dissociate slowly, whereas millimolar fragment hits bind weakly.
At standard temperature T, Kd connects directly to the standard Gibbs free energy of binding via ΔG° = RT ln(Kd), where every 10-fold improvement in Kd corresponds to roughly −1.36 kcal/mol (−5.7 kJ/mol) of binding free energy at 25 °C.