Which statement is true about Ka and Kd?

Which statement is true about Ka and Kd?

• A. Ka measures the association constant and Kd measures the dissociation constant
• B. Ka measures dissociation and Kd measures association
• C. Both measure dissociation
• D. Both measure occupancy

Answer: (C)

In binding equilibria, Ka and Kd correspond to opposite directions of the same process: Ka is the association constant, defined as Ka = [LR]/([L][R]), while Kd is the dissociation constant, defined as Kd = [L][R]/[LR]. They are reciprocals (Ka = 1/Kd). This means that a higher Ka (or a lower Kd) reflects tighter binding between ligand and receptor. Occupancy, or the fraction of receptors bound, depends on both Ka (or Kd) and the actual ligand concentration, via f = [LR]/[Rtot] = [L]/([L] + Kd). So the true statement is that Ka measures association and Kd measures dissociation; they are not both dissociation, and they do not directly measure occupancy.

In binding equilibria, Ka and Kd correspond to opposite directions of the same process: Ka is the association constant, defined as Ka = [LR]/([L][R]), while Kd is the dissociation constant, defined as Kd = [L][R]/[LR]. They are reciprocals (Ka = 1/Kd). This means that a higher Ka (or a lower Kd) reflects tighter binding between ligand and receptor. Occupancy, or the fraction of receptors bound, depends on both Ka (or Kd) and the actual ligand concentration, via f = [LR]/[Rtot] = [L]/([L] + Kd). So the true statement is that Ka measures association and Kd measures dissociation; they are not both dissociation, and they do not directly measure occupancy.