On a hyperbolic binding curve, Kd is defined as

On a hyperbolic binding curve, Kd is defined as

• A. Ligand concentration at 100% occupancy
• B. The affinity constant
• C. The ligand concentration at 50% occupancy
• D. The maximum binding capacity

Answer: (B)

On a hyperbolic binding curve, the key relationship is that the ligand concentration that yields half of the binding sites occupied defines the dissociation constant. In other words, at the point where occupancy is 50%, the ligand concentration equals Kd. This is why Kd is a measure of affinity: the smaller the Kd, the tighter the binding (higher affinity). Note that Kd is the dissociation constant, not the association (affinity) constant, which would be Ka = 1/Kd. Maximum binding capacity is governed by Bmax, not Kd. So the ligand concentration at 50% occupancy best describes Kd.

On a hyperbolic binding curve, the key relationship is that the ligand concentration that yields half of the binding sites occupied defines the dissociation constant. In other words, at the point where occupancy is 50%, the ligand concentration equals Kd. This is why Kd is a measure of affinity: the smaller the Kd, the tighter the binding (higher affinity). Note that Kd is the dissociation constant, not the association (affinity) constant, which would be Ka = 1/Kd. Maximum binding capacity is governed by Bmax, not Kd. So the ligand concentration at 50% occupancy best describes Kd.