Kd is the dissociation constant that reflects

Kd is the dissociation constant that reflects

• A. The concentration of ligand at which 50% of receptors are occupied
• B. The rate of receptor internalization
• C. The total number of receptors
• D. The maximum response

Answer: (A)

The key idea is how the dissociation constant reflects binding affinity and occupancy. Kd is the ligand concentration required to have half of the receptors bound at equilibrium. That means when [L] equals Kd, about 50% of receptors are occupied. A smaller Kd means higher affinity because less ligand is needed to reach that 50% occupancy, while a larger Kd means lower affinity. The occupancy follows roughly the relation fraction bound ≈ [L] / (Kd + [L]), so the 50% point occurs at [L] = Kd. This concept is about binding tightness, not how fast receptors are internalized, how many receptors exist, or how big the maximal response is. Those other factors relate to trafficking, receptor density, and signaling efficacy, respectively.

The key idea is how the dissociation constant reflects binding affinity and occupancy. Kd is the ligand concentration required to have half of the receptors bound at equilibrium. That means when [L] equals Kd, about 50% of receptors are occupied. A smaller Kd means higher affinity because less ligand is needed to reach that 50% occupancy, while a larger Kd means lower affinity. The occupancy follows roughly the relation fraction bound ≈ [L] / (Kd + [L]), so the 50% point occurs at [L] = Kd.

This concept is about binding tightness, not how fast receptors are internalized, how many receptors exist, or how big the maximal response is. Those other factors relate to trafficking, receptor density, and signaling efficacy, respectively.