An allosteric activator increases receptor affinity and/or efficacy; what is the expected effect on the agonist dose–response curve?

• A. The curve shifts to the left and the maximum response may increase.
• B. The curve shifts to the left with no change in maximum response.
• C. The curve becomes steeper without shifting the EC50.
• D. The curve remains unchanged.

Answer: (A)

An allosteric activator that raises affinity and/or efficacy makes the receptor respond at lower agonist concentrations and can boost the maximum possible response. Practically, this shows up as a leftward shift in the dose–response curve (potency increases because EC50 decreases). If the activator also enhances efficacy and there are enough receptors available to realize it, the maximal response can also be higher. So the best description is a left-shifted curve with the potential for a higher maximum. The other patterns don’t reflect both increased potency and possible higher efficacy.