On a binding curve, what is typically graphed on the x-axis?

On a binding curve, what is typically graphed on the x-axis?

• A. Receptor density.
• B. Time.
• C. Efficacy.
• D. Log concentration of the agonist.

Answer: (D)

The x-axis is the logarithm of the agonist concentration. Binding follows a saturable, Langmuir-type relationship, where occupancy increases with concentration and levels off at maximum binding. Using a log concentration on the x-axis spreads a wide range of concentrations into a more readable sigmoidal curve and places the half-occupancy point (the Kd) at its midpoint, making affinity comparisons straightforward. Time would be used for studying how binding changes over time, not for the x-axis of a binding isotherm, and efficacy is a property of the response, not the concentration axis. Receptor density affects the maximum possible binding but doesn’t determine what’s plotted on the x-axis.

The x-axis is the logarithm of the agonist concentration. Binding follows a saturable, Langmuir-type relationship, where occupancy increases with concentration and levels off at maximum binding. Using a log concentration on the x-axis spreads a wide range of concentrations into a more readable sigmoidal curve and places the half-occupancy point (the Kd) at its midpoint, making affinity comparisons straightforward. Time would be used for studying how binding changes over time, not for the x-axis of a binding isotherm, and efficacy is a property of the response, not the concentration axis. Receptor density affects the maximum possible binding but doesn’t determine what’s plotted on the x-axis.