What is the difference between Ki and IC50 in enzyme inhibition?

The value Ki is the dissociation constant describing the binding affinity between the inhibitor and the enzyme, while IC50 is the concentration of inhibitor required to reduce the enzymatic activity to half of the uninhibited value.

Both values can be used as quantitative indexes for the inhibitor potency. However, Ki is an intrinsic measure of affinity, which is independent of enzyme concentration (although it may depend on substrate concentration due to different mechanisms of inhibition); whereas IC50 is an operational parameter to describe the effective strength of a particular inhibitory substance, which is dependent on the enzyme concentration. Besides, IC50 is always larger than Ki.

By definition, IC50 is the “total” concentration of inhibitor needed to reach 50% inhibition; while Ki is the “free” concentration of inhibitor required to reach 50% enzyme saturation.

In an equilibrium of 50% inhibition, [I]t = [I]b + [I]f, where [I]t is the total concentration of inhibitor (i.e. IC50); [I]b is the concentration of inhibitor bound to the enzyme, which equals to 50% of total enzyme concentration (i.e. E/2) in this case; and [I]f is the concentration of free inhibitor, that is Ki.

Hence, IC50 = E/2 + Ki.

Therefore, IC50 is dependent on the enzyme concentration, and is always larger than Ki.