What's the difference between Kd and IC50?

Question

What's the difference between the equilibrium binding constant ($K_d$) and Inhibitory Concentration 50 $IC_{50}$? They both seem to mark halfway points. Is the only difference that $K_d$ refers to any binding, whereas $IC_{50}$ refers exclusively to inhibition?

Answer 1

First, let's review the definitions of $K_d$ and $IC_{50}$.

$K_d$

$K_d$ refers to the dissociation rate at equilibrium and is calculated by $\frac{K_{on}}{K_{off}}$. This is the rate at which the peptide is bound to the target protein ($K_{on}$) divided by the rate at which the peptide unbinds or comes off the target protein ($K_{off}$). So when you look at binding curves, it's the 50% bound/unbound point on the curve (the inflection point).

$IC_{50}$

$IC_{50}$ indeed refers specifically to the inhibition of binding or inhibition of a function at which 50% of the peptide is bound and 50% is not bound (or 50% of the function is active and 50% is inactive).

Comparison

In some cases $K_{d}$ and $IC_{50}$ may be very similar values even identical, but in other cases $K_{d}$ and $IC_{50}$ values actually differ.

Pragmatically, $K_{d}$ values generally are considered more "constant" and vary less with changes in concentration of the target protein and peptide. They are more independent from the assay set up and type. In contrast, an $IC_{50}$ value can range depending on the assay set up (not considered constant values). For example, the $IC_{50}$ values in Fluorescence Polarization depend heavily on what "probe" molecule was used and the concentrations of the target protein and peptide.