dissociation constant
The dissociation constant, Kd, is a single number that captures how tightly a ligand binds. Concretely, it is the concentration of free ligand at which exactly half of the target's binding sites are filled — so a smaller Kd means a tighter grip, because it takes less drug to reach the halfway point.
Kd is the ratio of the off-rate to the on-rate (koff/kon) for the binding reaction at equilibrium, and it has units of concentration (often nM or µM). It is the quantitative face of affinity: a Kd of 1 nM is high affinity, while a Kd in the millimolar range is weak, as seen for many initial fragment hits.
Because Kd equals the half-occupancy concentration, it directly links binding to biology: at a free drug concentration of Kd, receptor occupancy is 50%; at ten times Kd it is about 91%. This makes Kd a useful anchor when reasoning about what dose is needed to engage a target, though plasma protein binding and tissue distribution shift the free concentration the target actually sees.
If a ligand's Kd is 10 nM, then at 10 nM free ligand half its target's sites are occupied; raising the concentration to 100 nM fills about 91% of them.
Kd is the free-ligand concentration giving 50% occupancy.
Kd describes binding at equilibrium; IC50 and EC50 describe functional responses in a particular assay. They often track each other but are conceptually different and need not be equal.