All kinds of ligands (natural ligand, agonist, partial agonist, antagonist) can occupy a binding site on a receptor. Note that occupation of a receptor by a ligand does not automatically mean that a receptor becomes activated. Nevertheless, the proportion of receptor occupancy (RO) can be described by the following equation:
RO = [L] / (Kd + [L])
- L = ligand concentration
- Kd = dissociation constant (= K-1 / K+1)
The RO always gives a number between 0 and 1 (0-100%), as it is the proportion of receptor occupancy. The occupancy at different concentrations of ligand provides information on the amount of receptors and the affinity of the receptor for its ligand. If the ligand concentration is equal to the Kd then half of the total receptors is occupied (‘half maximum occupancy’).
Note that other abbreviations for RO, and [L] and Kd are also reported, using respectively PA, [A], and Ka. This results in the equation:
RO = [L] / (Ka + [L])
The use of Ka may be confusing, as Ka also stands for the association constant, but here (and often in literature) the equilibrium dissociation constant is meant.