All kinds of ligands (natural ligand, agonist, partial agonist, antagonist) can occupy a binding site on a receptor.
Occupation of a receptor by a ligand does not automatically mean, that a receptor becomes activated.The receptor occupancy PA can be described by the following equation:
L = ligand concentration
Ka = binding constant (= K-1
The PA always gives a number between 0 and 1.
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 Ka then half of the total receptors is occupied (‘half maximum occupancy’).