TRC: Occupancy

Occupancy

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 (R_O) can be described by the following equation:

R_O = [L] / (K_d + [L])

L = ligand concentration
K_d = dissociation constant (= K_-1 / K₊₁)

The R_O 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 K_d then half of the total receptors is occupied (‘half maximum occupancy’).

Note that other abbreviations for R_O, and [L] and Kd are also reported, using respectively P_A, [A], and Ka. This results in the equation:

R_O = [L] / (K_a + [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.

Occupancy

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