The rest of the discussion assumes that [L] is equal to EC50. If we are at EC50, then the system is also at 50% of Emax.
According to Clark's theory, the only way to achieve 50% of Emax is for half of the receptors to be bound by the ligand (a full agonist) as the receptor‐ligand complex (R‐L). If half of the receptors are bound, then an equal number (half) of the receptors are not bound and exist in the form of free receptor (R).
Remember that the binding of a ligand to a receptor is a reversible process. This equilibrium binding is quantified by the dissociation equilibrium constant (KD). The equation for KD is shown below.
As has been stated, at 50% Emax, [R‐L] = [R]. In the equation for the equilibrium dissociation constant, two terms cancel and leave the relationship KD = [L].
At 50% E50, [L] also equals EC50. Through a simple substitution, EC50 = KD.
This relationship is important. Experimentally, KD is not easy to measure directly, but through a log [L]‐response plot, EC50 is easy fairly easy to determine. Because EC50 and KD are equal, a log [L]‐ response plot is a relatively simple method for indirectly determining the equilibrium dissociation constant (KD) of a receptor and its agonist or partial agonist. OPTIONAL‐Please participate in the online discussion forum.