The parameter EC50 abbreviates for ‘half maximal effective concentration’. In a pharmacological context, this can be the concentration of a drug that is necessary to cause half of the maximum possible effect. In a binding experiment, the EC50 is the concentration of ligand (see Concentrations) at which half of the Target is present in the bound state.
Both the EC50 and the Kd serve to quantify interactions and for example compare the Binding Affinity of different ligands (see Ligand). In the context of TRIC measurements, using the Kd Fit Model for data evaluation will yield a Kd, while using the Hill Model will yield an EC50 value. See the respective articles for more details.
However, there is an important difference: the Kd is a physical property of the interaction and independent of the target concentration. Kds can therefore generally be compared across technologies as long as the general interaction conditions are the same.
The EC50, on the other hand, by definition, always depends on the target concentration. It is often measured via the effect it induces, for example in cell-based assays, and allows only the comparison of ligands measured in the same experimental setup. In situations where the target concentration is lower than the Kd , Kd and EC50 will have the same value, provided the interaction follows a 1:1 stoichiometry, is non-cooperative (see Cooperativity) and is determined by the law of mass action. In situations where the target concentration is in the range of or above the Kd, Kd and EC50 can differ significantly.
If a given interaction has a Kd value of 2 nM and the target is present at a concentration of 2 µM, a ligand concentration of 1 µM is necessary to reach a state where half of all target molecules are in the bound state. By definition the EC50 amounts to 1 µM (500*Kd) in this case.