The dissociation constant K_d describes the equilibrium between the bound and unbound state. Therefore, the K_d value is a measure for the affinity of a binding site to a ligand. This constant is independent of the concentrations of the interaction partners.

In contrast to the K_d value, the EC₅₀ describes the effective dose of ligand at which half of all target molecules are present in the bound state (EC₅₀ stands for half maximal effective concentration).

For interactions that follow a simple 1:1 equilibrium binding model (no cooperativity), the K_d value and the EC₅₀ are connected via the equation

It follows that EC₅₀ and K_d have the same value at target concentrations far below the K_d. If the target concentration is however in the range of the K_d or above, K_d and EC₅₀ can differ significantly. Because of their concentration dependence, EC₅₀ values are connected to a specific experimental setup and do not transfer to other methods. 

Example

If a given interaction has a K_d of 2 nM (a quite high affinity) and the target is present at a concentration of 200 nM, a ligand concentration of ~100 nM would be necessary to reach a state where half of all target molecules are in the bound state. By definition, the EC₅₀ amounts to ~100 nM in this case.

Figure 1: The EC₅₀ for a system with a 2 nM K_d at 200 nM target concentration is ~100 nM.

Note that the K_d of this interaction is still 2 nM, as this parameter rather describes the physical strength of a molecular interaction. After all the K_d can also be expressed as a function of the thermodynamic equilibrium of an interaction (ΔG = RT ln(K_d)). The common definition of the K_d as the ligand concentration at which half of all target molecules are in the bound state only applies at target concentrations well below the K_d, i.e. where K_d ≈ EC₅₀.