The Equilibrium Binding Constant and Binding Strength

In the cell, proteins randomly collide with other molecules, such as other proteins, nucleic acids, and small-molecule ligands.

If the binding is non-specific, few non-covalent interactions between the molecules result in a brief association.

If a specific ligand binds to the protein, it forms extensive non-covalent interactions along complementary surfaces. Such complexes are stable, staying bound for a long time, before dissociating.

The strength of a binding interaction is reported in terms of its equilibrium constant, K_b, also called the binding or association constant.

K_b can be calculated from the ratio of the concentration of protein-ligand complex over the concentrations of unbound protein and ligand found at equilibrium.

Given its relation with the free energy change due to binding, a large K_b means a large decrease in delta G, indicating strong affinity between protein and ligand.

Two competing processes are important for protein-ligand binding: the association of a protein and a ligand to form a complex and the dissociation of the complex into the reactants.

The association constant, k_on, is a measure of the number of binding events per second between a protein and its ligand; it can be used to calculate the rate of ligand binding to the protein at a given concentration.

Conversely, k_off, is a measure of the number of dissociation events; it can be used to calculate how quickly the complex comes apart.

When the rate of association equals the rate of dissociation, an equilibrium is reached, where the net concentrations of products and reactants remain constant.

Thus, at equilibrium, k_on times the product of the equilibrium concentrations of the protein and the ligand equals k_off times the equilibrium concentration of the protein-ligand complex.

Rearranging this expression also shows that the ratio of k_on to k_off equals K_b at equilibrium.