Binding

This is the term used to refer to the interaction between a ligand (L) and a receptor or other protein target (P). The shape of a binding curve (binding plotted against ligand concentration) is usually hyperbolic, and is described by the Hill-Langmuir equation. Binding is usually reversible and saturable, reaching a maximum at very high ligand concentrations when every protein target molecule present is occupied by a ligand for the maximum possible proportion of the total time.

There are two components to reversible binding: association and dissociation, the rates of which are determined by the product of the concentration(s) of the species involved in the reaction ([L] and [P] for association, [LP] for dissociation) and the rate constants for association (k_on) and dissociation (k_off). At equilibrium, rates of association and dissociation are equal. Dividing k_off by k_on yields a value for K_D, a measure of the binding affinity.