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.

For an antagonist, binding behaviour is determined entirely by the binding affinity, or more specifically by the components that make up the binding affinity. However, for an agonist, in addition to binding affinity, the agonist’s efficacy may also influence binding behaviour. The reasons for this are complex and lie beyond what it is necessary for clinical healthcare providers to understand. In brief, when agonist binding results in a structural change in the target protein to generate an activated receptor state, application of an agonist with high efficacy can drive the equilibrium existing between unbound, bound/inactive, and bound/activated receptor forms towards the bound/activated state such that half of the total receptor population can be bound by agonist at concentrations of the agonist far below the K_D, representing the binding affinity of agonist for the receptor in its inactive state. This occurs for reasons that are, in some ways, similar to those responsible for the difference between the K_M and the K_D for an enzyme substrate in a simple enzyme system, wherein the K_M represents the concentration of substrate at which the enzyme is working at half of its maximum velocity, and K_D is the concentration of substrate that binds to 50% of the total enzyme protein at equilibrium, in the absence of any subsequent metabolism. For an enzyme with a high V_max (or, more correctly, a high k_cat representing the rate constant for the reaction step converting substrate to product), the K_M can be higher, and perhaps very much higher, than the K_D, but is similar to the substrate’s K_D when the k_cat is small and the enzyme works at a relatively slow rate.