The two-step model of ligand-receptor interaction.

The theoretical properties of a "two-step" receptor, consisting of two independent molecular entities, a regulatory (R) and an effector (E) unit, which are supposed to diffuse freely in the plane of the fluid and mosaic membrane, are presented. The ligand (L)-hormone, drug, neurotransmitter-binds to the regulatory unit (first step), so that it then becomes able to interact with the effector unit (second step) which is then activated.