General thermodynamic considerations of receptor interactions

Assuming that the biological response is directly proportional to the fractional degree of receptor occupancy, the mathematical relationships between free ligand concentration, receptor occupation and biological activity are developed for a number of equilibrium models. The models considered include simple 1:1 binding with and without conformational changes in the receptor, the coupled binding of two distinct effectors to a single macromolecule, and a system involving indirect coupling between two effectors that bind to two distinct components of the receptor system. This latter model is elaborated into the concept of a domain of receptor-enzyme pairs such that occupation of a single receptor may activate the entire domain of enzymes. This model can explain discrepancies between activation and binding isotherms as has been found with some beta-adrenergic agonist-sensitive adenylate cyclase systems.