Old and new ways to calculate the affinity of agonists and antagonists interacting with G-protein-coupled monomeric and dimeric receptors: the receptor-dimer cooperativity index.

Almost all existing models that explain heptahelical G-protein-coupled receptor (GPCR) operation are based on the occurrence of monomeric receptor species. However, an increasing number of studies show that many G-protein-coupled heptahelical membrane receptors (HMR) are expressed in the plasma membrane as dimers. We here review the approaches for fitting ligand binding data that are based on the existence of receptor monomers and also the new ones based on the existence of receptor dimers. The reasons for equivocal interpretations of the fitting of data to receptor dimers, assuming they are monomers, are also discussed. A recently devised model for receptor dimers provides a new approach for fitting data that eventually gives more accurate and physiological relevant parameters. Fitting data using the new procedure gives not only the equilibrium dissociation constants for high- and low-affinity binding to receptor dimers but also a "cooperativity index" that reflects the molecular communication within the dimer. A comprehensive way to fit binding data from saturation isotherms and from competition assays to a dimer receptor model is reported and compared with the traditional way of fitting data. The new procedure can be applied to any receptor forming dimers; from receptor tyrosine kinases to intracellular receptors (e.g., estrogen receptor) and in general for ligand binding to proteins forming dimers.