Conformational Selection and Allosteric Regulation upon Ligand Binding in G-Protein Coupled Receptors

Abstract

G-protein coupled receptors (GPCRs) are allosteric membrane proteins mediating cellular signaling. GPCRs exhibit multiple inactive and active conformations, and the population balance between these conformations is altered upon binding of signaling molecules (or ligands). However, the nature of the conformational ensemble or the mechanism of the conformational transitions is not well understood. We have applied a multiscale computational approach combining a coarse-grained discrete conformational sampling method with fine-grained molecular dynamics to investigate the effect of various ligands binding on the ensemble of conformations sampled by human β2-adrenergic receptor (β2AR). We show that the receptor, in the absence of any ligand, samples an extensive conformational space that includes breathing of the orthosteric ligand binding site and shear motion of the transmembrane helices 5 and 6 against the other helices. The shear motion is similar to the reorganization of the intracellular regions of TM3, TM5, and TM6 observed in the crystal structure of the active state of GPCRs. Upon ligand binding a shift in population density, as well as a reduction in the number of conformations sampled by the receptor is observed. Binding of agonist norepinephrine or partial agonist salbutamol leads to the selection of a subset of conformations that include both active and inactive state conformations, while inverse agonist carazolol selects only the inactive state conformation. Possible mechanisms for the allosteric regulation of GPCR activity by ligand binding were identified by correlating receptor-ligand interactions with receptor sidechain orientation near the G-Protein coupling region.