Journal of the American Chemical Society, Vol.135, No.25, 9465-9474, 2013
The Role of Ligands on the Equilibria Between Functional States of a G Protein-Coupled Receptor
G protein-coupled receptors exhibit a wide variety of signaling behaviors in response to different ligands. When a small label was incorporated on the cytosolic interface of transmembrane helix 6 (Cys-265), F-19 NMR spectra of the beta(2) adrenergic receptor (beta(2)AR) reconstituted in maltose/neopentyl glycol detergent micelles revealed two distinct inactive states, an activation intermediate state en route to activation, and, in the presence of a G protein :mimic, a predominant active state. Analysis of the spectra as a function of temperature revealed that for all ligands, the activation intermediate is entropically favored and enthalpically disfavored. beta(2)AR enthalpy changes toward activation are notably lower than those observed with rhodopsin, a likely consequence of basal activity and the fact that the ionic lock and other interactions stabilizing the inactive state of beta(2)AR are weaker. Positive entropy changes toward activation likely reflect greater mobility (configurational entropy) in the cytoplasmic domain, as confirmed through an order parameter analysis. Ligands greatly influence the overall changes in enthalpy and entropy of the system and the corresponding changes in population and amplitude of motion of given states, suggesting a complex landscape of states and substates.