Molecular Dynamics Simulations of the Effect of the G-Protein and Diffusible Ligands on the β2-Adrenergic Receptor

G-protein-coupled receptors have extraordinary therapeutic potential as targets for a broad spectrum of diseases. Understanding their function at the molecular level is therefore essential. A variety of crystal structures have made the investigation of the inactive receptor state possible. Recently released X-ray structures of opsin and the β2-adrenergic receptor (β2AR) have provided insight into the active receptor state. In addition, we have contributed to the crystal structure of an irreversible agonist–β2 adrenoceptor complex. These extensive studies and biophysical investigations have revealed that agonist binding leads to a low-affinity conformation of the active state that is suggested to facilitate G-protein binding. The high-affinity receptor state, which promotes signal transduction, is only formed in the presence of both agonist and G-protein. Despite numerous crystal structures, it is not yet clear how ligands tune receptor dynamics and G-protein binding. We have now used molecular dynamics simulations to elucidate the distinct impact of agonist and inverse agonist on receptor conformation and G-protein binding by investigating the influence of the ligands on the structure and dynamics of a complex composed of β2AR and the C-terminal end of the Gαs subunit (GαCT). The simulations clearly showed that the agonist isoprenaline and the inverse agonist carazolol influence the ligand-binding site and the interaction between β2AR and GαCT differently. Isoprenaline induced an inward motion of helix 5, whereas carazolol blocked the rearrangement of the extracellular part of the receptor. Moreover, in the presence of isoprenaline, β2AR and GαCT form a stable interaction that is destabilized by carazolol.