Stereoselective binding of agonists to the β2-adrenergic receptor: insights into molecular details and thermodynamics from molecular dynamics simulations
The β2-adrenergic receptor (β2-AR) is one of the most studied G-protein-coupled receptors. When interacting with ligand molecules, it exhibits a binding characteristic that is strongly dependent on ligand stereoconfiguration. In particular, many experimental and theoretical studies confirmed that stereoisomers of an important β2-AR agonist, fenoterol, are associated with diverse mechanisms of binding and activation of β2-AR. The objective of the present study was to explore the stereoselective binding of fenoterol to β2-AR through the application of an advanced computational methodology based on enhanced-sampling molecular dynamics simulations and potentials of interactions tailored to investigate the stereorecognition effects. The results remain in very good, quantitative agreement with the experimental data (measured in the context of ligand–receptor affinities and their dependence on the temperature), which provides an additional validation for the applied computational protocols. Additionally, our results contribute to the understanding of stereoselective agonist binding by β2-AR. Although the significant role of the N2936.55 residue is confirmed, we additionally show that stereorecognition does not depend solely on the N293–ligand interactions; the stereoselective effects rely on the co-operation of several residues located on both the 6th and 7th transmembrane domains and on extracellular loops. The magnitude and character of the contributions of these residues may be very diverse and result in either enhancing or reducing the stereoselective effects. The same is true when considering the enthalpic and entropic contributions to the binding free energies, which also are dependent on the ligand stereoconfiguration.