Agonist binding by the β(2)-adrenergic receptor: an effect of receptor conformation on ligand association–dissociation characteristics

The β(2)-adrenergic receptor (β(2)-AR), a G protein-coupled receptor (GPCR), is a physiologically important transmembrane protein that is a target for drugs used for treatment of asthma and cardiovascular diseases. Study of the first steps of ligand recognition and the molecular basis of ligand binding to the orthosteric site is essential for understanding the pharmacological properties of the receptor. In this work we investigated the characteristic features of the agonist association–dissociation process to and from the different conformational forms of β(2)-AR by use of advanced molecular modeling techniques. The investigation was focused on estimating the free energy profiles (FEPs) corresponding to the process of a full agonist ((R,R)-fenoterol) and an inverse agonist (carazolol) binding and unbinding to and from β(2)-AR. The two different conformational forms of β(2)-AR, i.e. active β(2)-AR–PDB: 3P0G and inactive β(2)-AR–PDB: 2RH1 were included in this stage of the study. We revealed several significant qualitative differences between FEPs characteristic of both conformational forms. Both FEPs suggest the existence of three transient binding sites in the extracellular domain of β(2)-AR. Comparison of the residues surrounding these transient binding sites in both β(2)-AR states revealed the importance of the aromatic residues F194, H93(2.64), H296(6.58), and H178 (extracellular part of β(2)-AR) in the early stages of the binding process. In addition, slightly different exit and entry paths are preferred by the ligand molecule in the extracellular part of β(2)-AR, depending on the conformation of the receptor. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s00249-015-1010-4) contains supplementary material, which is available to authorized users.