G protein-coupled receptors (GPCRs, see Glossary) are an important class of membrane proteins targeted by approximately one third of the drugs currently on the market 1, 2. They are activated by a wide variety of signaling molecules of different nature: from proteins or peptides (i.e., chemokine receptors), to small neurotransmitters and neuromodulators including nucleosides and nucleotides [3]. The latter group is where we find the four receptors activated by adenosine, consisting of the A1, A
Classical AR agonists and antagonists share several interactions within the orthosteric binding pocket, since they are derived of a similar planar heterocycle (Box 2), from where modifications can confer high affinity, selectivity and/or intrinsic activity (i.e., the ribose moiety). The crystal structures of the A2AAR confirmed that the central heterocycle of both agonists and antagonists resides in the same binding pocket, dominated by H bonds with the completely conserved N2536×55(N/N/N/N) in
Mutations in the ELs do not only influence binding of orthosteric ligands, but may also play a role in ligand kinetics. In addition, this region plays a structural role through a series of cysteine bridges, and has been related to selectivity among certain receptor subtypes. Finally, mutational studies and lately crystal structures [11] suggest that this region might be the binding site of PAMs, in analogy to other GPCRs [51]. Here, we discuss the mutational data of the EL region in ARs.
Allosteric modulation of GPCRs is gaining acceptance as a new approach for drug development, since allosteric ligands typically display higher target selectivity compared to orthosteric ligands 3, 58. In ARs, two distinct receptor regions have been revealed as sites for allosteric modulation (Figure 3): the EL2 region and the sodium-binding pocket.
The intracellular side of the TM bundle is more conserved within the GPCR superfamily than the extracellular domain [2]. This region undergoes the most pronounced conformational changes upon receptor activation and is implicated in the binding of the intracellular G protein. Four motifs play a major role here (Figure 4): NPxxY in TM7, the DRY motif (TM3), the ionic lock and the TDY triad, as observed in the G protein-bound crystal structure [19].
The effects of point mutations of the four ARs on ligand binding affinities, functional potencies, and efficacies constitute a valuable source of pharmacological information. We analyzed the existing data and mapped it on the collection of available AR crystal structures, allowing for a comprehension of receptor–ligand interactions and receptor activation outstanding within the GPCR families. A majority of the mutational data collected refers to orthosteric ligand binding, where combinations of
The authors declare that they have no conflict of interest. Ligand selectivity between receptor subtypes can be achieved by exploiting sequence differences within the binding site (selectivity hotspots), but it also can be due to residues in the loop regions distal from the binding site. Additionally, the latest crystal structures reveal large conformational differences in both regions between ARs. To what extent do these structural properties contribute to ligand selectivity? The stereospecific
The authors are members of the European COST Action CM1207 (GLISTEN), where this project was conceived. This work was supported by the Swedish Research Council (Willem Jespers and Hugo Gutiérrez-de-Terán, Grant 521-2014-2118). Gerard J.P. van Westen thanks the Dutch Research Council Toegepaste en Technische Wetenschappen (NWO-TTW) for financial support (Veni #14410). Eddy Sotelo thanks Consellería de Cultura, Educación e Ordenación Universitaria of the Galician Government: (grant: GPC2014/03),