Molecular basis for high affinity agonist binding in GPCRs

A characteristic of GPCRs in the G protein-coupled state is that the affinity of the agonist often increases significantly, but the molecular basis for this is unclear. We have determined six active-state structures of the β 1 -adrenoceptor (β 1 AR) bound to conformation-specific nanobodies in the presence of agonists of varying efficacy. A direct comparison with structures of β 1 AR in inactive states bound to the identical ligands showed a 24-42% reduction in the volume of the orthosteric binding site. Potential hydrogen bonds were also shorter, and there was up to a 30% increase in the number of atomic contacts between the receptor and ligand. GPCRs are highly conserved, so these factors will likely be essential in increasing the affinity of a wide range of structurally distinct agonists. One Sentence Summary High affinity agonist binding to G protein-coupled GPCRs results from an increase in the number and strength of protein-ligand interactions.