Binding pathway determines norepinephrine selectivity for the human βAR over βAR.

Beta adrenergic receptors (βARs) mediate physiologic responses to the catecholamines epinephrine and norepinephrine released by the sympathetic nervous system. While the hormone epinephrine binds βAR and βAR with similar affinity, the smaller neurotransmitter norepinephrine is approximately tenfold selective for the βAR. To understand the structural basis for this physiologically important selectivity, we solved the crystal structures of the human βAR bound to an antagonist carazolol and different agonists including norepinephrine, epinephrine and BI-167107. Structural comparison revealed that the catecholamine-binding pockets are identical between βAR and βAR, but the extracellular vestibules have different shapes and electrostatic properties. Metadynamics simulations and mutagenesis studies revealed that these differences influence the path norepinephrine takes to the orthosteric pocket and contribute to the different association rates and thus different affinities.