Structural insights into adrenergic receptor function and pharmacology

Abstract

It has been over 50years since Sir James Black developed the first beta adrenergic receptor (βAR) blocker to treat heart disease. At that time, the concept of cell surface receptors was relatively new and not widely accepted, and most of the tools currently used to characterize plasma membrane receptors had not been developed. There has been remarkable progress in receptor biology since then, including the development of radioligand binding assays, the biochemical characterization of receptors as discrete membrane proteins, and the cloning of the first G-protein-coupled receptors (GPCRs), which led to the identification of other members of the large family of GPCRs. More recently, progress in GPCR structural biology has led to insights into the three-dimensional structures of βARs in both active and inactive states. Despite all of this progress, the process of developing a drug for a particular GPCR target has become more complex, time-consuming and expensive.

Figure 1

Signaling pathways regulated by the β2AR. Solid lines indicate activation while dashed lines indicate inhibition. Abbreviations: AC - adenylyl cyclase, GRK -G protein coupled receptors kinase, Gs – the stimulatory G protein for adenylyl cyclase, Gi – pertussis toxin sensitive G proteins, MAPK – mitogen activated protein kinase, PI3K -phosphoinositide 3 kinase, AKT – a serionine/threonine protein kinase initially identified in the AKT virus.

Figure 2

Subtype specific ligand binding in the β2AR. Amino acids that differ between β1AR and β2AR are shown in yellow. The inverse agonist carazolol is shown with green carbons. A, The binding pocket of the human β2AR. Only one of the 15 amino acids that constitute the antagonist binding pocket (defined as being within 4A of the inverse agonist carazol) differs between β1AR and β2AR. Tyr 308 at the top of TM7 in the β2AR is Phe in the β1AR. B, The extracellular surface of the β2AR. C, The interior surface of the β2AR that has been split along the plane of the binding pocket, TMs1 through 5 on the right and TMs 6 and 7 on the left.

Figure 3

Active and inactive states of the β2AR. A, β2AR-Nb80 complex. CDR3 of Nb80 projects into the cytoplasmic core of the protein. B, Side view of superimposed active (orange) and inactive (blue) conformations of the β2AR. C. Cytoplasmic view of superimposed active and inactive states. D, Binding pocket of active and inactive states of the β2AR are shown separately (outer panels) and overlapped (center). Small changes in the ligand binding pocket lead to rearrangement of the conserved amino acids (Pro 211, Ile 121, Phe 282, and Asn 318). These changes are indicated by arrows in the center panel. Carazolol is shown with yellow carbons and the agonist BI-167107 is shown with green carbons.