Activation processes in ligand‐activated G protein‐coupled receptors: A case study of the adenosine A_2A receptor

Abstract

Here we review concepts related to an ensemble description of G-protein-coupled receptors (GPCRs). The ensemble is characterized by both inactive and active states, whose equilibrium populations and exchange rates depend sensitively on ligand, environment, and allosteric factors. This review focuses on the adenosine A receptor (A R), a prototypical class A GPCR. F Nuclear Magnetic Resonance (NMR) studies show that apo A R is characterized by a broad ensemble of conformers, spanning inactive to active states, and resembling states defined earlier for rhodopsin. In keeping with ideas associated with a conformational selection mechanism, addition of agonist serves to allosterically restrict the overall degrees of freedom at the G protein binding interface and bias both states and functional dynamics to facilitate G protein binding and subsequent activation. While the ligand does not necessarily "induce" activation, it does bias sampling of states, increase the cooperativity of the activation process and thus, the lifetimes of functional activation intermediates, while restricting conformational dynamics to that needed for activation.

Insights into adenosine A2A receptor activation through cooperative modulation of agonist and allosteric lipid interactions

“…The cumulative effects of bound phospholipids and agonist are observed to have profound consequences on the overall population of receptor conformations, which can be defined according to key interhelical TM3-TM6 and TM3-TM7 distances (Fig 10, S24 Fig, S25 Fig). Furthermore, MD simulations with the more potent NECA agonist clearly reveal that agonist-mediated receptor activation results in approximately equal formation of two distinct active-like receptor conformations, as previously proposed by NMR experiments [41]. One of these conformations has a comparatively wider separation between TM3, TM6, and TM7 helices and resembles the G protein-bound active crystal structure, while the other has relatively closer TM helices but is still active-like (Fig 10).…”

“…inactive form towards the active state has not yet been described with unbiased MD simulations and it remains unclear how agonists select or induce the active form of A2aR [41].…”

“…In particular, MD simulations have been successful in providing accurate molecular features of GPCR conformational space [36]. In general, the typical behaviour of class A GPCRs, when they are observed without bound G protein, is to fluctuate between inactive and intermediate(s) states without inducing the fully active conformation, even in the presence of an agonist [40][41][42][43][44][45][46][47][48]. Therefore, one of the current challenges with GPCRs is to understand the molecular basis for their agonist-mediated transition into the active state.…”

“…On the other hand, recent MD studies have obtained results on the important role of ligand binding in A2aR activation [44,67,69,70]. In addition, A2aR has been described as a weakly coupled system, due to the fact that binding of an agonist does not guarantee that the active state is reached under experimental conditions [41] and ligand binding may not be an exclusive driving force for achieving full receptor activation [44,67,69,70,78]. Thus, to our knowledge, the full agonist-mediated transition of A2aR from the https://doi.org/10.1371/journal.pcbi.1007818.g001…”

Quantitative Structural Assessment of Graded Receptor Agonism

“…Related to our work here on nuclear receptors, NMR studies have been used to define how ligand binding influences the conformational ensemble of GPCRs in ways not detectable in ligand-bound crystal structures. Onedimensional 19 F NMR studies of the GPCRs β2-adrenergic receptor (β2AR) and adenosine A2 receptor (A2AR) tagged with a 19 F NMR reporter molecule attached to two cysteine residues revealed that pharmacologically distinct classes of ligands or nanobodies differentially stabilize the receptor conformational ensemble into G-protein inactive and active states (Liu et al 2012, Kim et al 2013, Staus et al 2016, Ye et al 2016, Prosser et al 2017, Susac et al 2018. We similarly used 19 F NMR to show that pharmacologically distinct PPARγ ligands differentially affect the conformation of the AF-2 helix using a 19 F NMR reporter molecule attached to helix 12, and the conformations we observed are predictive of ligand efficacy (Chrisman et al 2018).…”