Advances in G protein-coupled receptor allostery: from function to structure

It is now widely accepted that G protein-coupled receptors (GPCRs) are highly dynamic proteins that adopt multiple active states linked to distinct functional outcomes. Furthermore, these states can be differentially stabilized not only by orthosteric ligands but also by allosteric ligands acting at spatially distinct binding sites. The key pharmacologic characteristics of GPCR allostery include improved selectivity due to either greater sequence divergence between receptor subtypes and/or subtype-selective cooperativity, a ceiling level to the effect, probe dependence (whereby the magnitude and direction of the allosteric effect change with the nature of the interacting ligands), and the potential for biased signaling. Recent chemical biology developments are beginning to demonstrate how the incorporation of analytical pharmacology and operational modeling into the experimental workflow can enrich structure-activity studies of allostery and bias, and have also led to the discovery of a new class of hybrid orthosteric/allosteric (bitopic) molecules. The potential for endogenous allosteric modulators to play a role in physiology and disease remains to be fully appreciated but will likely represent an important area for future studies. Finally, breakthroughs in structural and computational biology are beginning to unravel the mechanistic basis of GPCR allosteric modulation at the molecular level.

Allosteric agonists of 7TM receptors: expanding the pharmacological toolbox

Approximately 1% of the genome of higher organisms encodes seven-transmembrane (7TM) G-protein-coupled receptors, which control an extensive range of physiological processes and represent drug targets for nearly half of all drugs that are prescribed currently. To date, most drugs that target 7TM receptors interact via the same domain as the endogenous agonist, called the orthosteric site. However, the advent of functional screening assays has greatly increased the number of allosteric ligands identified. Such ligands bind to topographically distinct sites on 7TM receptors. In addition to modulating the affinity of orthosteric ligands, allosteric ligands can also alter the efficacy of orthosteric ligands and activate 7TM receptors in their own right. In this article, we briefly review the current status of putative allosteric agonists of 7TM receptors, and discuss the promises and challenges that this class of ligand might pose for pharmacologists and the drug-discovery industry.