G Protein-Mediated Allosteric Modulation of Ligand Binding in Class A GPCRs: Receptor-Ligand-Transducer Ensembles in Disease and Drug Discovery.

G protein-coupled receptors (GPCRs) are dynamic allosteric proteins whose signaling properties are governed by reciprocal communication between extracellular ligand-binding sites and intracellular transducer interfaces. Although classical pharmacological models established the concept that ligand binding and G protein coupling are thermodynamically linked, recent structural, biophysical, and computational studies have revealed a far more complex picture in which GPCRs exist as ensembles of interconverting conformational states. Accumulating evidence indicates that G proteins function not only as downstream signaling effectors but also as endogenous allosteric modulators. By reshaping receptor conformational landscapes, G protein coupling can influence the structure and dynamics of orthosteric ligand-binding pockets, thereby regulating ligand affinity, binding kinetics, and receptor selectivity. These findings support a bidirectional model of GPCR signaling in which information is transmitted not only from ligand-binding sites to intracellular signaling partners but also in the reverse direction through receptor-wide allosteric networks. Disease-associated mutations of endothelin A receptor (ETAR) provide in vivo evidence that structural perturbations located far from orthosteric ligand-binding sites can alter ligand recognition through long-range allosteric communication. In addition, emerging studies of positive allosteric modulators demonstrate the therapeutic potential of selectively stabilizing ligand-receptor-G protein complexes. These observations suggest that ligand recognition, receptor activation, and transducer coupling should be viewed as integrated properties of a dynamic receptor-ligand-transducer ensemble. This perspective provides a conceptual framework that links classical GPCR pharmacology, structural biology, disease mechanisms, and next-generation drug discovery.