Agonist activation to open the Gα subunit of the GPCR–G protein precoupled complex defines functional agonist activation of TAS2R5

G protein-coupled receptors (GPCRs) regulate multiple cellular responses and represent highly successful therapeutic targets. The mechanisms by which agonists activate the G protein are unclear for many GPCR families, including the bitter taste receptors (TAS2Rs). We ascertained TAS2R5 properties by live cell-based functional assays, direct binding affinity measurements using optical resonators, and atomistic molecular dynamics simulations. We focus on three agonists that exhibit a wide range of signal transduction in cells despite comparable ligand-receptor binding energies derived from direct experiment and computation. Metadynamics simulations revealed that the critical barrier to activation is ligand-induced opening of the G protein between the α-helical (AH) and Ras-like domains of Gα subunit from a precoupled TAS2R5-G protein state to the fully activated state. A moderate agonist opens the AH-Ras cleft from 22 Å to 31 Å with an energy gain of -4.8 kcal mol^-1, making GDP water-exposed for signaling. A high-potency agonist had an energy gain of -11.1 kcal mol^-1. The low-potency agonist is also exothermic for Gα opening, but with an energy gain of only -1.4 kcal mol^-1. This demonstrates that TAS2R5 agonist-bound functional potencies are derived from energy gains in the transition from a precoupled complex at the level of Gα opening. Our experimental and computational study provides insights into the activation mechanism of signal transduction that provide a basis for rational design of new drugs.