Sodium Ions Regulate GPCR Activation by Remodeling Allosteric Coupling Networks and Hydration Patterns

Sodium ions ( Na ⁺ ) are key modulators of G-protein coupled receptor (GPCR) function, yet their mechanistic role remains incompletely understood. Here, we reveal a novel mode of Na ⁺ -mediated inactivation in the dopamine D2 receptor (DRD2), where Na ⁺ reshapes long-range allosteric coupling networks and disrupts a continuous internal water column essential for activation. Using extensive molecular dynamics simulations and alchemical free energy calculations, we show that Na ⁺ induces inactive-like residue interactions in the active state and triggers the formation of a distinct hydration gap. We also identify previously unreported Na ⁺ binding sites and quantify their impact on the active-inactive state equilibrium by thermodynamic scanning. These findings provide mechanistic insights into Na ⁺ -driven allosteric regulation of GPCRs and highlight new opportunities for drug design targeting ion-sensitive receptor states.