Molecular dynamics of a biophysical model for β2-adrenergic and G protein-coupled receptor activation

This study analyzes 16 molecular dynamic simulations of a biophysical model for β2-adrenergic (B2AR) and G protein-coupled receptor (GPCR) activation. In this model, a highly conserved cysteine residue, C106 (C3.25 or CysIII:01), provides a free sulfhydryl or thiol group in an acid–base equilibrium between uncharged (RSH) and charged (RS−) states that functions as an electrostatic molecular switch for receptor activation. The transition of C106 in the B2AR between acid and base states significantly changes the helical/transmembrane (TM) domain interactions and the electrostatic interaction energy differences (ΔΔEEL). The ΔΔEEL changes correlate well with the experimentally observed ligand efficacies. The TM interaction energies display patterns compatible with those previously recognized as responsible for GPCR activation. Key differences between the agonist, epinephrine, and the antagonist, pindolol, are seen for the TM3 × 6, TM3 × 4, TM6 × 7 and TM1 × 7 interaction energies. Pindolol also produces a weaker ΔΔEEL interaction and less TM interaction energy changes, which are important differences between the agonist and antagonist ligands. The D115E mutant with pindolol displays a greater ΔΔEEL and TM interactions than for the wild-type B2AR with pindolol. This explains the higher activity of pindolol in the D115E mutant. The constitutively active D130A mutant displays TM interaction patterns similar to those for the activating ligands implying a common pattern for receptor activation. These findings support the broad concept of protean agonism and demonstrate the potential for allosteric modulation. They also demonstrate that this two-state model agrees with many previous experimental and theoretical observations of GPCRs.