Translocation Mechanism of Allosteric Sodium Ions in ?(2)-Adrenoceptor

The allosteric modulation of G-protein-coupled receptors (GPCRs) by sodium ions has received significant attention as the crystal structures of several receptors show the binding of sodium ions (Na+) at the conserved D2.50. Theoretical studies have shown that extracellular Na+ would enter the allosteric D2.50 via the orthosteric site. However, it remains unclear how the bound allosteric Na+ would leave the GPCRs. In this study, we performed molecular dynamics (MD) simulations to illustrate the energy barriers of Na+ transfer through the transmembrane helix bundle of beta 2AR. In contrast to the postulations from other GPCRs, the translocation of this allosteric Na+ into the intracellular side is found to be significantly difficult. Hence, the translocation direction could be receptor-specific.