Insights into the Role of Asp79^2.50 in β₂ Adrenergic Receptor Activation from Molecular Dynamics Simulations

Achieving a molecular-level understanding of G-protein-coupled receptor (GPCR) activation has been a long-standing goal in biology and could be important for the development of novel drugs. Recent breakthroughs in structural biology have led to the determination of high-resolution crystal structures for the β₂ adrenergic receptor (β₂AR) in inactive and active states, which provided an unprecedented opportunity to understand receptor signaling at the atomic level. We used molecular dynamics (MD) simulations to explore the potential roles of ionizable residues in β₂AR activation. One such residue is the strongly conserved Asp79^2.50, which is buried in a transmembrane cavity and becomes dehydrated upon β₂AR activation. MD free energy calculations based on β₂AR crystal structures suggested an increase in the population of the protonated state of Asp79^2.50 upon activation, which may contribute to the experimentally observed pH-dependent activation of this receptor. Analysis of MD simulations (in total >100 μs) with two different protonation states further supported the conclusion that the protonated Asp79^2.50 shifts the conformation of the β₂AR toward more active-like states. On the basis of our calculations and analysis of other GPCR crystal structures, we suggest that the protonation state of Asp^2.50 may act as a functionally important microswitch in the activation of the β₂AR and other class A receptors