# pH-Driven β2AR Dynamics Reveal Loop-Mediated Allosteric Communication

## TL;DR

This study uses simulations to show how pH changes affect the structure and communication in the β2-adrenergic receptor, a type of GPCR.

## Contribution

The study introduces pH-driven CpHMD simulations to reveal loop-mediated allosteric communication in β2AR.

## Key findings

- Loop regions like ICL3 and ECL2 show strong responses to pH changes in β2AR simulations.
- pH variations alter loop flexibility and hydrogen-bonding patterns, creating a 'see-saw-like' effect.
- Mutual information analysis shows pH-dependent communication changes between extracellular and intracellular loops.

## Abstract

Membrane protein structure and dynamics are highly sensitive
to
environmental conditions, including changes in pH that can alter the
protonation states of ionizable residues and, in turn, influence local
electrostatics and stability. Constant-pH molecular dynamics (CpHMD)
provides a framework to explore such effects by allowing dynamic proton
exchange during simulations. Here, we applied CpHMD at pH:6.5, 7.0,
and 8.0, alongside conventional MD, to examine how pH variations may
influence the local conformational behaviors of the β2-adrenergic receptor (β2AR). During the 1.2-μs-long
total simulation, loop regions rich in titratable residues, particularly
ICL3 and ECL2, showed the strongest responses to protonation changes.
CpHMD trajectories suggested a pH-dependent redistribution of loop
flexibility and hydrogen-bonding patterns, producing a “see-saw-like”
effect, while fixed-protonation Control runs showed more constrained
behavior. Across all simulations, the key GPCR microswitches, such
as the ionic lock, the Y–Y gate, the NPxxY and PIF motifs,
and the Trp286–Phe290 toggle pair, stayed within the ranges
expected for an inactive receptor. This suggests that pH changes mainly
influence local loop motions in the inactive receptor without pushing
it toward activation-like states. Finally, mutual information analysis
on both Cα atoms and dihedral angles revealed altered communication
between the extracellular and intracellular loops under different
pH environments. While limited in time scale, these results provide
a computational perspective on how protonation dynamics can modulate
the GPCR behavior and highlight the value of incorporating pH effects
in molecular-level investigations.