pK_a Calculations of GPCRs: Understanding Protonation States in Receptor Activation

The increase in the available G protein-coupled receptor (GPCR) structures has been pivotal in helping to understand their activation process. However, the role of protonation-conformation coupling in GPCR activation still needs to be clarified. We studied the protonation behavior of the highly conserved Asp^2.50 residue in five different class A GPCRs (active and inactive conformations) using a linear response approximation (LRA) pK_a calculation protocol. We observed consistent differences (1.3 pK units) for the macroscopic pK_a values between the inactive and active states of the A2AR and B2AR receptors, indicating the protonation of Asp^2.50 during GPCR activation. This process seems to be specific and not conserved, as no differences were observed in the pK_a values of the remaining receptors (CB1R, NT1R, and GHSR).