Adrenergic Receptors use Proton Conduction to Transduce Ligand Binding Energy into Activating Conformational Change

Abstract

It is proposed that proton conduction in class A G protein coupled receptors (GPCRs) assists in transducing orthosteric ligand binding energy into activating structural movement at their cytosolic domain. The binding site and cytosolic domain are 26 angstroms apart in the beta2 adrenergic receptor. How this action at a distance occurs is not fully understood. To investigate this problem, classical all-atom molecular dynamics simulations were used to simulate the behavior of the transmembrane (TM) domain under different orthosteric binding pocket conformations. The beta1 and beta2 adrenergic receptors were used as model GPCRs. Five different models were simulated for approximately 100ns each. Analysis of the simulations revealed that agonist (activating orthosteric ligand) binding causes a Grotthus-type water wire to form, connecting an aspartic acid and aspartate residue. Wire formation is the result of a bound agonist forcing changes in the bend angles of TM helices 4, 5, and 7. The changes in the bend angles form a hydrogen bond between a conserved aspartate in TM2 (D2.50) and the asparagine of the conserved NPxxY motif. Formation of this hydrogen bond results in a disruption of a solvent shell around D2.50, and as a consequence, a wire is formed. Water wires have been characterized to conduct protons in some membrane proteins Based on the simulation results, it is proposed that GPCR agonists drive proton conduction by reorienting internal waters in receptors through changes in TM helix bend angles. In this model, proton conduction in GPCRs occurs by a process similar to that in piezoelectric sensors. The forces applied by agonist binding reorients the internal dipoles of the receptor to make conduction energetically favorable and kinetically feasible. A mechanism is proposed to explain how proton conduction may activate receptors.