Towards a thermodynamic definition of efficacy in partial agonism: The thermodynamics of efficacy and ligand proton transfer in a G protein-coupled receptor of the rhodopsin class.
The thermodynamic binding profiles of agonist and antagonist complexes of the 4-hydroxypropanolamine partial agonist, prenalterol, on the chronotropic adrenergic response in guinea-pig right atria were determined over a 15 °C temperature range. The tissue response was compared with data on the ethanolamine agonist, isoprenaline, given by binding studies in a number of rat tissues. Utilising the residue conservatism surrounding the known active conformers bound to either of two aspartate residues (α-helices II, III) in both receptors (β(1), β(2)) and species (guinea-pig, rat and human), no significant deformation in the extended side chain could be found in prenalterol's agonist binding compared to isoprenaline. Antagonist binding gave a highly favourable entropy contribution at 30.0 °C of -4.7±1.2 kcal/mol. The enthalpy change between bound agonist and antagonist complexes, a function of the efficacy alone, was -6.4±1.1 kcal/mol, coincident with the calculated intrinsic preference of a primary/secondary amine-aspartate interaction for a neutral hydrogen-bonded form over its ion pair state, giving values of 6.3-6.6 kcal/mol with calculations of good quality, a figure expected to be close to that shown within a hydrophobic environment. Delivery of a proton to a conserved aspartate anion (α-helix II) becomes the critical determinant for agonist action with resultant proton transfer stabilisation dominating the enthalpy change. A proposed monocation-driven ligand proton pumping mechanism within the ternary complex is consistent with the data, delivery between two acid groups being created by the movement of the cation and the counter-movement of the ligand protonated amine moving from Asp 138 (α-helix III) to Asp 104 (α-helix II).
Broadley
Kenneth J
KJ
The Welsh School of Pharmacy, The University of Cardiff, PO Box 13, Cardiff CF10 3XF, Wales, UK.
Sykes
Shane C
SC
Davies
Robin H
RH
D000318:Adrenergic beta-Agonists / Q000737:chemistry / Q000378:metabolism*; D000818:Animals; D006168:Guinea Pigs; D006325:Heart Atria / Q000378:metabolism; D066298:In Vitro Techniques; D008024:Ligands; D008956:Models, Chemical; D008958:Models, Molecular; D015394:Molecular Structure; D011485:Protein Binding; D017494:Proton Pumps; D018342:Receptors, Adrenergic, beta-1 / Q000378:metabolism; D018343:Receptors, Adrenergic, beta-2 / Q000378:metabolism; D043562:Receptors, G-Protein-Coupled / Q000819:agonists* / Q000037:antagonists & inhibitors; D012243:Rhodopsin / Q000737:chemistry*; D013816:Thermodynamics
D000318:Adrenergic beta-Agonists; D008024:Ligands; D017494:Proton Pumps; D018342:Receptors, Adrenergic, beta-1; D018343:Receptors, Adrenergic, beta-2; D043562:Receptors, G-Protein-Coupled; D012243:Rhodopsin