The Ligand–Receptor–G-Protein Ternary Complex as a GTP-Synthase.Steady-State Proton Pumping and Dose–Response Relationships for β -Adrenoceptors

Steady-state solutions are developed for the rate of G α.GTP production in a synthase model of the ligand–receptor–G-protein ternary complex activated by a ligand–receptor proton pumping mechanism. The effective rate, k31, defining the proton transfer, phosphorylation and G α.GTP release is a controlling rate of the synthase in the presence of a ligand with an efficient mode of signal activation, the ligand–receptor interaction taking place under effectively equilibrium conditions. The composite rate, however, becomes an amplifying factor in any dose–response relationship. The amplification is a triple product of the rate, k31, the equilibrium constant associated with the activation of the proton signal, Kactand the fraction of agonist conformer transmitting the signal, f*. Where the rate of activation of the proton signal becomes critically inefficient, the rate of activation, kact 1replacesk31Kact . A correlation between β1-adrenergic receptor-stimulated GDP release and adenylate cyclase activation shows that this correlation is not unique to an exchange reaction. Within the initiating Tyr–Arg–Tyr receptor proton shuttle mechanism, the position of Argr156∥∥All cited receptor residues (prefix r) are referenced to their position in the β1-adrenoceptor (human). For convenient reference to X-ray data, the G-protein numbering is referenced to the G α t and Gβ1 -subunits. es the high-(Rp) and low-(Ru) ligand-binding affinities. These states are close to R*and R0of the equilibrium model (De Lean et al., 1980, J. Biol. Chem.255, 7108–7117). An increased rate of hydrogen ion diffusion into a receptor mutant can give rise to constitutive activity while increased rates of G-protein release and changes in receptor state balance can contribute to the resultant level of action. Constitutive action will arise from a faster rate of G-protein release alone if proton diffusion in the wild-type receptor contributes to a basal level of G-protein activation. Competitive ligand–receptor occupancy for constitutive mutants shows that, where the rate of G-protein activation from the proportion of ligand-occupied receptors is less than the equivalent rate that would be generated from this fraction by proton diffusion, inverse agonism will occur. Rate-dependent dose–responses developed for the proposed synthase mechanism give explicit definition to the operational model for partial agonism (Black & Leff, 1983, Proc. Roy. Soc. Lond. B220, 141–162). When comparable ligands have effectively identical conformational states at the transition state for signal activation, the antagonist component of the binding “in vitro” can be derived by multiplying the apparent binding constant by (1−e) wheree is the maximum stimulatory response. This component should be consistent throughout the tissues.