The ligand-receptor-G-protein ternary complex as a GTP-synthase. Steady-state proton pumping and dose-response relationships for beta-adrenoceptors

Steady-state solutions are developed for the rate of G alpha GTP production in a synthase model of the ligand-receptor-G-protein ternary complex activ ated by a ligand-receptor proton pumping mechanism. The effective rate, k(3 1), defining the proton transfer, phosphorylation and G alpha.GTP release i s a controlling rate of the synthase in the presence of a ligand with an ef ficient mode of signal activation, the ligand-receptor interaction taking p lace under effectively equilibrium conditions. The composite rate, however, becomes an amplifying factor in any dose-response relationship. The amplif ication is a triple product of the rate, k(31), the equilibrium constant as sociated with the activation of the proton signal, K-act and the fraction o f agonist conformer transmitting the signal,f*. Where the rate of activatio n of the proton signal becomes critically inefficient, the rate of activati on, k(act1) replaces k(31)K(act). A correlation between beta(1)-adrenergic receptor-stimulated GDP release and adenylate cyclase activation shows that this correlation is not unique to an-exchange reaction. Within the initiat ing Tyr-Arg-Tyr receptor proton shuttle mechanism, the position of Arg(r156 )parallel to dictates the high-(R-p) and low-(R-u) ligand-binding affinitie s. These states are close to R* and R-o of the equilibrium model(De Lean et al., 1980, J. Biol. Chem. 255, 7108-7117). An increased rate of hydrogen i on diffusion into a receptor mutant can give rise to constitutive activity while increased rates of G-protein release and changes in receptor state ba lance can contribute to the resultant level of action. Constitutive action will arise from a faster rate of G-protein release alone if proton diffusio n in the wild-type receptor contributes to a basal level of G-protein activ ation. 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 generate d 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. Sec. Lend. B 220, 141-162). When comparable ligands have effectively identical conformational states at the transition state fo r signal activation, the antagonist component of the binding "in vitro" can be derived by multiplying the apparent binding constant by(1 - e) where e is the maximum stimulatory response. This component should be consistent th roughout the tissues. (C) 2000 Academic Press.