The encounter coupling model for β- adrenergic receptor/GTP-binding protein interaction in the S49 cell: Calculation of the encounter frequency

Experiments measuring epinephrine stimulation of the S49 cell have demonstrated that the rate of adenylate cyclase activation is partly dependent on the rate of turnover of epinephrine occupancy with respect to individual receptors. Specifically, it has been shown that a low occupancy of the full receptor population by epinephrine promotes a rate of adenylate cyclase activation significantly greater than that for a low number of receptors completely occupied by epinephrine with which the concentration of bound receptors is the same. This finding indicated that the interaction of individual receptors with GTP-binding protein (G) occurs on a time scale which is greater than the mean lifetime of the epinephrine-receptor complex; during this period of interaction (an “encounter”), a receptor can change its occupancy state in the presence of a high binding frequency agonist such as epinephrine. Here we present a general analysis, in an extension of the Collision Coupling Modelof Tolkovsky and Levitzki (Biochemistry17: 3795–3810, 1978), of the consequences of encounters (rather than pure collisions) for the relationships of receptor occupancy, receptor-agonist complex lifetime, and receptor-agonist efficiency to G/adenylate cyclase activation. Using this “encounter coupling” model of receptor/G interaction, it is demonstrated from a theoretical standpoint that the net rate of G activation can depend in part on the agonist binding frequency. The predicted dependence is consistent with the data on which the model is based, in which high binding frequency increases the activation rate. A special case of the “encounter coupling model” allows calculation of the frequency of encounters by an analysis of a previous experiment using epinephrine in which the rate of adenylate cyclase activation was measured in response to a small number of fully occupied, highly efficient receptors. Using those results and the model developed here, the encounter frequency was found to be on the order of 100/min in the intact S49 cell. This calculation relied on knowledge of the rate of inactivation of G/adenylate cyclase in intact cells. A method for the measurement of the adenylate cyclase inactivation rate is presented. Using this method, the adenylate cyclase inactivation rate constant was found to be between 0.8 and 3.0/min.