A Kinetic Model of GPCRs: Analysis of G protein Activity, Occupancy, Coupling and Receptor‐State Affinity Constants

Abstract

G protein-coupled receptors, upon agonist binding, accelerate the exchange of GDP for GTP in G proteins coupled to them. The activated G protein interacts with effector proteins to mediate diverse biological functions. We present a kinetic, quaternary complex model, based on a system of coupled linear first-order differential equations that accounts for GTPase activity, ligand binding to receptor, and guanine nucleotide binding to G protein. We solved the model numerically to predict the extents of G protein activation, receptor occupancy by ligand, and receptor coupling that result from varying the ligand concentration, presence of GDP and/or GTP, the ratio of G protein to receptor, and the equilibrium constants governing receptor pre-coupling and constitutive activity. We also simulated responses downstream from G protein activation using a transducer function. Our model shows that agonist-induced G protein activation can occur with either a net decrease or increase in total receptor-G protein coupling. We also demonstrate that affinity constants of the ligand for both the active and inactive states of the receptor can be derived to a close approximation from analysis of simulated responses downstream from receptor activation. Finally, we show that the concentration of GDP greatly affects the potency of agonists for stimulating the binding of non-hydrolyzable GTP analogs. Our analysis relates to agonist-stimulated [35S]GTPγS binding measurements and the interpretation of receptor-G protein interactions using fluorescence techniques. Finally, it validates our prior methods for estimating the active state affinity constants of ligands.