A kinetic model of GPCRs: analysis of G protein activity, occupancy, coupling and receptor-state affinity constants

• Published in: Journal of receptors and signal transduction 2015-07, Vol.35 (4), p.269-283
• Main Authors: Stein, Richard S. L., Ehlert, Frederick J.
• Format: Article
• Language: English
• Subjects: Active state affinity constant; Animals; Computer Simulation; constitutive activity; GTP-Binding Proteins - chemistry; GTP-Binding Proteins - metabolism; GTPase activity; guanine nucleotide; Humans; inverse agonism; Kinetics; Ligands; Models, Biological; quaternary complex; Receptors, G-Protein-Coupled - agonists; Receptors, G-Protein-Coupled - chemistry; Receptors, G-Protein-Coupled - metabolism; Signal Transduction
• ISSN: 1079-9893; 1532-4281
• DOI: 10.3109/10799893.2014.975250

Context: G protein-coupled receptors are vital macromolecules for a wide variety of physiological processes. Upon agonist binding, these receptors accelerate the exchange of GDP for GTP in G proteins coupled to them. The activated G protein interacts with effector proteins to implement downstream biological functions. Objective: We present a kinetic, quaternary complex model, based on a system of coupled linear first-order differential equations, which accounts for the binding attributes of the ligand, receptor, G protein and two types of guanine nucleotide (GDP and GTP) as well as for GTPase activity. Methods: 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. Results: Our model shows that agonist-induced G protein activation can occur with either a net decrease or increase in total receptor-G protein coupling. In addition, we 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. Discussion and conclusion: The latter result validates our prior methods for estimating the active state affinity constants of ligands, and our results on receptor coupling have relevance to studies investigating receptor-G protein interactions using fluorescence techniques.