title: A kinetic model of GPCRs: analysis of G protein activity, occupancy, coupling and receptor-state affinity constants.
abstract: BACKGROUND
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: a Department of Pharmacology, School of Medicine , University of California , Irvine , CA , USA.
authors.forename: Richard S L
authors.initials: RS
authors.lastname: Stein
authors.affiliation: a Department of Pharmacology, School of Medicine , University of California , Irvine , CA , USA.
authors.forename: Frederick J
authors.initials: FJ
authors.lastname: Ehlert
mesh_terms: D000818:Animals; D003198:Computer Simulation; D019204:GTP-Binding Proteins / Q000737:chemistry / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D008024:Ligands; D008954:Models, Biological; D043562:Receptors, G-Protein-Coupled / Q000819:agonists / Q000737:chemistry / Q000378:metabolism*; D015398:Signal Transduction
chemical_list: D008024:Ligands; D043562:Receptors, G-Protein-Coupled; D019204:GTP-Binding Proteins
keywords: Active state affinity constant; GTPase activity; constitutive activity; guanine nucleotide; inverse agonism; quaternary complex