Kinetics of receptor, ligand, and G protein interaction for signal transduction: A modeling study.

Normal cellular function requires that cells respond to stimuli in their environment. One of the most prevalent mechanisms by which cells respond to their environment involves the interaction of a ligand, receptor, and G-protein. In this work, the kinetics of the interaction between receptors and G-proteins are related to the physical characteristics of the tissue and ligand as well as the distribution of receptors and G-proteins in the plasma membrane. Additionally, this relationship is extended to examine responses in terms of G-protein activation. Monte Carlo techniques are used to simulate the reaction and diffusion of individual receptors and G-proteins in the plasma membrane to determine the effective reaction rate constant for the activation of G-proteins and the number of activated G-proteins. The rate constants determined from the simulation are then used in an ordinary differential equation model of activation to predict dose-response curves. For a random distribution of receptors and G-proteins, activation of G-proteins by collision with receptor-ligand complexes is found to have an effective rate constant $k\sb+$ that scales with the mean square displacement of a receptor-ligand complex, i.e. proportional to the diffusivity and inversely proportional to the ligand dissociation rate constant. The precoupling of receptors with G-proteins prior to ligand binding, the compartmentalization of receptors to domains, and the interconversion between active and inactive states are found to affect the effective activation rate and the level of G-protein activation. An ordinary differential equations model combining both literature binding models (ternary complex models) and activation models is developed. Values for the rate constant $k\sb+$ determined in the Monte Carlo simulations are used in the model to make predictions regarding dose-response curves and ligand efficacy, both of which are dramatically affected by the value for $k\sb+$ and the dynamics of GDP/GTP exchange and release. These results suggest experiments which may be used to further elucidate the details of signal transduction through G-protein coupled receptors in order to ultimately manipulate cellular responses.Ph.D.Applied SciencesBiological SciencesBiomedical engineeringCellular biologyChemical engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/130359/2/9722089.pd