A Physical Model Of Non-Specificity In P-GP's Selection Of Substrates

Nowadays, clinical resistance to chemotherapeutic drugs is a major problem in the treatment of tumor cells. Traditionally, tumor cells are considered to have a high spontaneous mutation rate, so drug-resistance tumor cells will emerge naturally. As the researches get to deepness, a new mechanism has been realized, in which Pump-glycoprotein (P-gp) plays important role. P-gp can extrude a wide variety of substrates including anti-tumor drugs out of cells, thus reduces the number of drugs which interact with intracellular targets. In this way, these cells are confered multidrug resistance.A lot of researches have been made on P-gp. Three-dimensional structure of P-gp at 0.38 nm resolution shows that P-gp has portals open to the lipid bilayer for drug entry. P-gp's substrates include anti-tumor drugs, antibiotics, etc. There are three important theoretical models on the mechanism of P-gp's pumping action. pH dependence model explains the experiments in which the interaction of P-gp's substrates with intracellular targets is pH dependent. Cleaner model explains some of P-gp's substrates are pumped out of cells directly. Both models above don't consider about P-gp's portals enough. Flippase model considers the portals and explains non-specificity of P-gp's substrates and the differential handling of its substrates based on their lipid solubility successfully. However, flippase model can't explain three problems:(1) the influence of ionic strength on P-gp's dimension (2) the main reason of substrates entering the lumen of P-gp efficiently (3) the consumption number of ATP hydrolyzed per substrate transported. To solve the three problems of flippase model, from the point of view that the basic interaction between biological molecules is electric and the change of free energy decides the direction and limit of an reaction, the electrostatic free energy of P-gp and the system P-gp—substrate are calculated based on Debye-Huckel theory. Some significant results are obtained.The thesis is divided into three chapters. In chapter 1, P-gp's structure, substrates' patterns, mechanism of pumping action, the role of anti-apoptosis, P-gp inhibitors and experimental methods are introduced, which provide the basis of the simplified model of P-gp's pumping action. In chapter 2, the background, application and derivation of Debye-Huckel theory are introduced, which are the theoretical basis of the calculation of electrostatic free energy. Chapter 3 is the main part of this paper. A simplified model of P-gp's pumping action and equivalent distribution of charge are obtained. A formula of the electrostatic free energy of P-gp without substrates is induced, which describes the influence of ionic strength and P-gp's dimension on the electrostatic free energy of P-gp. Furthermore, the change of electrostatic free energy is also induced in the process of substrates entering the lumen of P-gp. The main reason of substrates entering the lumen of P-gp with a high effectiveness is proposed. The energy minimum of transporting a substrate is obtained, which has the same order as the energy from one ATP hydrolyzed. It is illustrated that one ATP needs to be hydrolyzed per substrate transported. Two indirect ways are proposed to inhibit P-gp.