(254e) Stochastic Modeling of CTB-GM1 Binding Mechanisms

Cholera toxin (CTx) is a toxin protein, which can lead to lethal cholera. CTx is an AB₅ protein that is consisted of an enzymatic A-subunit and five identical cholera toxin B-subunits (CTB) [1]. CTB binds with gangliosides such as GM1 on host cell membrane, which facilitates the endocytosis of CTx and the development of cholera. Despite extensive experimental and theoretical work on understanding the CTB structure and function, there is a lack of systematic framework that relies on fundamental understanding of the binding mechanisms of CTB at the microscopic level and utilizes such information to elucidate the detailed mechanism of CTB-GM1 interactions, which is the prerequisite for CTx cell entry. It is worth noting that a pentavalent CTB can simultaneously binds to multiple copies of GM1 in cell membranes. Also, the positive binding cooperativity between each binding subunit can further enhance the overall binding avidity. Since microscopic binding events highly dependent on the current surface configuration, the kinetic Monte Carlo methodology is applied to simulate the complex interactions between CTB and GM1 microscopically.

The goal of this research focuses on modeling and simulation of CTB-GM1 interactions. In this work, GM1 is modeled as an entity constrained to a 2 dimensional host cell membrane, whereas CTB is modeled as an entity capable of binding and detaching from the membrane surface. Three types of microscopic events are considered: 1) molecular attachment, 2) detachment, and 3) migration events on host cell membranes. The implementation of the kinetic Monte Carlo methodology requires knowledge of association, dissociation and migration reaction constants. Previous work computed a range of values to these parameters until satisfactory agreement between the calculated and the experimental binding rates was achieved [2, 3]. Due to the overwhelming likelihood of migration events, migration has been decoupled from the event selection process in order to improve computational efficiency [4]. For molecular attachment, it is assumed that each lattice site is available for attachment, and thus the attachment rate is equal over the lattice. Conversely, for molecular detachment and migration events, the rates are dependent on the local environment. Each local environment comprises up to 6 nearest neighbors. This allows us to classify our local environment into seven classes (zero to six nearest neighbors) in order to increase the computational efficiency when calculating the rates associated with executing a kinetic Monte Carlo event. We will also apply the kinetic Monte Carlo model to examine other host cell receptors such as GM2, which exhibits a distinct CTB binding kinetics. The specific details of how the GM1 concentration affects the CTB-GM1 interactions as well as steric hindrance between the CTB molecules on the membrane surface will be presented.

Reference

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3. Lin, H.; Kitova, E.N.; Klassen, J.S. Measuring positive cooperativity using the direct ESI-MS Assay. Cholera toxin B subunit homopentamer binding to GM1 pentasaccharide. J. Am. Soc. Mass. Spectrom. 2014, 25, 104-110.
4. Crose, M.; Kwon, J.S.; Nayhouse, M.; Ni, D.; Christofides, P.D. Multiscale modeling and operation of PECVD of thin film solar cells. Chem. Eng. Sci. 2015, 136, 50-61.