The Mechanism of Polyvalent Ion Competition with Membranes and Membrane-Associated Proteins

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With this award, the Chemistry of Life Processes Program is funding Drs. Jeffery Klauda and Sergei Sukharev at the University of Maryland to study the binding competition of polyvalent ions with lipids and membrane-associated proteins. Calcium ions in solution play multiple critical roles as signaling agents and cofactors that mediate muscle contraction, nerve signal transmission, blood clotting, and, importantly, dead cell removal. Positively charged calcium ions assist in recognition of injured and dying cells by special biological processes, which depend on calcium binding to negatively-charged lipid molecules exposed during cell death. Specific proteins recognize these calcium-coated dying (apoptotic) cells and are key to removing these cells. However, some toxic metal ions, such as beryllium, can compete with, displace, and suppress natural functions mediated by calcium. Drs. Jeffery Klauda and Sergei Sukharev use a combined experimental and computational approach to understand the basic aspects of binding and competition of calcium and beryllium to model compounds, lipids, and proteins. This project provides insight into the mechanisms of how beryllium interferes with multiple calcium-dependent processes and specifically disrupts apoptotic cell recognition and the associated cleaning cascade in the body. This project provides training for undergraduate and graduate student researchers, and the research results are disseminated through presentations at scientific meetings and publications. This project involves educational outreach through summer research internships for high school students at minority-serving institutions. A series of magnet practicums will allow students to actively perform supervised research in the labs of Drs. Klauda and Sukharev. In addition, course lectures and practical examples from the project research are integrated into undergraduate and graduate engineering and science courses to increase engagement and exposure.Probing the structure and thermodynamics for ions binding to lipids in cellular membranes has been challenging. This project aims to use isothermal titration calorimetry (ITC) to obtain free energies of binding for calcium(II) and beryllium(II) to model compounds for lipids and peptide carbonyls/carboxylates. Fourier Transform-Infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopic techniques are used to determine the structure of ions binding to liposomes with the aid of quantum mechanical (QM) calculations of model clusters. QM and experimental measures (ITC, FTIR, Langmuir monolayer experiments, and NMR) are used to parametrize an additive and a polarizable Drude force field for ion-water, -lipid and -protein interactions. These force fields will be used in molecular dynamics (MD) simulations with enhanced sampling approaches to study the coordination and competitive binding of calcium(II) and beryllium(II). The final aspect of this project is probing the binding of model phosphatidylserine (PS) receptors to liposomes and/or apoptotic cells in the presence of calcium(II) and beryllium(II) with fluorescent microscopy, flow cytometry and electron paramagnetic resonance. In parallel, calcium(II) displacement by beryllium(II) in the calcium(II)-arranged PS contacts with these protein receptors is studied with MD simulations to probe the critical role of coordination and the strength of these interactions. Together these studies are expected to provide insight into the mechanism of how beryllium(II) disrupts intracellular signaling, including details of extracellular PS recognition and of Be(II)-disruption of the apoptotic cell cleaning cascade.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.