Molecular Mechanisms of Monovalent Cation Selectivity in Na+-Coupled Secondary Transporters from Free Energy Simulations with Implicit and Explicit Treatment of Electronic Effects

Abstract

The ability to efficiently discriminate different cations is essential for the proper physiological functioning of channels and transporters. Strict monovalent selectivity of Na+-coupled secondary transporters is essential to realize the strong coupling between the transport of the substrate and Na+ flux. Loss of selectivity would result in the substrate being pumped outward. The x-ray structures of Na+-dependent amino-acid transporters (LeuT and Glt)[1,2], provides a great opportunity to better understand the molecular basis of monovalent cation selectivity.Extensive all-atom free energy molecular dynamics simulations with hamiltonian construted with additive, polarizable force-fields and QM/MM descriptions of the binding pocket are performed at various occupancy states of the binding sites. In this work, we demonstrate that there is a collective effect of multiple binding sites on a total selectivity for Na+ over K+ (and Tl+) in LeuT and Glt transporters. The polarizable models of metal ions and protein (for sites Na1 and Na2 identified in structural studies) employed to provide better understanding for ion selectivity [3] as well as to mechanism of ion-substrate coupling in Glt transporter. The role of local connectivity, site rigidity and atomic polarization in monovalent cation selectivity is discussed. We will also present results from decomposition analysis.[1] Boudker, O., Ryan, R.M., Yernool, D., Shimamoto, K., Gouaux, E., 2007, Nature 445: 387-393.[2] Yamashita, A., Singh, S.K., Kawate, T., Jin, Y., Gouaux, E., 2005, Nature 437: 215-223.[3] Lev B., Salohub D., Noskov S., Computational Life Sciences,Vol. 2, No. 1, pp. 12-20.