Chloride binding site of neurotransmitter sodium symporters

Figure 4. The impact of a negative charge on the interaction network near the Na1 and S1 binding sites revealed by MD simulations. A and B show the evolution of the distances between the closest heavy atoms of the residues at positions 250 and 290 in (A) the E290S mutant and (B) the WT in the course of the MD trajectories; the corresponding constructs are depicted in C–F. The curves in A (E290S mutant) are from simulations carried out in the presence or absence of Cl⁻ (lines in cyan and orange, respectively), and the curves in B are for the corresponding constructs of the deprotonated (cyan) or protonated (orange) Glu290 in WT. Note that, in A, the dotted curves are from simulations with the n-octyl-β-glucopyranoside bound in the S2 site and found to reflect a similar trend as seen in the absence of n-octyl-β-glucopyranoside (solid curves). (C) In the E290S mutant, a negative charge provided by the bound Cl⁻ ion is key in maintaining the interaction network near the Na1 and S1 binding sites. (D) In the WT, this function is fulfilled by the deprotonated Glu290. In the absence of such a negative charge, water molecules are seen to penetrate to this region in both the WT and the E290S mutant, interacting directly with the residue at position 290 as well as with Asn286 and Tyr47. This water entry results in an equivalent rotation of the Gln250 side chain in both (E) the E290S mutant and (F) the WT.