Free-Energy Simulations Resolve the Low-Affinity Na+-High-Affinity Asp Binding Paradox in GltPh

Glutamate transporters clear up excess extracellular glutamate by cotransporting three Na+ and one H+ with the countertransport of one K+. The archaeal homologs are selective to aspartate and only cotransport three Na+. The crystal structures of GltPh from archaea have been used in computational studies to understand the transport mechanism. Although some progress has been made with regard to the ligand-binding sites, a consistent picture of transport still eludes us. A major concern is the discrepancy between the computed binding free energies, which predict high-affinity Na+-low-affinity aspartate binding, and the experimental results in which the opposite is observed. Here, we show that the binding of the first two Na+ ions involves an intermediate state near the Na1 site, where two Na+ ions coexist and couple to aspartate with similar strengths, boosting its affinity. Binding free energies for Na+ and aspartate obtained using this intermediate state are in good agreement with the experimental values. Thus, the paradox in binding affinities arises from the assumption that the ligands bind to the sites observed in the crystal structure following the order dictated by their binding free energies with no intermediate states. In fact, the presence of an intermediate state eliminates such a correlation between the binding free energies and the binding order. The intermediate state also facilitates transition of the first Na+ ion to its final binding site via a knock-on mechanism, which induces substantial conformational changes in the protein consistent with experimental observations.