Fig. 6. Thermodynamics of ion and substrate binding to Glt_Ph.

(A) Determination of binding ΔC_p. Binding enthalpies for aspartate in the presence of 10 mM Na⁺ (solid circles) or 1 M Na⁺ (open circles) are plotted as functions of temperature for outwardly-(top, blue) and inwardly- (bottom, red) constrained Glt_Ph. The data were fitted to straight lines with slopes corresponding to binding ΔC_p of: −297 and −131 cal mol⁻¹ K⁻¹, respectively, for the outward-facing transporter and −608 and −184 cal mol⁻¹ K⁻¹, respectively, for the inward-facing transporter. (B) Thermodynamic scheme of the transport cycle of Glt_Ph. Top and bottom show the thermodynamic schemes of Na⁺ and aspartate binding reactions with corresponding thermodynamic parameters^• for the outward- and inward-facing states, respectively. The signs of the parameters correspond to the directions of the reactions indicated by the arrowheads. Broken lines represent conformational changes between the outward- and inward-facing states. The units are kcal mol⁻¹ for ΔH and ΔG° and cal mol⁻¹ K⁻¹ for ΔC_p. Figure was adapted from Reyes, et al. (15).