TFB-TBOA binds to two states in WT Glt _Ph . All ITC experiments were performed at 15°C in buffers containing 500 mM Na-gluconate (red circles), NaCl (green squares), or NaNO₃ (blue triangles). Insets show the thermal power with the corresponding scales. All data were fitted to the two-state model; however, exact binding parameters cannot be reliably determined. (a–c) TFB-TBOA binding isotherms. (d–f) Aspartate competition isotherms in the presence of saturating TFB-TBOA concentrations (see Materials and methods).