Thermodynamic parameters of anion binding to halorhodopsin from Natronomonas pharaonis by isothermal titration calorimetry

Abstract

Halorhodopsin (HR), an inwardly directed, light-driven anion pump, is a membrane protein in halobacterial cells that contains the chromophore retinal, which binds to a specific lysine residue forming the Schiff base. An anion binds to the extracellular binding site near the Schiff base, and illumination makes this anion go to the intracellular channel, followed by its release from the protein and re-uptake from the opposite side. The thermodynamic properties of the anion binding in the dark, which have not been previously estimated, are determined using isothermal titration calorimetry (ITC). For Cl(-) as a typical substrate of HR from Natronomonas pharaonis, ΔG=-RT ln(1/K(d))=-15.9 kJ/mol, ΔH=-21.3 kJ/mol and TΔS=-5.4 kJ/mol at 35 °C, where K(d) represents the dissociation constant. In the dark, K(d) values have been determined by the usual spectroscopic methods and are in agreement with the values estimated by ITC here. Opsin showed no Cl(-) binding ability, and the deprotonated Schiff base showed weak binding affinity, suggesting the importance of the positively charged protonated Schiff base for the anion binding.