Deprotonation of Glu234 during the photocycle of Natronomonas pharaonis halorhodopsin

Halorhodopsin (HR) is a light-driven chloride pump in haloarchaea. Our previous low-temperature Fourier-transform infrared (FTIR) study of pharaonis HR reported that the chloride binding site is destabilized in the L1 intermediate because of the changes in its polar environment. The present FTIR spectroscopy of the wild-type and mutant pharaonis HR revealed that Glu234 becomes deprotonated in the L2 intermediate. This suggests that appearance of a negative charge at the extracellular surface prevents a chloride ion from translocation toward the extracellular side.