Proton Transport by Proteorhodopsin Requires that the Retinal Schiff Base Counterion Asp-97 Be Anionic

Abstract

At pH >7, proteorhodopsin functions as an outward-directed proton pump in cell membranes, and Asp-97 and Glu-108, the homologues of the Asp-85 and Asp-96 in bacteriorhodopsin, are the proton acceptor and donor to the retinal Schiff base, respectively. It was reported, however [Friedrich, T. et al. (2002) J. Mol. Biol., 321, 821−838], that proteorhodopsin transports protons also at pH K_a of 7.1, and became zero at the pH where Asp-97 is fully protonated.