Chloride ion binding to bacteriorhodopsin at low pH: an infrared spectroscopic study.
Bacteriorhodopsin (bR) and halorhodopsin (hR) are light-induced ion pumps in the cell membrane of Halobacterium salinarium. Under normal conditions bR is an outward proton transporter, whereas hR is an inward Cl- transporter. There is strong evidence that at very low pH and in the presence of Cl-, bR transports Cl- ions into the cell, similarly to hR. The chloride pumping activity of bR is connected to the so-called acid purple state. To account for the observed effects in bR a tentative complex counterion was suggested for the protonated Schiff base of the retinal chromophore. It would consist of three charged residues: Asp-85, Asp-212, and Arg-82. This quadruplet (including the Schiff base) would also serve as a Cl- binding site at low pH. We used Fourier transform infrared difference spectroscopy to study the structural changes during the transitions between the normal, acid blue, and acid purple states. Asp-85 and Asp-212 were shown to participate in the transitions. During the normal-to-acid blue transition, Asp-85 protonates. When the pH is further lowered in the presence of Cl-, Cl- binds and Asp-212 also protonates. The binding of Cl- and the protonation of Asp-212 occur simultaneously, but take place only when Asp-85 is already protonated. It is suggested that HCl is taken up in undissociated form in exchange for a neutral water molecule.
Kelemen
L
L
Institute of Biophysics, Biological Research Center of the Hungarian Academy of Sciences, Szeged, H-6701 Hungary.
Galajda
P
P
Száraz
S
S
Ormos
P
P
D001120:Arginine / Q000737:chemistry; D001224:Aspartic Acid / Q000737:chemistry; D001436:Bacteriorhodopsins / Q000737:chemistry / Q000378:metabolism*; D001665:Binding Sites; D055592:Biophysical Phenomena; D001703:Biophysics; D002712:Chlorides / Q000737:chemistry / Q000378:metabolism*; D019617:Halobacterium salinarum / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D017136:Ion Transport; D012545:Schiff Bases; D017550:Spectroscopy, Fourier Transform Infrared
D002712:Chlorides; D012545:Schiff Bases; D001224:Aspartic Acid; D001436:Bacteriorhodopsins; D001120:Arginine