title: Factors that differentiate the H-bond strengths of water near the Schiff bases in bacteriorhodopsin and Anabaena sensory rhodopsin.
abstract: Bacteriorhodopsin (BR) functions as a light-driven proton pump, whereas Anabaena sensory rhodopsin (ASR) is believed to function as a photosensor despite the high similarity in their protein sequences. In Fourier transform infrared (FTIR) spectroscopic studies, the lowest O-D stretch for D(2)O was observed at ∼2200 cm(-1) in BR but was significantly higher in ASR (>2500 cm(-1)), which was previously attributed to a water molecule near the Schiff base (W402) that is H-bonded to Asp-85 in BR and Asp-75 in ASR. We investigated the factors that differentiate the lowest O-D stretches of W402 in BR and ASR. Quantum mechanical/molecular mechanical calculations reproduced the H-bond geometries of the crystal structures, and the calculated O-D stretching frequencies were corroborated by the FTIR band assignments. The potential energy profiles indicate that the smaller O-D stretching frequency in BR originates from the significantly higher pK(a)(Asp-85) in BR relative to the pK(a)(Asp-75) in ASR, which were calculated to be 1.5 and -5.1, respectively. The difference is mostly due to the influences of Ala-53, Arg-82, Glu-194-Glu-204, and Asp-212 on pK(a)(Asp-85) in BR and the corresponding residues Ser-47, Arg-72, Ser-188-Asp-198, and Pro-206 on pK(a)(Asp-75) in ASR. Because these residues participate in proton transfer pathways in BR but not in ASR, the presence of a strongly H-bonded water molecule near the Schiff base ultimately results from the proton-pumping activity in BR.
authors.affiliation: 202 Building E, Career-Path Promotion Unit for Young Life Scientists, Graduate School of Medicine, Kyoto University, Yoshida-Konoe-cho, Kyoto 606-8501, Japan.
authors.forename: Keisuke
authors.initials: K
authors.lastname: Saito
authors.forename: Hideki
authors.initials: H
authors.lastname: Kandori
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Ishikita
mesh_terms: D017033:Anabaena / Q000737:chemistry*; D001436:Bacteriorhodopsins / Q000737:chemistry*; D018360:Crystallography, X-Ray; D017666:Deuterium Oxide / Q000737:chemistry; D006860:Hydrogen Bonding; D017434:Protein Structure, Tertiary; D012545:Schiff Bases
chemical_list: D012545:Schiff Bases; D001436:Bacteriorhodopsins; D017666:Deuterium Oxide