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.
Highlights
Bacteriorhodopsin (BR) functions as a proton pump, whereas Anabaena sensory rhodopsin (ASR) functions as a photosensor
In Fourier transform infrared (FTIR) spectroscopic studies, the lowest O-D stretch for D2O was observed at ϳ2200 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
The lowest calculated frequency was observed for OW402–H...OAsp-85, which is consistent with FTIR studies [7]
Summary
Bacteriorhodopsin (BR) functions as a proton pump, whereas Anabaena sensory rhodopsin (ASR) functions as a photosensor. Results: pKa of the conserved Asp residues near the Schiff base significantly differ between BR and ASR. The difference is mostly due to the influences of Ala-53, Arg-82, Glu-194 –Glu-204, and Asp-212 on pKa(Asp85) in BR and the corresponding residues Ser-47, Arg-72, Ser188-Asp-198, and Pro-206 on pKa(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 results from the proton-pumping activity in BR
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