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Biochemistry. 2019 Jul 2;58(26):2944-2952. doi: 10.1021/acs.biochem.9b00340. Epub 2019 Jun 11.

Role of Gln114 in Spectral Tuning of a Long-Wavelength Sensitive Visual Pigment.

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Department of Life Science and Applied Chemistry , Nagoya Institute of Technology , Showa-ku, Nagoya 466-8555 , Japan.
OptoBioTechnology Research Center , Nagoya Institute of Technology , Showa-ku, Nagoya 466-8555 , Japan.
Primate Research Institute , Kyoto University , Inuyama 484-8506 , Japan.


Visual pigments of the long-wavelength sensitive opsin group (L group) are anion sensitive in nature. Their highly conserved amino acid residues, H197 and K200, exclusively interact with a chloride ion (Cl-) in the chromophore-binding pocket. Substitution of H197 completely abolishes Cl- binding and results in an ∼30 nm spectral blue-shift. Recent attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy studies of monkey green sensitive pigment have provided insights into the role of Cl- binding in stabilizing the antiparallel β-sheet at extracellular loop 2 (ECL2). In addition to maintaining the dark state of L opsins, Cl- binding is also believed to play a crucial role in spectral tuning. Here, we used a combination of site-directed mutagenesis in combination with UV-visible spectroscopy to show that Q1142.65 that is positioned far from ECL2 is also a crucial residue for the Cl- effect in L opsins. Comprehensive FTIR spectroscopic analyses on both ion-binding-induced and light-induced structural changes revealed that Q1142.65 contributes to the stability of β-sheet structure indirectly even though Q1142.65 is not located in ECL2. Overall, these structure-function studies are important for understanding the functional role of Cl- binding in L opsins.

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