Spectral Tuning Mechanism of Primate Blue-sensitive Visual Pigment Elucidated by FTIR Spectroscopy

Kota Katayama,Kei Tsutsui,Hiroo Imai,Hideki Kandori,Yuki Nonaka

doi:10.1038/s41598-017-05177-4

Kota Katayama, Kei Tsutsui + Show 3 more

Open Access

https://doi.org/10.1038/s41598-017-05177-4

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Abstract

Protein-bound water molecules are essential for the structure and function of many membrane proteins, including G-protein-coupled receptors (GPCRs). Our prior work focused on studying the primate green- (MG) and red- (MR) sensitive visual pigments using low-temperature Fourier transform infrared (FTIR) spectroscopy, which revealed protein-bound waters in both visual pigments. Although the internal waters are located in the vicinity of both the retinal Schiff base and retinal β-ionone ring, only the latter showed differences between MG and MR, which suggests their role in color tuning. Here, we report FTIR spectra of primate blue-sensitive pigment (MB) in the entire mid-IR region, which reveal the presence of internal waters that possess unique water vibrational signals that are reminiscent of a water cluster. These vibrational signals of the waters are influenced by mutations at position Glu113 and Trp265 in Rh, which suggest that these waters are situated between these two residues. Because Tyr265 is the key residue for achieving the spectral blue-shift in λmax of MB, we propose that these waters are responsible for the increase in polarity toward the retinal Schiff base, which leads to the localization of the positive charge in the Schiff base and consequently causes the blue-shift of λmax.

Highlights

Fourier transform infrared (FTIR) spectroscopy has been extensively used to investigate the chromophore structure and chromophore-protein interactions and protein-bound water molecules[10,11,12,13]
We report the first difference in the FTIR spectra of the primate blue-sensitive pigment (MB) that contains vital information regarding the retinal chromophore, protein moiety, and internal water molecules
A pair of peaks at 1576 (−)/1560 (+) cm−1 corresponds to the ethylenic C=C stretching vibration of monkey blue (MB), and a spectral down-shift in this vibrational mode of the retinal chromophore at 1580–1500 cm−1 corresponds to a red-shift in the visible region[7], which is evident from the linear correlation between λmax and the negative frequency of the 11-cis form (Figure S2d)

Summary

Pigment Elucidated by FTIR

Protein-bound water molecules are essential for the structure and function of many membrane proteins, including G-protein-coupled receptors (GPCRs). We report FTIR spectra of primate blue-sensitive pigment (MB) in the entire mid-IR region, which reveal the presence of internal waters that possess unique water vibrational signals that are reminiscent of a water cluster. We reported light-induced differences in the FTIR spectra of primate green- (MG) and red- (MR) sensitive pigments at 77 K, which provided direct evidence of the presence of internal water molecules in the retinal binding. We report the first difference in the FTIR spectra of the primate blue-sensitive pigment (MB) that contains vital information regarding the retinal chromophore, protein moiety, and internal water molecules. Previous site-directed mutagenesis studies have reported that several mutations in the vicinity of the retinal chromophore of the human blue-sensitive pigment (HB) induced a spectral shift in λmax[17, 18]. Unique retinal-protein interactions can be clarified using the present comprehensive vibrational spectral analysis of cone pigments

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