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Issue 26, 2018
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Time-resolved FTIR study of light-driven sodium pump rhodopsins

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Light-driven sodium ion pump rhodopsin (NaR) is a new functional class of microbial rhodopsin. A previous flash photolysis study of Krokinobacter eikastus rhodopsin 2 (KR2) revealed the presence of three kinetically distinct intermediates: K, L/M, and O. Previous low-temperature Fourier-transform infrared (FTIR) spectroscopy of KR2 showed that photoisomerization from the all-trans to the 13-cis form is the primary event of the Na+ pumping photocycle, but structural information on the subsequent intermediates is limited. Here, we applied step-scan time-resolved FTIR spectroscopy to KR2 and Nonlabens dokdonensis rhodopsin 2 (NdR2). Both low-temperature static and time-resolved FTIR spectra resolved a K-like intermediate, and the corresponding spectra showed few differences. Strong hydrogen-out-of-plane (HOOP) vibrations, which appeared in the K intermediate, are common among other rhodopsins. It is, however, unique for NaR that such HOOP bands are persistent in late intermediates, such as L and O intermediates. This observation strongly suggests similar chromophore structures for the K, L, and O intermediates. In fact, an isotope-labeled study that used 12,14-D2 retinal revealed that the chromophore configuration of the O intermediate in NaR is 13-cis. In contrast to the vibrations of the chromophore, those of the protein differ among intermediates, and this is related to the sodium-pumping function. The molecular mechanism of the light-driven sodium pump is discussed on the basis of the present time-resolved FTIR results.

Graphical abstract: Time-resolved FTIR study of light-driven sodium pump rhodopsins

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Supplementary files

Article information

24 Apr 2018
11 Jun 2018
First published
13 Jun 2018

Phys. Chem. Chem. Phys., 2018,20, 17694-17704
Article type

Time-resolved FTIR study of light-driven sodium pump rhodopsins

H. Chen, K. Inoue, H. Ono, R. Abe-Yoshizumi, A. Wada and H. Kandori, Phys. Chem. Chem. Phys., 2018, 20, 17694
DOI: 10.1039/C8CP02599A

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