Issue 35, 2024

Low-temperature FTIR spectroscopy of the L/Q switch of proteorhodopsin

Abstract

Rhodopsins are photoreceptive membrane proteins containing a retinal chromophore, and the color tuning mechanism in rhodopsins is one of the important topics. Color switch is a color-determining residue at the same position, where replacement of red- and blue-shifting amino acids in two wild-type rhodopsins causes spectral blue- and red-shifts, respectively. The first and most famous color switch in microbial rhodopsins is the L/Q switch in proteorhodopsins (PRs). Green- or blue-absorbing PR (GPR or BPR) contains Leu and Gln at position 105 of the C-helix (TM3), respectively, and their replacement converted absorbing colors. The L/Q switch enables bacteria to absorb green or blue light in shallow or deep ocean waters, respectively. Although Gln and Leu are hydrophilic and hydrophobic residues, respectively, a comprehensive mutation study of position 105 in GPR revealed that the λmax correlated with the volume of residues, not the hydropathy index. To gain structural insights into the mechanism, we applied low-temperature FTIR spectroscopy of L105Q GPR, and the obtained spectra were compared with those of GPR and BPR. The difference FTIR spectra of L105Q GPR were similar to those of BPR, not GPR, implying that the L/Q switch converts the GPR structure into a BPR structure in terms of the local environments of the retinal chromophore. It includes retinal skeletal vibration, hydrogen-bonding strength of the protonated Schiff base, amide-A vibration (peptide backbone), and protein-bound water molecules. Consequently color is switched accompanying such structural alterations, and known as the L/Q switch.

Graphical abstract: Low-temperature FTIR spectroscopy of the L/Q switch of proteorhodopsin

Supplementary files

Article information

Article type
Paper
Submitted
01 Jun 2024
Accepted
05 Aug 2024
First published
07 Aug 2024

Phys. Chem. Chem. Phys., 2024,26, 22959-22967

Low-temperature FTIR spectroscopy of the L/Q switch of proteorhodopsin

T. Nishikino, T. Sugimoto and H. Kandori, Phys. Chem. Chem. Phys., 2024, 26, 22959 DOI: 10.1039/D4CP02248C

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