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Protonation states of highly conserved carboxylic acids in NeoR
(Ritsu Mizutori, Masahiro Sugiura, Hideki Kandori, Biophysics and Physicobiology 22, e220032 (2025), DOI: 10.2142/biophysico.bppb-v22.0032)
Neorhodopsin (NeoR), an enzymerhodopsin, has λmax at 690–700 nm, and four carboxylic acids near the retinal chromophore play important roles for the unique spectral red-shift. In this study, light-induced difference FTIR spectroscopy of these mutants successfully identified protonation states of the carboxylic acids. In the resting state of NeoR, E141 and D140 are protonated, whereas one negative charge is delocalized at E136 and E262 that contributes to unusual spectral red-shift in NeoR. In the UV-absorbing photoproduct state with 7-cis retinal, the protonation states of these residues remain unchanged; however, the broadened peak of E141 indicates increased structural fluctuations.