title: Proton release group of pharaonis phoborhodopsin revealed by ATR-FTIR spectroscopy.
abstract: The proton release mechanism has been one of the recent interesting topics in the field of microbial rhodopsins since it was established that a protonated water cluster is the proton release group(PRG) in bacteriorhodopsin (BR). pharaonis phoborhodopsin [ppR, also called pharaonis sensory rhodopsinII (pSRII)] is a photoreceptor for negative phototaxis in Natronomonas pharaonis, and in the absence of transducer protein, pHtrII, ppR can pump protons like BR. Fast, BR-like proton release was observed during the lifetime of the M intermediate (ppRM) at low pH, but it was slowed in the absence of Cl-[Iwamoto, M., et al. (2004) Biochemistry 43, 3195]. This observation suggests that Cl- binding controls the pKa of PRG in ppR and ppRM. In this paper, we studied the molecular mechanism of the PRG action in ppR by means of Cl(-)-induced and light-induced difference attenuated total reflection (ATR) FTIRspectroscopy in the aqueous phase. Cl(-)-induced difference ATR-FTIR spectra clearly demonstrated that binding of Cl- to ppR accompanies protonation of a carboxylic acid (C=O stretch at 1724 cm-1). The amino acid was identified as Asp193, because the corresponding band is shifted to 1705 cm-1 in the D193Emutant protein. Light-induced ppRM minus ppR difference ATR-FTIR spectra show the deprotonation signal of Asp193 (at 1724 cm-1) only in the presence of Cl-. The double-difference spectrum between the light-induced changes in the presence and absence of Cl- is a mirror image of the spectrum of Cl binding in the dark, indicating that ppRM formation accompanies deprotonation of Asp193 and dissociation of Cl- simultaneously. It was also shown that structural changes of arginine are involved in these processes by use of [15N]arginine-labeled ppR. We thus conclude that the PRG of ppR includes Asp193, whose pKa changes are controlled by Cl- and Arg72.
authors.affiliation: Department of Frontier Materials, Nagoya Institute of Technology, Showa-ku, Nagoya 466-8555, Japan.
authors.forename: Yuya
authors.initials: Y
authors.lastname: Kitade
authors.forename: Yuji
authors.initials: Y
authors.lastname: Furutani
authors.forename: Naoki
authors.initials: N
authors.lastname: Kamo
authors.forename: Hideki
authors.initials: H
authors.lastname: Kandori
mesh_terms: D018360:Crystallography, X-Ray; D025602:Halorhodopsins / Q000737:chemistry / Q000378:metabolism*; D008958:Models, Molecular; D011522:Protons; D025603:Sensory Rhodopsins / Q000737:chemistry / Q000378:metabolism*; D017550:Spectroscopy, Fourier Transform Infrared / Q000379:methods*
chemical_list: D025602:Halorhodopsins; D011522:Protons; D025603:Sensory Rhodopsins; C061508:sensory rhodopsin II protein, archaeal