title: Properties of the anion-binding site of pharaonis Halorhodopsin studied by ultrafast pump-probe spectroscopy and low-temperature FTIR spectroscopy.
abstract: Halorhodopsin (HR) is a light-driven chloride pump. Cl(-) is bound in the Schiff base region of the retinal chromophore, and unidirectional Cl(-) transport is probably enforced by the specific hydrogen-bonding interaction with the protonated Schiff base and internal water molecules. It is known that HR from Natronobacterium pharaonis (pHR) also pumps NO(3)(-) with similar efficiency, suggesting that NO(3)(-) binds to the Cl(-)-binding site. In the present study, we investigated the properties of the anion-binding site by means of ultrafast pump-probe spectroscopy and low-temperature FTIR spectroscopy. The obtained data were surprisingly similar between pHR-NO(3)(-) and pHR-Cl(-), even though the shapes and sizes of the two anions are quite different. Femtosecond pump-probe spectroscopy showed very similar excited-state dynamics between pHR-NO(3)(-) and pHR-Cl(-). Low-temperature FTIR spectroscopy of unlabeled and [zeta-(15)N]Lys-labeled pHR revealed almost identical hydrogen-bonding strengths of the protonated retinal Schiff base between pHR-NO(3)(-) and pHR-Cl(-), which is similarly strengthened after retinal isomerization. There were spectral variations for water stretching vibrations between pHR-NO(3)(-) and pHR-Cl(-), suggesting that the water molecules hydrate each anion. Nevertheless, the overall spectral features were similar for the two species. These observations strongly suggest that the anion-binding site has a flexible structure and that the interaction between retinal and the anions is weak, despite the presence of an electrostatic interaction. Such a flexible hydrogen-bonding network in the Schiff base region in HR appears to be in remarkable contrast to that in light-driven proton-pumping proteins.
authors.affiliation: Department of Frontier Materials, Nagoya Institute of Technology, Showa-ku, Nagoya 466-8555, Japan.
authors.forename: Keisuke
authors.initials: K
authors.lastname: Nakashima
authors.forename: Takumi
authors.initials: T
authors.lastname: Nakamura
authors.forename: Satoshi
authors.initials: S
authors.lastname: Takeuchi
authors.forename: Mikihiro
authors.initials: M
authors.lastname: Shibata
authors.forename: Makoto
authors.initials: M
authors.lastname: Demura
authors.forename: Tahei
authors.initials: T
authors.lastname: Tahara
authors.forename: Hideki
authors.initials: H
authors.lastname: Kandori
mesh_terms: D000838:Anions / Q000737:chemistry; D001665:Binding Sites; D025602:Halorhodopsins / Q000737:chemistry*; D013057:Spectrum Analysis / Q000379:methods*; D013696:Temperature
chemical_list: D000838:Anions; D025602:Halorhodopsins