title: Light-driven chloride ion transport by halorhodopsin from Natronobacterium pharaonis. 1. The photochemical cycle.
abstract: The photochemical cycle of the light-driven chloride pump, halorhodopsin from N. pharaonis, is described by transient optical multichannel and single-wavelength spectroscopy in the visible, and in the infrared. Titration of a blue-shift of the absorption maximum upon addition of chloride describes a binding site with a KD of 1 mM. The reaction sequence after the all-trans to 13-cis photoisomerization of the retinal in this chloride binding form is itself dependent on chloride. At 2 M chloride it is described by the scheme: HR-->K<==>L<==>N-->HR that relaxes in a few milliseconds, and is very similar to the photocycle of bacteriorhodopsin under conditions where the retinal Schiff base cannot deprotonate. At lower chloride concentrations, e.g., 0.1 M, however, a red-shifted state termed O appears between N and HR, in equilibrium with N. The absorption spectra of K, L, N, and O are very similar to their counterparts in the bacteriorhodopsin photocycle. As in their equivalents in bacteriorhodopsin, in the N state the retinal is still 13-cis, but it is reisomerized in the O state to all-trans.
authors.affiliation: Department of Physiology and Biophysics, University of California, Irvine 92717, USA.
authors.forename: G
authors.initials: G
authors.lastname: Váró
authors.forename: L S
authors.initials: LS
authors.lastname: Brown
authors.forename: J
authors.initials: J
authors.lastname: Sasaki
authors.forename: H
authors.initials: H
authors.lastname: Kandori
authors.forename: A
authors.initials: A
authors.lastname: Maeda
authors.forename: R
authors.initials: R
authors.lastname: Needleman
authors.forename: J K
authors.initials: JK
authors.lastname: Lanyi
mesh_terms: D001105:Archaea / Q000378:metabolism*; D001436:Bacteriorhodopsins / Q000378:metabolism*; D002712:Chlorides / Q000378:metabolism*; D025602:Halorhodopsins; D017136:Ion Transport; D007700:Kinetics; D008027:Light; D010777:Photochemistry
chemical_list: D002712:Chlorides; D025602:Halorhodopsins; D001436:Bacteriorhodopsins