Light-driven chloride ion transport by halorhodopsin from Natronobacterium pharaonis. 1. The photochemical cycle.
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.
Váró
G
G
Department of Physiology and Biophysics, University of California, Irvine 92717, USA.
Brown
L S
LS
Sasaki
J
J
Kandori
H
H
Maeda
A
A
Needleman
R
R
Lanyi
J K
JK
D001105:Archaea / Q000378:metabolism*; D001436:Bacteriorhodopsins / Q000378:metabolism*; D002712:Chlorides / Q000378:metabolism*; D025602:Halorhodopsins; D017136:Ion Transport; D007700:Kinetics; D008027:Light; D010777:Photochemistry
D002712:Chlorides; D025602:Halorhodopsins; D001436:Bacteriorhodopsins