Light-driven chloride ion transport by halorhodopsin from Natronobacterium pharaonis. 1. The photochemical cycle

Biochemistry. 1995 Nov 7;34(44):14490-9. doi: 10.1021/bi00044a027.


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.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Archaea / metabolism*
  • Bacteriorhodopsins / metabolism*
  • Chlorides / metabolism*
  • Halorhodopsins
  • Ion Transport
  • Kinetics
  • Light
  • Photochemistry


  • Chlorides
  • Halorhodopsins
  • Bacteriorhodopsins