LIGHT-DRIVEN CHLORIDE-ION TRANSPORT BY HALORHODOPSIN FROM NATRONOBACTERIUM-PHARAONIS .1. THE PHOTOCHEMICAL CYCLE

The photochemical cycle of the light-driven chloride pump, halorhodops in from N. pharaonis, is described by transient optical multichannel a nd 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 K-D of 1 mM. The reaction seq uence 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 -->/hv K double left right arrow L double left right arrow N --> HR that relaxes in a few millise conds, and is very similar to the photocycle of bacteriorhodopsin unde r conditions where the retinal Schiff base cannot deprotonate. At lowe r chloride concentrations, e.g., 0.1 M, however, a red-shifted state t ermed O appears between N and HR, in equilibrium with N. The absorptio n spectra of K, L, N, and O are very similar to their counterparts in the bacteriorhodopsin photocycle. As in their equivalents in bacterior hodopsin, in the N state the retinal is still 13-cis, but it is reisom erized in the O state to all-trans.