Spectroscopic, photochemical and biochemical properties of halorhodopsin are summarized. Halorhodopsin is present in the plasma membrane of
Halobacterium halobium and acts as a light-driven Cl
--pump. Halorhodopsin has an all-
trans retinal as its chromophore which binds to apoprotein, halo-opsin through a protonated Schiff base. The absorption spectrum of halorhodopsin and its photoreaction depend on the chloride concentration in the medium. The λ
max of halorhodopsin in the presence and absence of NaCl are 576 and 567 nm, respectively. The Cl
--dependency of the absorption spectrum suggests the existence of two sites for binding Cl
-; one is a “Cl
--binding site” specific for Cl
- and the other is an “anionbinding site” non-specific for Cl
-. The Cl
--binding site is responsible for the wavelength regulation of the absorption spectrum, while the anion-binding site is not. On the basis of these two binding sites, a light-driven Cl
--pump mechanism has been suggested as follows : During the photoreaction of halorhodopsin, a Cl
- is released from the Cl
--binding site to the inside of the cell, then another Cl
- in the anionbinding site shifts to the Cl
--binding site and finally a Cl
- in the medium outside of the cell is bound in the anion-binding site. Thus one Cl
- in the medium can be transported into the inside of the cell by the aid of the photoreaction of one halorhodopsin molecule.
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