title: Cl(-) concentration dependence of photovoltage generation by halorhodopsin from Halobacterium salinarum.
abstract: The photovoltage generation by halorhodopsin from Halobacterium salinarum (shR) was examined by adsorbing shR-containing membranes onto a thin polymer film. The photovoltage consisted of two major components: one with a sub-millisecond range time constant and the other with a millisecond range time constant with different amplitudes, as previously reported. These components exhibited different Cl(-) concentration dependencies (0.1-9 M). We found that the time constant for the fast component was relatively independent of the Cl(-) concentration, whereas the time constant for the slow component increased sigmoidally at higher Cl(-) concentrations. The fast and the slow processes were attributed to charge (Cl(-)) movements within the protein and related to Cl(-) ejection, respectively. The laser photolysis studies of shR-membrane suspensions revealed that they corresponded to the formation and the decay of the N intermediate. The photovoltage amplitude of the slow component exhibited a distorted bell-shaped Cl(-) concentration dependence, and the Cl(-) concentration dependence of its time constant suggested a weak and highly cooperative Cl(-)-binding site(s) on the cytoplasmic side (apparent K(D) of approximately 5 M and Hill coefficient > or =5). The Cl(-) concentration dependence of the photovoltage amplitude and the time constant for the slow process suggested a competition between spontaneous relaxation and ion translocation. The time constant for the relaxation was estimated to be >100 ms.
mesh_terms: D055592:Biophysical Phenomena; D001703:Biophysics; D002713:Chlorine / Q000494:pharmacology*; D019617:Halobacterium salinarum / Q000378:metabolism*; D025602:Halorhodopsins / Q000096:biosynthesis* / Q000737:chemistry*; D007477:Ions; D007700:Kinetics; D008027:Light; D008954:Models, Biological; D008962:Models, Theoretical; D010777:Photochemistry / Q000379:methods*; D013997:Time Factors
chemical_list: D025602:Halorhodopsins; D007477:Ions; D002713:Chlorine