Iodide influence on the photochemical reaction cycle of pharaonis halorhodopsin

K. Magyari1, V. Simon1 and G. Váró2

1 Department of Physics, University “Babes-Bolyai” Cluj-Napoca, Romania
2 Institute of Biophysics, Biological Research Center of the Hungarian Academy of Sciences, Szeged, H-6701, Hungary

	Sodium salt of chloride and iodide were used to elucidate the effect of the size of the ion on the binding to pharaonis halorhodopsin and its transport during the photocycle of this retinal protein. Spectroscopic titration revealed an apparent strong binding constant of 2mM for chloride and 5 mM for iodide. In the case of iodide a second week binding constant of about 10 M could be estimated. This second binding constant was similar to that observed earlier for nitrate[2]. By changing the halide anions, only the transitions in the second half of the photocycle were affected, which contained intermediates N, O, and HR’. The O to HR’ transition become faster with increasing ion volume. With increasing ion concentration the N-O-HR’ equilibrium changed in such a way that the accumulated O tended to decrease. This tendency was overruled in iodide, by the appearance of the second binding constant. The increasing iodide concentration, up to 100 mM decreases the accumulation of the intermediate O, but at higher ion concentration the amount of O increases, although its decay become faster.

References

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