Electric-field induced pK-changes in bacteriorhodopsin.

Bacteriorhodopsin, the only protein of the purple membranes of halobacteria [ 11, acts as a light-driven proton pump, translocating protons from the inner to the outer side of the plasma membrane [2]. The lightinduced asymmetric proton distribution contributes to the membrane potential by up to lo-40 mV [3]. Assuming a membrane thickness of 50 A, this potential difference corresponds to an electric field strength of 2-8 X lo6 V/m. Since the protein is polyionic it is possible that photocycle and proton transport are affected by a direct action of the membrane electric field on the conformation of bacteriorhodopsin. An electric field (l-30 X 10’ V/m and l-30 ps duration) causes a cycle of structural changes within the bacteriorhodopsin of purple membrane suspensions [4]. The data suggest an induced-dipole mechanism which involves a restricted rotational displacement of the chromophore by an angle of >2O”C toward the membrane normal [4]. We show here that the field-induced structural changes in purple membrane involve alterations of the pK-values of at least two types of proton binding sites of bacteriorhodopsin. The comparison of the time course of the pH changes indicates that the electric field-induced pK-shifts are opposite in direction to the light-induced pK-changes. This observation suggests a possible negative feedback of the increased electric membrane field on the proton transporting conformations of bacteriorhodopsin.