Relation Between Surface Charge Changes in Purple Membranes and the Photoreaction Cycle in Bacteriorhodopsin (Halobacteria, Bioenergetics, Surface Potentials)

摘要

The surface potential of purple membrane fragments, determined from the distribution of the aqueous-free and the membrane-bound positively charged, paramagnetic, amphiphilic probe 4-(dodecyl dimethyl ammonium)1-oxyl-2, 2,6,6-tetramethyl piperidine bromide, varied almost 60 mV as a function of ionic strength and 50 mV as a function of pH of the medium. Light induced changes in surface potential followed the changes observed in the M sub 412 intermediate of the photocycle of bacteriorhodopsin as a function of pH, temperature, and response to antibiotics Beauvericin and Valinomycin. The number of induced charges per M sub 412 appearing at the surface of purple membranes decreases from 0.75 to 0.6 as the surface potential becomes more negative. The stoichiometric values would be twice as large if the charge changes were localized exclusively on one side of the purple membrane. Laser flash induced kinetics of the rise and decay of surface charge changes lagged behind the kinetics of the rise and decay of M sub 412 which is associated with the reversible deprotonation of the retinal Schiff base nitrogen in the chromophore. It is suggested that the light induced charge changes monitor a dissociable amino acid residue which may be a step in the movement of protons across the purple membrane. (ERA citation 05:014255)