Optical control of enzymatic conversion of sucrose to glucose by bacteriorhodopsin incorporated into self-assembled phosphatidylcholine vesicles

This report describes an alternative method to modulate enzyme activity optically through pH modulation. Bacteriorhodopsin (BR) is used to regulate the activity of an enzyme which it does not naturally regulate. In this study BR controlled the pH-dependent conversion of sucrose to glucose by the enzyme, invertase. BR is oriented in the wall of phosphatidylcholine vesicles so that at pH values greater than 4–5 it pumps protons into the vesicles and at pH values less than 4 it pumps protons out of the vesicle when illuminated with light of the proper wavelength. The bulk-solution pH correspondingly changes as a result. At an initial pH of 5.57 the change in pH of the solution due to pumping by the bacteriorhodopsin-vesicle complex was 0.17 pH units. To study this effect on the enzyme, samples containing BR vesicles and invertase were assayed for enzyme activity under dark and illuminated conditions. It was found that changes in enzyme activity, upon illumination, were generally larger than predicted for the corresponding changes in the bulk solution pH generated by the BR vesicles.