A Correlation between Photocycle and Photoelectric Response of Bacteriorhodopsin Monomers

Abstract

The bacteriorhodopsin (BR) molecules in the purple membrane (PM) are organized into a two-dimensional hexagonal lattice of trimers which are surrounded by about 30 lipid molecules. The protein-protein and proteinlipid interactions may affect their structure and function. It is therefore interesting to solubilize the PM into BR monomers and to investigate its functional characteristics. The BR monomers were reconstituted with dimyristoylphosphatidylcholine (DMPC) to form vesicles. We studied the pH dependence of the photoelectric response in a bilayer lipid membrane (BLM) system and of the kinetics of the M412 and the 0640 photocycle intermediates both in PM fragments and in BR monomer vesicles. The results show that the conformation of BR in monomeric state is more readily affected by the pH of the medium. Especially in pH higher than 9.4 the M412 slow-decaying component becomes very slow and even the polarity of the photovoltage signal is reversed. It appears that BR has two different conformational forms and their distribution depends on the pH of the medium. The two forms have different photocycle intermediates with different lifetimes and even the direction of proton pumping is reversed.