Photogenerated movement of protons through bacteriorhodopsin in relation to the analysis of photoelectrical responses

Abstract

Bacteriorhodopsin, a protein–retinal complex, forms two-dimensional patches located on the membrane of a photosynthetic bacterium. These patches, called purple-membrane fragments, act as light-driven proton pumps converting the light energy into a transmembrane gradient of proton electrochemical potential.Purple-membrane fragments are adsorbed on the surface of bimolecular lipid membranes doped with a lipophilic proton carrier and separating two aqueous solutions of controlled pH. We have measured the photocurrents associated with the photogenerated proton movements through the bacteriorhodopsin. In particular the influence of the light energy, the pH of the aqueous media and the applied voltage have been investigated, the main result being a reversal in the direction of proton pumping.In order to account for these results, a model of phototransport of protons through the bacteriorhodopsin is proposed. According to the model, the pathway for proton movement is composed of two chains of amino acids acting as proton-conducting wires which connect the aqueous solutions to sites able to exchange protons with the photoactive heart. Undoubtedly the photoactive heart contains the Schiff base of the retinal. From this transport model it appears that the degree of protonation of the donor sites which exchange protons with the photoactive heart plays a central role in the working of the pump.A kinetic expression of the stationary photocurrent is derived from the model and accounts for the experimental data.