Abstract
Light driven proton translocating activity of bacteriorhodopsin was examined as a function of a pH gradient (ΔpH) and a membrane potential difference (ΔΨ) using planar lipid bilayer method. Variation of ΔΨ exerted a considerably larger effect on the rate of proton translocation than energetically equivalent magnitude of ΔpH. These features are consistent with the structural data, particularly in view of an asymmetric environment provided by the key amino acid residues with different pKa. The relatively small effect of ΔpH is explained in terms of the proton uptake residue, Asp96, and the proton ejecting residue, Asp85, whose pKa's are known to be about 10 and 3 in the unphotolyzed state, respectively. On the other hand, proton transfer from Asp96 to the Schiff base during the decay of the M intermediate can account for the large effect of ΔΨ on the rate of proton translocation. With these experimental data and explanations in mind, I further propose a simplified model based on an asymmetric potential profile of a multi ion channel. In this model, the uni-directional translocation is induced by a stochastic transition between two states of the multi ion channel. Thus, the active pumping can be explained in the theoretical framework of an asymmetric multi ion channel which undergoes stochastic non-equilibrium transitions between two states. A simple numerical simulation could qualitatively reproduce the experimental data. The simple principle for proton pumping described here may be applied in development of an artificial ion pump molecule.
