DELTA-MU-H-RELAXATION CHANNEL MODEL( DEPENDENCY OF PROTON TRANSLOCATION BY BACTERIORHODOPSIN AND A STOCHASTIC ENERGIZATION)

The effect of a pH gradient (Delta pH) and a membrane potential differ ence (Delta psi) on the rate of proton translocation by bacteriorhodop sin was examined. Contrary to the ''proton well'' hypothesis, variatio n of Delta psi, exerted a considerably larger effect on the rate of pr oton translocation than the energetically equivalent magnitude of Delt a pH (maximum Delta pH examined was about 2, while the absolute pH val ue was 5-9.) These apparently puzzling features are, however, consiste nt with the structural data, particularly in view of an asymmetric env ironment provided by the key amino acid residues with different pK(a) values. The relatively small effect of Delta pH is explained in terms of the proton uptake residue, Asp96, and the proton-ejecting residue, Asp85, whose pK(a) values are known to be about 10 and 3 in the ground state, respectively. On the other hand, proton transfer from Asp96 to the Schiff base during the decay of the M intermediate can account fo r the large effect of Delta psi on the rate of proton translocation. W ith these experimental data and explanations in mind, we further propo se a simplified stochastic model for proton pumping where an asymmetri c environment, which in turn provides an asymmetric potential field fo r protons, plays an essential role for vectorial proton translocation. A simple numerical simulation could qualitatively reproduce the exper imental data. These results suggest that some common principle may exi st in the mechanisms of ion pumps and molecular motors, and it may be applied in development of an artificial ion pump molecule.