Nonlinear voltage dependence of the light-driven proton pump current of bacteriorhodopsin.

The light-driven proton pump current generated by bacteriorhodopsin reconstituted in asymmetric planar bilayer membranes was investigated. The current-voltage dependence was found to be nonlinear and can be approximated by an exponential at least below +50 mV. The current changed e-fold when the membrane potential was changed by 80 mV. The voltage dependence was analyzed in terms of a barrier model. This analysis revealed an effective displacement of 0.63 elementary charges across the membrane during the rate-limiting step. Comparison of this value with the results from flash-induced photovoltage signals suggests that one proton is pumped per cycle.