Linear relations between proton current and pH gradient in bacteriorhodopsin liposomes

The dependence of proton movement across the membrane of bacteriorhodopsin liposomes on the pH gradient was investigated. Under the appropriate experimental conditions, endogenous proton (or hydroxyl) leakage, proton movement catalyzed by protonophore or nigericin, and light-driven proton translocation depend linearly on the pH gradient. This justifies the use of linear proton flux vs. protonmotive force relations in a recent mosaic thermodynamic description of ion translocation in bacteriorhodopsin liposomes [Westerhoff, H. V., Scholte, B. J., & Hellingwerf, K. J. (1979) Biochim. Biophys. Acta 547, 544-560]. Since bacteriorhodopsin liposomes are a model system for all biological energy transducing systems in which proton pumps are involved, these findings also explain linear relations between proton flux and protonmotive force observed in and postulated for those systems. In cases where the membrane potential is not clamped at a low value, an initial phase of rapid proton movement occurs, followed by a phase of slower proton movement. The rate of proton movement during the slow phase is again linear with the pH gradient. Such a linear relation is not observed for the fast phase. Since the rapid proton movement phase is also observed in liposomes without bacteriorhodopsin, it is not due (only) to dissociation of scalar protons from bacteriorhodopsin. We suggest that during the initial phase of proton movement, the proton flux is not yet electrically compensated by the fluxes of other ions.