The relation between the electrochemical proton gradient and active transport in Escherichia coli membrane vesicles

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Abstract
E. coli membrane vesicles generate a large electrochemical proton gradient (.DELTA..hivin..mu.H+) under appropriate conditions, and some of the properties of .DELTA..hivin..mu.H+ and its component forces [i.e., the membrane potential (.DELTA..PSI.) and the chemical gradient of protons (.DELTA.pH)] were previously described. The relationship between .DELTA..hivin..mu.H+, .DELTA..PSI. and .DELTA.pH and the active transport of specific solutes is examined. Addition of lactose or G-6-P to membrane vesicles containing the appropriate transport systems results in partial collapse of .DELTA.pH, indicating that respiratory energy can drive active transport via the pH gradient across the membrane. Titration studies with valinomycin and nigericin indicate that at pH 5.5 there are 2 general classes of transport systems: those that are driven primarily by .DELTA..hivin..mu.H+ (lactose, proline, serine, glycine, tyrosine, glutamate, leucine, lysine, cysteine and succinate) and those that are driven primarily by .DELTA.pH (G-6-P, D-lactate, glucuronate and gluconate). At pH 7.5 all of these transport systems are driven by .DELTA..PSI., which comprises the only component of .DELTA..hivin..mu.H+ at this external pH. The effect of external pH on the steady-state levels of accumulation of different solutes is examined, and none of the pH profiles correspond to those observed for .DELTA..hivin..mu.H+, .DELTA..PSI. or .DELTA.pH. At external pH values > 6.0-6.5, .DELTA..hivin..mu.H+ is insufficient to account for the concentration gradients established for each substrate unless the stoichiometry between protons and accumulated solutes is greater than unity. The results confirm many facets of the chemiosmotic hypothesis, but they also extend the concept in certain important respects and allow explanations for some earlier observations which may preclude the involvement of chemiosmotic phenomena in active transport.