Glucose 6-phosphate transport in membrane vesicles isolated from Escherichia coli: effect of imposed electrical potential and pH gradient.

Abstract

Imposition of a membrane potential (delta psi, interior negative) or a pH gradient (delta pH, interior alkaline) across the membrane of Escherichia coli DF2000 leads to a marked, transient increase in glucose 6-phosphate transport that varies systematically with pH. Outwardly directed potassium diffusion gradients in the presence of valinomycin (i.e., generation of delta psi, interior negative) drive glucose 6-phosphate transport at pH 7.5 but much less effectively at pH 5.5, although the magnitudes of the transient delta psi generated are comparable at both pH values. Similarly, imposition of delta psi (interior negative) retards the rate of passive, carrier-mediated glucose 6-phosphate efflux down a concentration gradient at pH 7.5 but not at pH 5.5. In contrast, imposition of delta pH (interior alkaline) by means of outwardly directed acetate diffusion gradients drives glucose 6-phosphate accumulation at pH 5.5 but is relatively ineffective at pH 7.5. The results are independent of the pK of glucose 6-phosphate and provide strong support for the argument that the glucose 6-phosphate porter catalyzes an electrically neutral reaction at acid pH and an electrogenic reaction at alkaline pH. In addition, they are entirely consistent with the hypothesis that the proton/glucose 6-phosphate stoichiometry increases at alkaline pH [Rottenberg, H. (1976) FEBS Lett. 66, 159; Ramos, S., & Kaback, H.R. (1977) Biochemistry 16, 854, 4271].