Kinetics of lactose transport into Escherichia coli in the presence and absence of a protonmotive force

Abstract

The kinetics of galactoside movements into and out of whole cells of Escherichia coli under metabolizing and metabolically-inhibited conditions were described by Winkler and Wilson [ 11. They found that the inhibition of metabolism caused the very high app. Km for efflux to fall to the lower values characteristic of the app. Km for infhrx, whereas the app. Km for influx was not detectably changed. These results were substantially confirmed in [2], but in a neglected paper [3], the presence of uncoupler was reported to cause the app. Km for influx of thiomethylgalactoside to increase to a much higher value. In the presence of a protonmotive force membrane vesicles accumulate lactose with an app. Km similar to that for influx into whole cells [5,6]. In the absence of added respirable substrate there is essentially no protonmotive force and galactosides are not accumulated, though they may still bind to the carrier [6]. Under the latter conditions, the binding constant for lactose was 200-fold greater than the app. Km for active uptake [6]. Similar results have been reported in [7]. This paper describes experiments designed to examine the kinetics of the lactose carrier in the absence of a protonmotive force but under conditions of net transport. The results rule out the possibility that the difference between the app. Km and Kd observed in [6] could be due to measurements being made under different conditions (i.e., that the app.