Anion and cation fluxes in Escherichia coli during glucose fermentation and their biochemical implications

The end products of glucose fermentation by Escherichia coli include organic acids such as formic, acetic, lactic, and succinic acid. Anions are transported out of the cell along with protons by a symport mechanism mediated by the F_OF₁-ATPase operating in the hydrolysis mode. The present work measures and analyzes new data on H⁺ efflux, K⁺ influx, and anionic formate, acetate, lactate, and succinate efflux rates in whole cells of E. coli grown at 2 g L^-1 and 8 g L^-1 glucose under fermentative conditions at pH 7.5, with 2 g L^-1 added glucose. The experiments are performed both in the presence and absence of 0.2 mM N N'-dicyclohexylcarbodiimide (DCCD), and in the presence and absence of external formate. At 2 g L^-1 glucose in the growth medium, and addition of 2 g L^-1 glucose, the mean DCCD-sensitive H⁺ efflux rate of (1.83 ± 0.20) × 10^-3 mmol min^-1 is almost exactly compensated by a (succinate + K⁺) transport rate of (1.94 ± 0.12) × 10^-3 mmol min^-1. This equivalence also holds in other experimental conditions investigated, e.g. in the presence of 10 mM external formate, and with 8 g L^-1 glucose in the growth medium and 2 g L^-1 added glucose. Comparison of ion transport rates between wild type and mutant atpB (a) and atpE (c) E. coli cells at a growth pH of 7.5 with 2 g L^-1 added glucose are also made. Biochemical implications of these findings are discussed, and a structural model involving cotransport and supercomplex formation with the F_OF₁-ATPase that is consistent with the experimental observations is presented. Finally, the great scope for future work, and the innovations needed for further progress on these key aspects of bioenergetics and transport across biomembranes are discussed.