Hydrogenase activity and proton-motive force generation by Escherichia coli during glycerol fermentation

Abstract

Proton motive force (Δp) generation by Escherichia coli wild type cells during glycerol fermentation was first studied. Its two components, electrical-the membrane potential (∆φ) and chemical-the pH transmembrane gradient (ΔpH), were established and the effects of external pH (pHex) were determined. Intracellular pH was 7.0 and 6.0 and lower than pHex at pH 7.5 and 6.5, respectively; and it was higher than pHex at pH 5.5. At high pHex, the increase of ∆φ (-130 mV) was only partially compensated by a reversed ΔpH, resulting in a low Δp. At low pHex ∆φ and consequently Δp were decreased. The generation of Δp during glycerol fermentation was compared with glucose fermentation, and the difference in Δp might be due to distinguished mechanisms for H(+) transport through the membrane, especially to hydrogenase (Hyd) enzymes besides the F0F1-ATPase. H(+) efflux was determined to depend on pHex; overall and N,N'-dicyclohexylcarbodiimide (DCCD)-inhibitory H(+) efflux was maximal at pH 6.5. Moreover, ΔpH was changed at pH 6.5 and Δp was different at pH 6.5 and 5.5 with the hypF mutant lacking all Hyd enzymes. DCCD-inhibited ATPase activity of membrane vesicles was maximal at pH 7.5 and decreased with the hypF mutant. Thus, Δp generation by E. coli during glycerol fermentation is different than that during glucose fermentation. Δp is dependent on pHex, and a role of Hyd enzymes in its generation is suggested.