Extracellular oxidoreduction potential modifies carbon and electron flow in Escherichia coli

Wild-type Escherichia coli K-12 ferments glucose to a mixture of ethanol an d acetic, lactic, formic, and succinic acids. In anoxic chemostat culture a t four dilution rates and two different oxidoreduction potentials (ORP), th is strain generated a spectrum of products which depended on ORP. Whatever the dilution rate tested, in low reducing conditions (-100 mV), the product ion of formate, acetate, ethanol, and lactate was in molar proportions of a pproximately 2.5:1:1:0.3, and in high reducing conditions (-320 mV), the pr oduction was in molar proportions of 2:0.6:1:2. The modification of metabol ic fluxes was due to an ORP effect on the synthesis or stability of some fe rmentation enzymes; thus, in high reducing conditions, lactate dehydrogenas e-specific activity increased by a factor of 3 to 6. Those modifications we re concomitant,vith a threefold decrease in acetyl-coenzyme A (CoA) needed for biomass synthesis and a 0.5- to 5-fold decrease in formate flux. Calcul ations of carbon and cofactor balances have shown that fermentation was bal anced and that extracellular ORP did not modify the oxidoreduction state of cofactors. From this, it was concluded that extracellular ORP could regula te both some specific enzyme activities and the acetyl-CoA needed for bioma ss synthesis, which modifies metabolic fluxes and ATP yield, leading to var iation in biomass synthesis.