The pH-dependent energetic uncoupling ofZymomonas by acetic acid
pH-dependent energetic uncoupling ofZymomonas by acetic acid occurs by virtue of the permeability of the plasma membrane to the undissociated form of acetic acid (HAc) and the acidification of the cytoplasm resulting from the uptake of HAc and the consequential diversion of energy away from biosynthetic processes (growth) in order to maintain constant intracellular pH. Energetic uncoupling is manifested by an increase in specific productivity. The degree of uncoupling caused by HAc depends on a rather complex interaction between several different variables including membrane permeability, the transmembrane δpH and the concentraiton of undissociated form of acetic acid in the medium. Within the pH range of 5.0–5.5, maximal energic uncoupling is produced by 30–38 mM HAc. For practical purposes, in terms of the concentration of acetic acid, this corresponds to about 5 and 15 g/L at pH 5.0 and 5.5, respectively. Assuming any upper limit concentration of acetic acid in hydrolysate fermentation media of about 12 g/L, inhibition of Z.mobilis in terms of both ethanol yield and productivity is avoided by controlling the pH in the range of 5.5–6.0.
Index EntriesZymomonas acetic acid fuel ethanol energetic uncoupling specific productivity
undissociated acetic acid
specific growth rate (h−1)
growth yield co-efficient (g dry wt cells/g glucose)
product yield (g ethanol/g glucose)
average volumetric productivity (g ethanol/L/h)
specific productivity (g ethanol/g cell/h).
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