Transport of Substrates and Metabolites and Their Effect on Cell Metabolism (in Butyric‐Acid and Methylotrophic Fermentations)^a

Abstract

The two components, delta pH and delta psi, of the membrane protonmotive force (delta p) effect and are affected by the transport of many substrates and metabolites. Because the integrity (or restoration) of the delta p requires the expenditure of metabolic energy, such transport processes affect the overall cell bioenergetics. However, the transport or high concentrations of certain substrates and metabolites can have more serious effects on cell metabolism because they partially or completely abolish either or both the delta pH and delta psi. If the cells cannot eventually restore the collapsed component(s) of the delta p, complete growth inhibition and cell death become inevitable. In the butanol/acetone fermentation of Clostridium acetobutylicum, the transport and the presence of key metabolites (acetic and butyric acids, and butanol) have serious and some necessary effects on the delta p. Acetic and butyric acids act as uncouplers of the delta pH, thereby reducing the internal pH. Using other acid uncouplers (such as acetoacetate, which is metabolized by the cells, or FCCP, which is not metabolized by the cells), we found that a lower pHo combined with the metabolic-energy drain of the uncoupling effect and high internal acid concentrations are implicated in the mechanism(s) of solventogenesis. Thus, the production or presence (or both) of the two acids (acetic and butyric) is beneficial to the initiation of solvent production. The transport mechanisms of CH3OH, CH2O, and HCOOH in obligate CH3OH utilizers (methylotrophs) were also discussed in detail. We showed that CH3OH is actively transported by the cells at the expense of metabolic energy and that its transport significantly affects the dynamics of continuous bioreactors. The accumulation of CH2O was found to be driven by the membrane delta p. Finally, formate was accumulated by the delta pH according to the general transport mechanism of short-chain fatty acids. The inhibition of growth by formate was explained by its uncoupling effect on the cells. Growth inhibition by CH3OH appeared to be related to the severe reduction of the membrane delta pH and cell pHi by relatively low CH3OH concentrations.

Complex and extensive post-transcriptional regulation revealed by integrative proteomic and transcriptomic analysis of metabolite stress response in Clostridium acetobutylicum

“…These findings suggest that the cells are using posttranscriptional regulation to upregulate proteins necessary for repairing DNA damaged by the butanol insult, and continue cell division despite the severe, overall, growth inhibition. Butanol stress has a strong negative effect on membrane functionality as it affects the membrane fluidity [ 20 ], which in turn affects membrane transport and the transmembrane potential and Δ pH [ 21 , 22 ]. Several ABC transporters and permeases (Fig.…”

Small and Low but Potent: the Complex Regulatory Role of the Small RNA SolB in Solventogenesis in Clostridium acetobutylicum

“…Growth curves for all strains (Fig. S2) reveal that cell growth peaked at lower cell densities for 824(p94_solB) and 824 ΔsolB than for the WT and plasmid control strains; this stunted growth is not surprising given the abnormally high levels of acid metabolites and the corresponding low pH (due to a lack of pH control in these simple batch cultures), which is known to inhibit cell growth (27,37). As a result, glucose consumption in both strains [824(p94_solB) and 824 ΔsolB] was reduced about 3-fold as well ( Table 2).…”

Characterizing metabolic stress-induced phenotypes ofSynechocystisPCC6803 with Raman spectroscopy

“…This leads to the accumulation of carboxylic acids (e.g. acetate), which can be toxic and require a reorganization of the cell membrane for survival (Papoutsakis et al, 1987;Russell, 1992;Lasko et al, 1997;Roe et al, 2002). Thus, cells have the ability to respond to such chemical stresses, adapt for survival, and even acquire tolerance (Nicolaou, Gaida & Papoutsakis, 2010).…”