Osmotic stress as a factor for regulating E. coli hydrogenase activity and enhancing H2 production during mixed carbon sources fermentation

Role of the Escherichia coli FocA and FocB formate channels in controlling proton/potassium fluxes and hydrogen production during osmotic stress in energy-limited, stationary phase fermenting cells

Escherichia coli FocA and FocB formate channels export formate or import it for further disproportionation by the formate hydrogenlyase (FHL) complex to H2 and CO2. Here, we show that under pH and osmotic stress FocA and FocB play important roles in regulating proton and potassium fluxes and couple this with H2 production in stationary-phase cells. Using whole-cell assays with glucose as electron donor, a focB mutant showed a 50 % decrease in VH2, while N'N'-dicyclohexylcarbodiimide (DCCD) treatment of osmotically stressed cells underlined the role of FOF1 ATPase in H2 production. At pH 7.5 and under osmotic stress FocB contributed to the proton flux but not to the potassium flux. At pH 5.5 both formate channels contributed to the proton and potassium fluxes. Particulalry, a focA mutant had 40 % lower potassium flux whereas the proton flux increased approximately two-fold. Moreover, at pH 5.5H2 production was totally inhibited by DCCD in the focA mutant. Taken together, our results suggest that depending on external pH, the formate channels play an important role in osmoregulation by helping to balance proton/potassium fluxes and H2 production, and thus assist the proton FOF1-ATPase in maintenance of ion gradients in fermenting stationary-phase cells.

Proton conductance and regulation of proton/potassium fluxes in Escherichia coli FhlA-lacking cells during fermentation of mixed carbon sources

Escherichia coli uptake potassium ions with the coupling of proton efflux and energy utilization via proton FOF1-ATPase. In this study contribution of formate hydrogen lyase (FHL) complexes in the proton/potassium fluxes and the formation of proton conductance (CMH+) were investigated using fhlA mutant strain. The proton flux rate (JH+) decreased in fhlA by ∼ 25 % and ∼70 % during the utilization of glucose and glycerol, respectively, at 20 h suggesting H+ transport via or through FHL complexes. The decrease in JK+ in fhlA by ∼40 % proposed the interaction between FHL and Trk secondary transport system during mixed carbon fermentation. Moreover, the usage of N,N'-dicyclohexylcarbodiimide (DCCD) demonstrated the mediation of FOF1-ATPase in this interaction. CMH+ was 13.4 nmol min-1 mV-1 in WT at 20 h, which decreased by 20 % in fhlA. Taken together, FHL complexes have a significant contribution to the modulation of H+/K+ fluxes and the CMH + for efficient energy transduction and regulation of the proton motive force during mixed carbon sources fermentation.