Novel insights into the bioenergetics of mixed-acid fermentation: Can hydrogen and proton cycles combine to help maintain a proton motive force?

Abstract

Escherichia coli possesses four [NiFe]-hydrogenases that catalyze the reversible redox reaction of 2H(+) + 2e(-) ↔ H2. These enzymes together have the potential to form a hydrogen cycle across the membrane. Their activity, operational direction, and interaction with each other depend on the fermentation substrate and particularly pH. The enzymes producing H2 are likely able to translocate protons through the membrane. Moreover, the activity of some of these enzymes is dependent on the F0 F1 -ATPase, thus linking a proton cycle with the cycling of hydrogen. These two cycles are suggested to have a primary basic role in modulating the cell's energetics during mixed-acid fermentation, particularly in response to pH. Nevertheless, the mechanisms underlying the physical interactions between these enzyme complexes, as well as how this is controlled, are still not clearly understood. Here, we present a synopsis of the potential impact of proton-hydrogen cycling in fermentative bioenergetics.

Impact of membrane-associated hydrogenases on the FOF1-ATPase in Escherichia coli during glycerol and mixed carbon fermentation: ATPase activity and its inhibition by N,N′-dicyclohexylcarbodiimide in the mutants lacking hydrogenases

“…This is probably related with bacterial optimal growth at pH 7.5. Moreover, the F O F 1 -ATPase has a different role in maintaining proton motive force depending on Hyd enzymes as suggested [13,22]. The results with the hyaB hybC selC triple mutant suggest the requirement of Hyd-1 and Hyd-2 for the activity of the F O F 1 -ATPase during glycerol fermentation, and cooperation or interaction between these enzymes is more obvious at slightly alkaline pH (see Fig.…”

“…This interesting idea seems to be not simple and might have different explanations. Hyd enzymes can affect the production of fermentation end products and they might impact on the F O F 1 -ATPase, maintaining a proton-motive force [13,22]. Therefore, the relationship of Hyd enzymes with the F O F 1 -ATPase was studied during the glycerol and mixed carbon sources fermentation.…”

“…The higher ATPase activity of hyaB hybC selC and selC mutants upon the mixed carbon (glucose and glycerol) fermentation at pH 7.5 (see Fig. 3a) presumably can be explained by that, during mixed carbon fermentation, the F O F 1 -ATPase is involved in the formation of H 2 cycling through the membrane as suggested [22]. It is possible that, when Hyd-3 is not active as a part of FHL, the F O F 1 -ATPase operates for regulation of intracellular pH.…”

“…Some requirement of F O F 1 and its relationship with Hyd enzymes in E. coli were suggested upon both glucose and glycerol fermentations [7,13]. It was shown that Hyd-3 and Hyd-4 activities are related with the F O F 1 -ATPase at pH 7.5 and this relationship may be due to Hyd-4 interaction with the F O F 1 -ATPase to provide reducing equivalents (H + + e À ) for energy transfer to the secondary transport system [21,22]. Indeed, it has been also established that H 2 production by E. coli during glucose and glycerol fermentation at slightly alkaline and acidic pHs, respectively, is inhibited by N,N 0 -dicyclohexylcarbodiimide (DCCD), an inhibitor of the F O F 1 -ATPase [7].…”

Optimizing strategy for Escherichia coli growth and hydrogen production during glycerol fermentation in batch culture: Effects of some heavy metal ions and their mixtures

“…Four membrane-associated [Ni-Fe]-hydrogenase (Hyd) enzymes, Hyd 1 (hya), Hyd 2 (hyb), Hyd 3 (hyc) and Hyd 4 (hyf), are responsible for H 2 production and oxidation by E. coli; they can be reversible and operate in opposite directions during glucose or glycerol fermentation [9,[11][12][13][14][15]. Interestingly, four Hyd enzymes together have the potential to form a H 2 cycle across the bacterial membrane of E. coli [16]. However, responsible enzymes in E. coli under different technological conditions including glycerol as a cheap substrate and at different pHs are not clear yet [1,2,11,12,14].…”