FocA and its central role in fine-tuning pH homeostasis of enterobacterial formate metabolism

During enterobacterial mixed-acid fermentation, formate is generated from pyruvate by the glycyl-radical enzyme pyruvate formate-lyase (PflB). In , especially at low pH, formate is then disproportionated to CO and H by the cytoplasmically oriented, membrane-associated formate hydrogenlyase (FHL) com...

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Bibliographic Details
Published in:Microbiology (Society for General Microbiology) 2022-10, Vol.168 (10)
Main Authors: Kammel, Michelle, Pinske, Constanze, Sawers, R Gary
Format: Article
Language:English
Subjects:
Anions - metabolism
Carbon Dioxide - metabolism
Enterobacteriaceae - metabolism
Escherichia coli - metabolism
Escherichia coli Proteins - metabolism
Formate Dehydrogenases - genetics
Formate Dehydrogenases - metabolism
Formates - metabolism
Homeostasis
Hydrogen-Ion Concentration
Hydrogenase - metabolism
Membrane Transport Proteins - metabolism
Nitrites - metabolism
Protons
Pyruvates - metabolism
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Summary:During enterobacterial mixed-acid fermentation, formate is generated from pyruvate by the glycyl-radical enzyme pyruvate formate-lyase (PflB). In , especially at low pH, formate is then disproportionated to CO and H by the cytoplasmically oriented, membrane-associated formate hydrogenlyase (FHL) complex. If electron acceptors are available, however, formate is oxidized by periplasmically oriented, respiratory formate dehydrogenases. Formate translocation across the cytoplasmic membrane is controlled by the formate channel, FocA, a member of the formate-nitrite transporter (FNT) family of homopentameric anion channels. This review highlights recent advances in our understanding of how FocA helps to maintain intracellular formate and pH homeostasis during fermentation. Efflux and influx of formate/formic acid are distinct processes performed by FocA and both are controlled through protein interaction between FocA's N-terminal domain with PflB. Formic acid efflux by FocA helps to maintain cytoplasmic pH balance during exponential-phase growth. Uptake of formate against the electrochemical gradient (inside negative) is energetically and mechanistically challenging for a fermenting bacterium unless coupled with proton/cation symport. Translocation of formate/formic acid into the cytoplasm necessitates an active FHL complex, whose synthesis also depends on formate. Thus, FocA, FHL and PflB function together to govern formate homeostasis. We explain how FocA achieves efflux of formic acid and propose mechanisms for pH-dependent uptake of formate both with and without proton symport. We propose that FocA displays both channel- and transporter-like behaviour. Whether this translocation behaviour is shared by other members of the FNT family is also discussed.
ISSN:1350-0872
1465-2080
DOI:10.1099/mic.0.001253