Cited by 221Microbial Metabolic Engineering and BioproductionAnaerobic Digestion and Biogas ProductionBiofuel production and bioconversion

Energy Conservation Associated with Ethanol Formation from H 2 and CO 2 in Clostridium autoethanogenum Involving Electron Bifurcation

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Mock, Johanna, Zheng, Yanning, Mueller, Alexander P., Ly, San, Tran, Loan, Segovia, Simon, Nagaraju, Shilpa, Köpke, Michael +2 more·2015·Journal of Bacteriology

Abstract

UNLABELLED: Most acetogens can reduce CO2 with H2 to acetic acid via the Wood-Ljungdahl pathway, in which the ATP required for formate activation is regenerated in the acetate kinase reaction. However, a few acetogens, such as Clostridium autoethanogenum, Clostridium ljungdahlii, and Clostridium ragsdalei, also form large amounts of ethanol from CO2 and H2. How these anaerobes with a growth pH optimum near 5 conserve energy has remained elusive. We investigated this question by determining the specific activities and cofactor specificities of all relevant oxidoreductases in cell extracts of H2/CO2-grown C. autoethanogenum. The activity studies were backed up by transcriptional and mutational analyses. Most notably, despite the presence of six hydrogenase systems of various types encoded in the genome, the cells appear to contain only one active hydrogenase. The active [FeFe]-hydrogenase is electron bifurcating, with ferredoxin and NADP as the two electron acceptors. Consistently, most of the other active oxidoreductases rely on either reduced ferredoxin and/or NADPH as the electron donor. An exception is ethanol dehydrogenase, which was found to be NAD specific. Methylenetetrahydrofolate reductase activity could only be demonstrated with artificial electron donors. Key to the understanding of this energy metabolism is the presence of membrane-associated reduced ferredoxin:NAD(+) oxidoreductase (Rnf), of electron-bifurcating and ferredoxin-dependent transhydrogenase (Nfn), and of acetaldehyde:ferredoxin oxidoreductase, which is present with very high specific activities in H2/CO2-grown cells. Based on these findings and on thermodynamic considerations, we propose metabolic schemes that allow, depending on the H2 partial pressure, the chemiosmotic synthesis of 0.14 to 1.5 mol ATP per mol ethanol synthesized from CO2 and H2. IMPORTANCE: Ethanol formation from syngas (H2, CO, and CO2) and from H2 and CO2 that is catalyzed by bacteria is presently a much-discussed process for sustainable production of biofuels. Although the process is already in use, its biochemistry is only incompletely understood. The most pertinent question is how the bacteria conserve energy for growth during ethanol formation from H2 and CO2, considering that acetyl coenzyme A (acetyl-CoA), is an intermediate. Can reduction of the activated acetic acid to ethanol with H2 be coupled with the phosphorylation of ADP? Evidence is presented that this is indeed possible, via both substrate-level phosphorylation and electron transport phosphorylation. In the case of substrate-level phosphorylation, acetyl-CoA reduction to ethanol proceeds via free acetic acid involving acetaldehyde:ferredoxin oxidoreductase (carboxylate reductase).

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Crossref · 10.1128/jb.00399-15

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Authors
Mock, J., Zheng, Y., Mueller, A. P., Ly, S., Tran, L., Segovia, S., Nagaraju, S., Köpke, M., Dürre, P., & Thauer, R. K.
Affiliations
  • Max Planck Institute for Terrestrial Microbiology (DE)
  • LanzaTech Inc., Skokie, Illinois, USA
  • Universität Ulm (DE)
Venue
Journal of Bacteriology
Published
2015-09-15
Volume
197
Issue
18
Pages
2965-2980
ISSN
  • 0021-9193 (ISSN-L)
  • 1098-5530
OpenAlex
W1841900872
Cited by
220
References
88
Citations by year
2026: 13 · 2025: 17 · 2024: 18 · 2023: 25 · 2022: 26 · 2021: 18 · 2020: 27 · 2019: 21 · 2018: 20 · 2017: 13 · 2016: 16 · 2015: 6
FWCI
7.96
Type
journal article
Language
en
Open access
Locations
  • Journal of Bacteriology (publishedVersion)
  • PubMed Central (publishedVersion)
  • PubMed (publishedVersion)
  • Max Planck Institute for Plasma Physics (submittedVersion)
  • MPG.PuRe (Max Planck Society) (submittedVersion)
Topics
  • Life Sciences › Biochemistry, Genetics and Molecular Biology › Molecular Biology › Microbial Metabolic Engineering and Bioproduction
  • Physical Sciences › Engineering › Building and Construction › Anaerobic Digestion and Biogas Production
  • Physical Sciences › Engineering › Biomedical Engineering › Biofuel production and bioconversion
Keywords
Clostridium autoethanogenum, ethanol production, electron bifurcation, energy conservation, acetogenic bacteria, hydrogenase, [FeFe]-hydrogenase, methylenetetrahydrofolate reductase, ethanol dehydrogenase, Wood-Ljungdahl pathway, ATP production, syngas conversion, biofuel production
MeSH terms
Acetyl Coenzyme A, Adenosine Diphosphate, Ethanol, Bacterial Proteins, Carbon Dioxide, Clostridium, Electron Transport, Energy Metabolism, Hydrogen, Membrane Proteins, Oxidoreductases, Phosphoproteins, Gene Expression Regulation, Bacterial, Gene Expression Regulation, Enzymologic, Acetic Acid

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