CO2-Fixing One-Carbon Metabolism in a Cellulose-Degrading Bacterium Clostridium thermocellum

Katherine Chou, Wei Xiong, Lauren Magnusson, Pin-Ching Maness, Paul Lin, Lisa Warner, James Liao

Research output: Contribution to journalArticlepeer-review

45 Scopus Citations


Clostridium thermocellum can ferment cellulosic biomass to formate and other end products, including CO2. This organism lacks formate dehydrogenase (Fdh), which catalyzes the reduction of CO2 to formate. However, feeding the bacterium 13C-bicarbonate and cellobiose followed by NMR analysis showed the production of 13C-formate in C. thermocellum culture, indicating the presence of an uncharacterized pathway capable of converting CO2 to formate. Combining genomic and experimental data, we demonstrated that the conversion of CO2 to formate serves as a CO2 entry point into the reductive one-carbon (C1) metabolism, and internalizes CO2 via two biochemical reactions: the reversed pyruvate:ferredoxin oxidoreductase (rPFOR), which incorporates CO2 using acetyl-CoA as a substrate and generates pyruvate, and pyruvate-formate lyase (PFL) converting pyruvate to formate and acetyl-CoA. We analyzed the labeling patterns of proteinogenic amino acids in individual deletions of all five putative PFOR mutants and in a PFL deletion mutant. We identified two enzymes acting as rPFOR, confirmed the dual activities of rPFOR and PFL crucial for CO2 uptake, and provided physical evidence of a distinct in vivo "rPFOR-PFL shunt" to reduce CO2 to formate while circumventing the lack of Fdh. Such a pathway precedes CO2 fixation via the reductive C1 metabolic pathway in C. thermocellum. These findings demonstrated the metabolic versatility of C. thermocellum, which is thought of as primarily a cellulosic heterotroph but is shown here to be endowed with the ability to fix CO2 as well.

Original languageAmerican English
Pages (from-to)13180-13185
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number46
StatePublished - 15 Nov 2016

Bibliographical note

Publisher Copyright:
© 2016, National Academy of Sciences. All rights reserved.

NREL Publication Number

  • NREL/JA-2700-67201


  • C-isotopic tracing
  • Clostridium thermocellum
  • Formate
  • One-carbon metabolism
  • Pyruvate:ferredoxin oxidoreducase


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