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

Authors

Wei Xiong, National Renewable Energy Labratory
Paul P. Lin, University of California
Lisa Warner, Boise State UniversityFollow
James C. Liao, University of California
Pin-Ching Maness, National Renewable Energy Laboratory
Katherine J. Chou, National Renewable Energy Laboratory

Document Type

Article

Publication Date

11-15-2016

DOI

http://dx.doi.org/10.1073/pnas.1605482113

Abstract

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.

Copyright Statement

This document was originally published in Proceedings of the National Academyj of Sciences of the United States of America by National Academy of Sciences. Copyright restrictions may apply. doi: 10.1073/pnas.1605482113

Publication Information

Xiong, Wei; Lin, Paul P.; Warner, Lisa; Liao, James C.; Maness, Pin-Ching; and Chou, Katherine J.. (2016). "CO₂-Fixing One-Carbon Metabolism in a Cellulose-Degrading Bacterium Clostridium thermocellum". Proceedings of the National Academy of Sciences of the United States of America, 113(46), 13180-13185.