title: CO2-fixing one-carbon metabolism in a cellulose-degrading bacterium Clostridium thermocellum.
abstract: Clostridium thermocellum can ferment cellulosic biomass to formate and other end products, including CO2This organism lacks formate dehydrogenase (Fdh), which catalyzes the reduction of CO2to formate. However, feeding the bacterium13C-bicarbonate and cellobiose followed by NMR analysis showed the production of13C-formate in C. thermocellum culture, indicating the presence of an uncharacterized pathway capable of converting CO2to formate. Combining genomic and experimental data, we demonstrated that the conversion of CO2to formate serves as a CO2entry point into the reductive one-carbon (C1) metabolism, and internalizes CO2via two biochemical reactions: the reversed pyruvate:ferredoxin oxidoreductase (rPFOR), which incorporates CO2using 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 CO2uptake, and provided physical evidence of a distinct in vivo "rPFOR-PFL shunt" to reduce CO2to formate while circumventing the lack of Fdh. Such a pathway precedes CO2fixation 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 CO2as well.
authors: Biosciences Center, National Renewable Energy Laboratory, Golden, CO 80401.
authors: Wei
authors: W
authors: Xiong
authors: Department of Chemical and Biomolecular Engineering, University of California, Los Angeles, CA 90095.
authors: Paul P
authors: PP
authors: Lin
authors: Biosciences Center, National Renewable Energy Laboratory, Golden, CO 80401.
authors: Lauren
authors: L
authors: Magnusson
authors: National Bioenergy Center, National Renewable Energy Laboratory, Golden, CO 80401.
authors: Lisa
authors: L
authors: Warner
authors: Department of Chemical and Biomolecular Engineering, University of California, Los Angeles, CA 90095.|Academia Sinica, Taipei City, Taiwan 115.
authors: James C
authors: JC
authors: Liao
authors: Biosciences Center, National Renewable Energy Laboratory, Golden, CO 80401; katherine.chou@nrel.gov pinching.maness@nrel.gov.
authors: Pin-Ching
authors: PC
authors: Maness
authors: Biosciences Center, National Renewable Energy Laboratory, Golden, CO 80401; katherine.chou@nrel.gov pinching.maness@nrel.gov.
authors: Katherine J
authors: KJ
authors: Chou
mesh_terms: D019149:Bioreactors; D002244:Carbon / Q000378:metabolism; D002245:Carbon Dioxide / Q000378:metabolism*; D002482:Cellulose / Q000378:metabolism*; D048013:Acetivibrio thermocellus / Q000187:drug effects / Q000235:genetics / Q000254:growth & development / Q000378:metabolism*; D005285:Fermentation; D006859:Hydrogen / Q000378:metabolism; D017693:Sodium Bicarbonate / Q000494:pharmacology
chemical_list: D002245:Carbon Dioxide; D002244:Carbon; D006859:Hydrogen; D017693:Sodium Bicarbonate; D002482:Cellulose
keywords: 13C-isotopic tracing; Clostridium thermocellum; formate; one-carbon metabolism; pyruvate:ferredoxin oxidoreducase