title: Electrode-assisted acetoin production in a metabolically engineeredEscherichia colistrain.
abstract: BACKGROUND
  This paper describes the metabolic engineering ofEscherichia colifor theanaerobicfermentation of glucose to acetoin. Acetoin has well-established applications in industrial food production and was suggested to be a platform chemical for a bio-based economy. However, the biotechnological production is often hampered by the simultaneous formation of several end products in the absence of an electron acceptor. Moreover, typical production strains are often potentially pathogenic. The goal of this study was to overcome these limitations by establishing an electrode-assisted fermentation process inE. coli. Here, the surplus of electrons released in the production process is transferred to an electrode as anoxic and non-depletable electron acceptor.
  
  
  RESULTS
  In a first step, the central metabolism was steered towards the production of pyruvate from glucose by deletion of genes encoding for enzymes of central reactions of the anaerobic carbon metabolism (ΔfrdA-DΔadhEΔldhAΔpta-ack). Thereafter, the genes for the acetolactate synthase (alsS) and the acetolactate decarboxylase (alsD) were expressed in this strain from a plasmid. Addition of nitrate as electron acceptor led to an anaerobic acetoin production with a yield of up to 0.9 mol acetoin per mol of glucose consumed (90% of the theoretical maximum). In a second step, the electron acceptor nitrate was replaced by a carbon electrode. This interaction necessitated the further expression ofc-type cytochromes fromShewanella oneidensisand the addition of the soluble redox shuttle methylene blue. The interaction with the non-depletable electron acceptor led to an acetoin formation with a yield of 79% of the theoretical maximum (0.79 mol acetoin per mol glucose).
  
  
  CONCLUSIONS
  Electrode-assisted fermentations are a new strategy to produce substances of biotechnological value that are more oxidized than the substrates. Here, we show for the first time a process in which the commonly used chassis strainE. coliwas tailored for an electrode-assisted fermentation approach branching off from the central metabolite pyruvate. At this early stage, we see promising results regarding carbon and electron recovery and will use further strain development to increase the anaerobic metabolic turnover rate.
authors.affiliation: Department of Applied Biology, Institute for Applied Biosciences, Karlsruhe Institute of Technology, Fritz-Haber-Weg 2, 76131 Karlsruhe, Germany.
authors.forename: Andreas H
authors.initials: AH
authors.lastname: Förster
authors.affiliation: Department of Applied Biology, Institute for Applied Biosciences, Karlsruhe Institute of Technology, Fritz-Haber-Weg 2, 76131 Karlsruhe, Germany.
authors.forename: Sebastian
authors.initials: S
authors.lastname: Beblawy
authors.affiliation: Department of Applied Biology, Institute for Applied Biosciences, Karlsruhe Institute of Technology, Fritz-Haber-Weg 2, 76131 Karlsruhe, Germany.
authors.forename: Frederik
authors.initials: F
authors.lastname: Golitsch
authors.affiliation: Department of Applied Biology, Institute for Applied Biosciences, Karlsruhe Institute of Technology, Fritz-Haber-Weg 2, 76131 Karlsruhe, Germany.|Department of Microbiology of Natural and Technical Interfaces, Institute of Functional Interfaces, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.
authors.forename: Johannes
authors.initials: J
authors.lastname: Gescher
keywords: Acetoin; Bulk chemicals; Electrode-assisted fermentation; Escherichia coli; Metabolic engineering