title: Coenzyme A-transferase-independent butyrate re-assimilation in Clostridium acetobutylicum-evidence from a mathematical model.
abstract: The hetero-dimeric CoA-transferase CtfA/B is believed to be crucial for the metabolic transition from acidogenesis to solventogenesis in Clostridium acetobutylicum as part of the industrial-relevant acetone-butanol-ethanol (ABE) fermentation. Here, the enzyme is assumed to mediate re-assimilation of acetate and butyrate during a pH-induced metabolic shift and to faciliate the first step of acetone formation from acetoacetyl-CoA. However, recent investigations using phosphate-limited continuous cultures have questioned this common dogma. To address the emerging experimental discrepancies, we investigated the mutant strain Cac-ctfA398s::CT using chemostat cultures. As a consequence of this mutation, the cells are unable to express functional ctfA and are thus lacking CoA-transferase activity. A mathematical model of the pH-induced metabolic shift, which was recently developed for the wild type, is used to analyse the observed behaviour of the mutant strain with a focus on re-assimilation activities for the two produced acids. Our theoretical analysis reveals that the ctfA mutant still re-assimilates butyrate, but not acetate. Based upon this finding, we conclude that C. acetobutylicum possesses a CoA-tranferase-independent butyrate uptake mechanism that is activated by decreasing pH levels. Furthermore, we observe that butanol formation is not inhibited under our experimental conditions, as suggested by previous batch culture experiments. In concordance with recent batch experiments, acetone formation is abolished in chemostat cultures using the ctfa mutant.
authors.affiliation: Institute of Computer Science, Department of Systems Biology & Bioinformatics, University of Rostock, Ulmenstr. 69, 18057, Rostock, Germany, thomas.millat@nottingham.ac.uk.
authors.forename: Thomas
authors.initials: T
authors.lastname: Millat
authors.forename: Christine
authors.initials: C
authors.lastname: Voigt
authors.forename: Holger
authors.initials: H
authors.lastname: Janssen
authors.forename: Clare M
authors.initials: CM
authors.lastname: Cooksley
authors.forename: Klaus
authors.initials: K
authors.lastname: Winzer
authors.forename: Nigel P
authors.initials: NP
authors.lastname: Minton
authors.forename: Hubert
authors.initials: H
authors.lastname: Bahl
authors.forename: Ralf-Jörg
authors.initials: RJ
authors.lastname: Fischer
authors.forename: Olaf
authors.initials: O
authors.lastname: Wolkenhauer
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D002087:Butyrates / Q000378:metabolism*; D046969:Clostridium acetobutylicum / Q000378:metabolism*; D003065:Coenzyme A / Q000378:metabolism*; D015256:Coenzyme A-Transferases / Q000235:genetics / Q000378:metabolism; D017353:Gene Deletion; D006863:Hydrogen-Ion Concentration; D008962:Models, Theoretical; D016254:Mutagenesis, Insertional
chemical_list: D001426:Bacterial Proteins; D002087:Butyrates; D015256:Coenzyme A-Transferases; C083979:ctfA protein, Clostridium acetobutylicum; C083978:ctfB protein, Clostridium acetobutylicum; D003065:Coenzyme A