Coenzyme A-transferase-independent butyrate re-assimilation in Clostridium acetobutylicum—evidence from a mathematical model
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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.
KeywordsClostridium acetobutylicum ctfA mutant Acid re-assimilation pH-induced metabolic shift Mathematical modelling
The authors acknowledge support by the German Federal Ministry for Education and Research (BMBF FKZ 0315782D) and by the Biotechnology and Biological Sciences Research Council (UK) (BBSRC No. BB/I004475/1), as part of the European Transnational Network ‘Systems Biology of Microorganisms’ (SysMo) within the COSMIC consortium. N.P.M. also acknowledges funding received from the Biotechnology and Biological Sciences Research Council, as part of the BBSRC Sustainable Bioenergy Centre (BSBEC) initiative, for the programme ‘Second Generation, Sustainable Biofuels’ (BBSRC No. BB/G016224/1). We thank Ulf W. Liebal and Carola Berger for their critical comments and fruitful discussions. The responsibility for the content of this manuscript lies with the authors.
Conflict of interests
The authors declare that they have no conflict of interest.
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