title: Systematic engineering of the central metabolism in Escherichia coli for effective production of n-butanol.
abstract: BACKGROUND
  Microbes have been extensively explored for production of environment-friendly fuels and chemicals. The microbial fermentation pathways leading to these commodities usually involve many redox reactions. This makes the fermentative production of highly reduced products challenging, because there is a limited NADH output from glucose catabolism. Microbial production of n-butanol apparently represents one typical example.
  
  
  RESULTS
  In this study, we addressed the issue by adjustment of the intracellular redox state in Escherichia coli. This was initiated with strain BuT-8 which carries the clostridial CoA-dependent synthetic pathway. Three metabolite nodes in the central metabolism of the strain were targeted for engineering. First, the pyruvate node was manipulated by enhancement of pyruvate decarboxylation in the oxidative pathway. Subsequently, the pentose phosphate (PP) pathway was amplified at the glucose-6-phosphate (G6P) node. The pathway for G6P isomerization was further blocked to force the glycolytic flux through the PP pathway. It resulted in a growth defect, and the cell growth was later recovered by limiting the tricarboxylic acid cycle at the acetyl-CoA node. Finally, the resulting strain exhibited a high NADH level and enabled production of 6.1 g/L n-butanol with a yield of 0.31 g/g-glucose and a productivity of 0.21 g/L/h.
  
  
  CONCLUSIONS
  The production efficiency of fermentative products in microbes strongly depends on the intracellular redox state. This work illustrates the flexibility of pyruvate, G6P, and acetyl-CoA nodes at the junction of the central metabolism for engineering. In principle, high production of reduced products of interest can be achieved by individual or coordinated modulation of these metabolite nodes.
authors.affiliation: Department of Chemical Engineering, Feng Chia University, 100 Wenhwa Road, Taichung, 40724 Taiwan Republic of China.
authors.forename: Mukesh
authors.initials: M
authors.lastname: Saini
authors.affiliation: Department of Chemical Engineering, National Chung Hsing University, Taichung, 402 Taiwan Republic of China.
authors.forename: Si-Yu
authors.initials: SY
authors.lastname: Li
authors.affiliation: Department of Chemical Engineering, Feng Chia University, 100 Wenhwa Road, Taichung, 40724 Taiwan Republic of China.
authors.forename: Ze Win
authors.initials: ZW
authors.lastname: Wang
authors.affiliation: Department of Medical Laboratory Science and Biotechnology, China Medical University, No. 91, Hsueh-Shih Road, Taichung, 40402 Taiwan Republic of China.
authors.forename: Chung-Jen
authors.initials: CJ
authors.lastname: Chiang
authors.affiliation: Department of Chemical Engineering, Feng Chia University, 100 Wenhwa Road, Taichung, 40724 Taiwan Republic of China ; Department of Health and Nutrition Biotechnology, Asia University, Taichung, 41354 Taiwan Republic of China ; Department of Medical Research, China Medical University Hospital, Taichung, 40447 Taiwan Republic of China.
authors.forename: Yun-Peng
authors.initials: YP
authors.lastname: Chao
keywords: Escherichia coli; Metabolic engineering; The redox state; n-Butanol