Abstract
BackgroundPoly(3-hydroxybutyrate) (PHB), have been considered to be good candidates for completely biodegradable polymers due to their similar mechanical properties to petroleum-derived polymers and complete biodegradability. Escherichia coli has been used to simulate the distribution of metabolic fluxes in recombinant E. coli producing poly(3-hydroxybutyrate) (PHB). Genome-scale metabolic network analysis can reveal unexpected metabolic engineering strategies to improve the production of biochemicals and biofuels.ResultsIn this study, we reported the discovery of a new pathway called threonine bypass by flux balance analysis of the genome-scale metabolic model of E. coli. This pathway, mainly containing the reactions for threonine synthesis and degradation, can potentially increase the yield of PHB and other acetyl-CoA derived products by reutilizing the CO2 released at the pyruvate dehydrogenase step. To implement the threonine bypass for PHB production in E. coli, we deregulated the threonine and serine degradation pathway and enhanced the threonine synthesis, resulting in 2.23-fold improvement of PHB titer. Then, we overexpressed glyA to enhance the conversion of glycine to serine and activated transhydrogenase to generate NADPH required in the threonine bypass.ConclusionsThe result strain TB17 (pBHR68) produced 6.82 g/L PHB with the yield of 0.36 g/g glucose in the shake flask fermentation and 35.92 g/L PHB with the yield of 0.23 g/g glucose in the fed-batch fermentation, which was almost 3.3-fold higher than the parent strain. The work outlined here shows that genome-scale metabolic network analysis can reveal novel metabolic engineering strategies for developing efficient microbial cell factories.Electronic supplementary materialThe online version of this article (doi:10.1186/s12934-015-0369-3) contains supplementary material, which is available to authorized users.
Highlights
Poly(3-hydroxybutyrate) (PHB), have been considered to be good candidates for completely biodegradable polymers due to their similar mechanical properties to petroleum-derived polymers and complete biodegradability
In recombinant E. coli, an arcA mutation was used for PHB accumulation under microaerobic conditions using glucose or glycerol as a carbon source [15, 16]
In addition to the classical pathway from PEP-pyruvate to the PHB precursor acetyl-CoA through pyruvate dehydrogenase complex, there is a much longer pathway for acetylCoA production including PEP carboxylation, threonine synthesis and degradation, serine formation from glycine and serine deamination. We named this pathway threonine bypass as threonine synthesis and degradation are the main parts of the pathway
Summary
Poly(3-hydroxybutyrate) (PHB), have been considered to be good candidates for completely biodegradable polymers due to their similar mechanical properties to petroleum-derived polymers and complete biodegradability. Escherichia coli has been used to simulate the distribution of metabolic fluxes in recombinant E. coli producing poly(3-hydroxybutyrate) (PHB). Recombinant E. coli strains harboring the R. eutropha PHA biosynthesis genes have been used for the production of PHB [8,9,10]. A range of metabolic engineering studies have been carried out with the objective to improve the product yield and produce PHB from cheap carbon source (whey, hemicellulose) by using recombinant E. coli [12]. The strategy of reducing mix acid accumulation was employed to enhance the PHB production and the yield was increased by 4.3-fold [14]. In recombinant E. coli, an arcA mutation was used for PHB accumulation under microaerobic conditions using glucose or glycerol as a carbon source [15, 16]
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