Rational Metabolic Engineering of Escherichia coli for Ethylene Glycol Production from D-Xylose
The microbial production of renewable ethylene glycol (EG) has been gaining attention recently. This compound is mainly used as an antifreeze agent and precursor for polymer synthesis. EG has been successfully produced biosynthetically from D-xylose through several novel synthetic pathways. The first report on the biosynthesis of EG employs the Dahms pathway in Escherichia coli. Conversion of D-xylose is initiated by an oxidation reaction to produce D-xylonic acid catalyzed by a heterologous xylose dehydrogenase (Xdh). D-xylonic acid is further converted to glycolaldehyde and pyruvate through the native D-xylonic acid metabolism in E. coli. Pyruvate is then assimilated into the central carbon metabolism, while glycolaldehyde is either reduced to ethylene glycol or oxidized to glycolic acid. The yield achieved through this pathway is at 70 % of the theoretical. In this study, EG yield is further improved by implementing metabolic engineering strategies. D-xylonic acid accumulation was reduced when Xdh was placed under a weak promoter and tighter control of gene expression. This involves expression of Xdh under the tac promoter in a low-copy vector, and introduction of multiple copies of lac repressor gene. In addition, the suitable aldehyde reductase (ALR) for the EG pathway was identified in E. coli. EG yield improved after overexpression of the native YjgB. Finally, the EG yield was further increased by blocking the competing glycolic acid formation. The lactaldehyde dehydrogenase activity of E. coli was deactivated by disruption of the aldA gene, which encodes for AldA. The chromosomal yjgB, which encodes for YjgB, was also disrupted in the host genome. The combination of these genetic manipulations resulted in improved growth, D-xylose consumption, and EG yields of the final strain WTXB. The yield reached up to 99 % from D-xylose, which is the highest yield of EG from D-xylose so far. The main drawback in WTXB is the low productivity. Additional studies and pathway optimization are still necessary to enhance the overall performance of WTXB in order to reach industrial viability. This work was supported by Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education (No. 2009-0093816), and Korea Research Fellowship Program through the NRF funded by the Ministry of Science, ICT and Future Planning (No. 2015H1D3A1062172).