Engineering Escherichia coli for Production of Glycolic Acid from D-Xylose through Dahms Pathway

Glycolic acid (GA) is the simplest form of α-hydroxy acid and it is extensively used in cosmetics and dermatology for its exfoliating and moisturizing properties. Its corresponding polymeric form, polyglycolic acid (PGA), has excellent gas barrier properties which makes it a good packaging material. In addition, the use of PGA in PET bottles and medical sutures has also led to increasing demand for GA. Recent reports on the biosynthetic production of ethylene glycol from D-xylose have identified GA as a by-product during fermentation. On the other hand, studies that were aimed at GA production in recombinant microorganisms have employed the glyoxylate shunt.  In this study, GA production was implemented by recruiting the Dahms pathway in E. coli. D-xylose is oxidized to D-xylonic acid by a heterologous xylose dehydrogenase (Xdh). D-xylonic acid is the converted to glycolaldehyde and pyruvate through the inherent D-xylonic acid metabolism in E. coli. Pyruvate is further assimilated into the central carbon metabolism, while glycolaldehyde is either reduced to ethylene glycol or oxidized to GA. The metabolic engineering strategies carried out to produce mainly GA include (1) optimization of the expression levels of enzymes catalyzing the conversion of D-xylose to glycolaldehyde, (2) recruitment of alternative dehydratase, (3) blocking ethylene glycol formation by deletion of relevant aldehyde reductases, (4) blocking GA oxidation reaction by disruption of the glcD gene which codes for glycolate oxidase, and (5) overexpression of glyoxylate shunt enzymes glyoxylate reductase (ycdW gene), isocitratelyase (aceA gene), and isocitrate dehydrogenase kinase/ phosphatease (aceK gene) to channel the TCA cycle towards GA formation. 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).