Engineering Escherichia coli for glycolic acid production from D-xylose through the Dahms pathway and glyoxylate bypass

Glycolic acid (GA) is an ai-hydroxy acid used in cosmetics, packaging, and medical industries due to its excellent properties, especially in its polymeric form. In this study, Escherichia coli was engineered to produce GA from D-xylose by linking the Dahms pathway, the glyoxylate bypass, and the partial reverse glyoxylate pathway (RGP). Initially, a GA-producing strain was constructed by disrupting the xylAB and glcD genes in the E. coli genome and overexpressing the xdh(Cc) from Caulobacter crescentus. This strain was further improved through modular optimization of the Dahms pathway and the glyoxylate bypass. Results for module 1 showed that the rate-limiting step of the Dahms pathway was the xylonate dehydratase reaction, and the overexpression of yagF was sufficient to overcome this bottleneck. Furthermore, the appropriate aldolase gene for module 1 was proven to be yagE. The results also show that overexpression of the lactaldehyde dehydrogenase gene, aldA, is needed to increase the GA production while the overexpression of glyoxylate reductase gene, ycdW, was only essential when the glyoxylate bypass was active. On the other hand, the module 2 enzymes AceA and AceK were vital in activating the glyoxylate bypass, while the RGP enzymes were dispensable. The final strain (GA19) produced 4.57 g/L GA with a yield of 0.46 g/g from D-xylose. So far, this is the highest value achieved for GA production in engineered E. coli through the Dahms pathway.