The synthetic xylulose-1 phosphate pathway increases production of glycolic acid from xylose-rich sugar mixtures

Abstract

Background: Glycolic acid (GA) is a two-carbon hydroxyacid with applications in the cosmetic, textile, and medical industry. Microbial GA production from all sugars can be achieved by engineering the natural glyoxylate shunt. The synthetic (d)-xylulose-1 phosphate (X1P) pathway provides a complementary route to produce GA from (d)-xylose. The simultaneous operation of the X1P and glyoxylate pathways increases the theoretical GA yield from xylose by 20 %, which may strongly improve GA production from hemicellulosic hydrolysates.

Results: We herein describe the construction of an E. coli strain that produces GA via the glyoxylate pathway at a yield of 0.31 , 0.29 , and 0.37 g/g from glucose, xylose, or a mixture of glucose and xylose (mass ratio: 33:66 %), respectively. When the X1P pathway operates in addition to the glyoxylate pathway, the GA yields on the three substrates are, respectively, 0.39 , 0.43 , and 0.47 g/g. Upon constitutive expression of the sugar permease GalP, the GA yield of the strain which simultaneously operates the glyoxylate and X1P pathways further increases to 0.63 g/g when growing on the glucose/xylose mixture. Under these conditions, the GA yield on the xylose fraction of the sugar mixture reaches 0.75 g/g, which is the highest yield reported to date.

Conclusions: These results demonstrate that the synthetic X1P pathway has a very strong potential to improve GA production from xylose-rich hemicellulosic hydrolysates.

Keywords: Escherichia coli; Glucose; Glycolic acid; Synthetic pathway; Xylose.

Fig. 1

Synthetic (blue) (D)-xylulose-1 phosphate (X1P) and natural (black) pathways for the production of glycolic acid from (d)-xylose and (d)-glucose. Relevant genes are depicted in blue next to the reactions that they encode. Bold black arrows indicate overexpression of the activity. DHAP dihydroxyacetone phosphate, GAP glyceraldehyde-3P, α-KG α-ketoglutarate)

Fig. 2

Production of glycolic acid (GA) by optimized E. coli strains during growth on a synthetic mixture. a Strain Pen905 (∆aceB ∆glcDEFGB ∆gcl ∆edd-eda ∆iclR ∆arcA ∆icd ∆xylB expressing pGS and pX1P). b Strain Pen979 (∆aceB ∆glcDEFGB ∆gcl ∆edd-eda ∆iclR ∆arcA ∆icd ∆xylB galP ^proD expressing pGS and pX1P). Initial glucose and xylose concentrations were 2.5 and 5 g/l, respectively. Cultivation was carried out in shake flasks at 30 °C