Xylitol Production from Corncob Hydrolysate by an Engineered Escherichia coli M15 as Whole-Cell Biocatalysts

Purpose Xylitol is used in the food and pharmaceutical industries as a sweetener, and its consumption rate has remarkably increased over the years. Currently, xylitol is produced through a chemical reduction method. However, considering the global climate change issue, the development of eco-friendly, renewable, and sustainable substrates for the production of high-value platform chemicals such as xylitol is the need of the hour. Hence, the present study aimed to use a microbial process for xylitol production. Methods Escherichia coli M15 platform was used to overexpress the D-xylose reductase (XR) gene from a mesophilic yeast, Candida tropicalis GRA1. The 37-KDa CtXR sequence exhibited a highly conserved active site structure, where a tetrad of residues (Tyr51, Lys80, His113, and Asp46) was located at the base of the substrate-binding pocket. To mitigate the rate-limiting step of cofactor supply in the bioconversion of xylose to xylitol, overexpression of XR genes coupled with auxiliary substrates was used toward in vivo cofactor regeneration. Results In the presence of xylose as the only carbon source, the M15 CtXR produced 2.1 g L-1 of xylitol. The xylitol titer showed a steady-state increase with the addition of auxiliary substrates glucose and glycerol to 3.4 and 6.4 g L-1, respectively. Further, M15 CtXR as a whole-cell biocatalyst in alkali pretreated detoxified corncob hydrolysate produced xylitol titer of 3.7 g L-1. Conclusion Therefore, we successfully produced xylitol from corncob hydrolysates using the engineered E. coli M15 as whole-cell biocatalysts. Further, this process can be up scaled for the synthesis of eco-friendly high-value green chemicals. [GRAPHICS] .