Xylitol Production By Lignocellulosic Inhibitory Compounds-Resistant Strain And Genetic Engineering For Ethanol Production

Lignocellulosic materials are cost-effective resources for bio-production of value-added products such as xylitol and ethanol. However, the microbial utilization of lignocellulose is limited because of the toxic compounds present in the hydrolysate. In this work, a lignocellulosic inhibitory compounds-resistant strain, Pichia guilliermondii, was isolated and was used for xylitol production from xylose. Different inoculation sizes, mixed carbon sources, and concentrations of inhibitors were investigated. After optimization of these fermentation conditions, oxygen supply conditions, i.e., various initial volumetric oxygen transfer coefficient (KLa) values, and the behavior of key enzymes of xylose metabolism, i.e., xylose reductase (XR), xylitol dehydrogenase (XDH) and glucose-6-phosphate dehydrogenase (G6PD), were studied in detail. The results indicated that an inoculation size of 1.8g l-1 and a glucose:xylose ratio (g:g) of less than 1:10 was favorable to the P. guilliermondii fermentation in the presence of lignocellulosic inhibitory compounds. In culture medium with high concentration of inhibitors (2g l-1 furfural, 2g l-1 5-hydroxymethyl furfural and 3g l-1 acetic acid), this yeast degraded furfural and HMF rapidly and maintained the fermentation ability of more than 90% compared to the control without inhibitors addition. This strain was found to be very sensitive to oxygen supply during xylose fermentation. A very low initial KLa value, i.e., within the range of 0.033-0.37h-1, showed a significant effect on the cell growth, substrate uptake, xylitol production as well as the activities of key enzymes (XR, XDH and G6PD). The highest xylitol yield on xylose (0.61gg-1) was attained at an initial KLa value of 0.075h-1. The G6PD specific activity was enhanced with the increase of intail KLa; the very low initial KLa inhibited the activity of XDH, thus inhibited the degradation of xylitol. Furthermore, a XR/XDH ratio <1 was found in this yeast during xylitol production.At the same time, a lignocellulosic inhibitory compounds-resistant bacterium was screened out with its known genomic background. A plasmid with pdc and adhB from Zymomonas mobilis was constructed in the aim of its genetic engineering towards ethanol production.