From wheat straw to xylitol: Bioconversion by a genetically engineered Candida guilliermondii strain

Wheat straw is a major crop in Europe and represents a promising raw material for the production of high value products. One of these products is xylitol, a sugar alcohol commonly used as a sweetener in food and also pharmaceuticals. In order to produce xylitol from wheat, two essential processes play a crucial role: 1) biomass pretreatment and 2) microbial fermentation. Biomass pretreatment (e. g. Steam Explosion) is necessary to obtain a sugar solution for the microbial fermentation. This process also releases metabolic inhibitors that drastically reduce the efficiency of the subsequent fermentation step. In our laboratory, we aim to use the yeast Candida guilliermondii for the production of xylitol in a natural manner. This is considered as a sustainable production platform to produce xylitol in an environmentally friendly manner. In this work, we engineered the xylitol metabolism of C. guilliermondii by modulating the activity of the two main enzymes involved in this process, xylose reductase and xylitol dehydrogenase. Our results suggest that the described strategy is successful in modulating the activity of these enzymes and the xylitol metabolism. Additionally, we provide a descriptive analysis of C. guilliermondii tolerance towards the main growth inhibitors released during the pretreatment of wheat straw, which will facilitate the optimization of the whole process. This work was financed by the European Union Program ‘IWB/EFRE-Regionalprogramm 2014-2020’ (Project name: Combined Agro-Forest Biorefinery – CAFB) funds of the European Regional Development Fund (ERDF) and the Federal State of Upper Austria.