Xylo-Oligosaccharide Utilization by Engineered Saccharomyces cerevisiae to Produce Ethanol

Abstract

The engineering of xylo-oligosaccharide-consuming Saccharomyces cerevisiae strains is a promising approach for more effective utilization of lignocellulosic biomass and the development of economic industrial fermentation processes. Extending the sugar consumption range without catabolite repression by including the metabolism of oligomers instead of only monomers would significantly improve second-generation ethanol production This review focuses on different aspects of the action mechanisms of xylan-degrading enzymes from bacteria and fungi, and their insertion in S. cerevisiae strains to obtain microbial cell factories able of consume these complex sugars and convert them to ethanol. Emphasis is given to different strategies for ethanol production from both extracellular and intracellular xylo-oligosaccharide utilization by S. cerevisiae strains. The suitability of S. cerevisiae for ethanol production combined with its genetic tractability indicates that it can play an important role in xylan bioconversion through the heterologous expression of xylanases from other microorganisms.

Keywords: Saccharomyces cerevisiae; lignocellulosic ethanol; xylanases; xylo-oligosaccharides; xylose.

FIGURE 1

Schematic overview of the xylose degradation pathway associated with the central carbon metabolism in yeast. The orange box indicates the redox-cofactor-independent xylose isomerase (XI), red boxes indicate the pyridine-nucleotide-dependent xylose reductase (XR) and xylitol dehydrogenase (XDH). Purple boxes indicate the five reaction steps of the Weimberg pathway catalyzed by xylose dehydrogenase (XylB), xylonolactonase (XylC), xylonate dehydratase (XylD), 3-keto-2deoxy-xylonate dehydratase (XylX), and α-ketoglutarate semialdehyde dehydrogenase (XylA). Abbreviations: PPP—pentose phosphate pathway, TCA—tricarboxylic acid cycle, G3P—glyceraldehyde 3-phosphate, DHAP—dihydroxyacetone phosphate. Adapted from Borgström et al. (2019).

FIGURE 2

Xylan degradation scheme. The arrows represent each enzyme activity for a determined substrate. Adapted from Bhardwaj et al. (2019).