Engineered Microbial Strains for Co-fermentation of C5 and C6 Sugars to Produce Cellulosic Ethanol.

Efficient fermentation of both hexose (C6) and pentose (C5) sugars derived from lignocellulosic biomass is essential for the sustainable production of cellulosic ethanol. However, conventional yeast Saccharomyces cerevisiae lacks native pathways for pentose utilization and is susceptible to various environmental stresses caused by hydrolysate-rich fermentation conditions. This chapter outlines the metabolic engineering strategies for introducing and optimizing C5 pathways alongside native C6 metabolism. It highlights advances in recombinant strain development, including synthetic pathway design, cofactor balancing, and transporter engineering. We also review approaches for consolidated bioprocessing that improve overall productivity by combining saccharification and fermentation in a single batch. These approaches include the secretion or cell surface display of cellulases and hemicellulases and optimizing their expression levels. Furthermore, this chapter introduces tolerance mechanisms against stresses derived from fermentation conditions including high temperature, osmolarity, and ethanol, as well as lignocellulose hydrolysate-derived inhibitors such as acetic acid and furfural. By connecting innovations in metabolic design and stress response, the chapter provides a comprehensive view of the current progress and future directions in developing yeast strains to co-ferment C5/C6 sugars under industrially practical conditions.