Recent progress in engineering yeast producers of cellulosic ethanol.

The production of second-generation (2 G) bioethanol, a key sector in industrial biotechnology, addresses the demand for sustainable energy by utilizing lignocellulosic biomass. Efficient fermentation of all sugars from lignocellulose hydrolysis is essential to enhance ethanol titers, improve biomass-to-biofuel yields, and lower costs. This review compares the potential of recombinant yeast strains for 2 G bioethanol production, focusing on their ability to metabolize diverse sugars, particularly xylose. Saccharomyces cerevisiae, engineered for enhanced pentose and hexose utilization, is compared with the nonconventional yeasts Scheffersomyces stipitis, Kluyveromyces marxianus, and Ogataea polymorpha. Key factors include sugar assimilation pathways, cofermentation with glucose, oxygen requirements, tolerance to hydrolysate inhibitors, and process temperature. Saccharomyces cerevisiae shows high ethanol tolerance but requires genetic modification for xylose use. Scheffersomyces stipitis ferments xylose naturally but lacks robustness. Kluyveromyces marxianus offers thermotolerance and a broad substrate range with lower ethanol yields, while O. polymorpha enables high-temperature fermentation but yields modest ethanol from xylose. The comparative analysis clarifies each yeast's advantages and limitations, supporting the development of more efficient 2 G bioethanol production strategies. Strain selection must balance ethanol yield, stress tolerance, and temperature adaptability to meet industrial requirements for cost-effective lignocellulosic bioethanol production.