Advances, challenges, and opportunities in engineering C5 and C6 sugar transporters in yeast for bio-based industrial biotechnology.

Harnessing biomass for bio-based industrial biotechnology is vital for addressing global energy needs and mitigating climate change. In this context, microorganisms are the cornerstone of biorefineries based on renewable materials, with applications in bioenergy, agriculture, biomedicine, and other sectors. By engineering metabolic pathways, microorganisms can be tailored to improve yields, tolerate industrial conditions, and selectively produce valuable compounds. Through advances in metabolic engineering and synthetic biology, engineered strains of the yeast Saccharomyces cerevisiae have been successfully developed to efficiently convert the pentose sugars D-xylose and L-arabinose. Despite this important breakthrough, the efficient transport of these sugars remains a major limitation. Sugar sensing and transport in yeast are regulated at both transcriptional and post-translational levels. D-xylose is not recognized as a fermentable carbon source, leading to downregulation of transporter expression, removal from the cytoplasmic membrane, and degradation via ubiquitination in the absence of extracellular glucose. Additionally, transporters exhibit lower affinity for C5 sugars compared to D-glucose, resulting in strong D-glucose repression. To address these challenges, cutting-edge strategies have been successfully employed, including rational protein engineering, directed evolution, and machine learning approaches, to expand the repertoire of C5 transporters available for engineering in S. cerevisiae. Specific D-xylose transporters have been redesigned, with key residues identified to reduce D-glucose affinity, while studies have demonstrated improvements in transporter stability and sugar uptake rates. This review summarizes the key bottlenecks in C5 sugar transport and highlights the major advances and progress made toward creating robust microbial platforms capable of sustainable and efficient bio-based production.