Overcoming glucose repression through cellobiose fermentation enhances bioconversion of pectin-rich biomass in...

Pectin-rich biomass is a promising substrate for microbial fermentation due to its high fermentable sugar content. However, simultaneous utilization of galacturonic acid with other sugars is limited by low pH and glucose repression. In this study, Saccharomyces cerevisiae was engineered to co-ferment cellobiose, xylose, and galacturonic acid under acidic conditions without pH adjustment. Adaptive laboratory evolution yielded a strain (DH1) carrying two mutations (M128I and L167H) in the cellodextrin transporter, enhancing cellobiose uptake and tolerance to galacturonic acid at pH 3.5. DH1 showed 6-fold higher cellobiose consumption and approximately two-fold improved galacturonic acid utilization. Replacing glucose with cellobiose alleviated glucose repression, improved intracellular NADPH availability, and enabled efficient co-utilization of mixed sugars. When engineered for lactic acid production, DH1 achieved a titer of 38.2 g/L, a 46.4% increase over the glucose-fed control. These findings highlight a robust microbial platform for converting pectin-rich waste into value-added chemicals under industrially relevant low-pH conditions.