From mannan to bioethanol: cell surface co-display of β-mannanase and β-mannosidase on yeast Saccharomyces cerevisiae

Background: Mannans represent the largest hemicellulosic fraction in softwoods and also serve as carbohydrate stores in various plants. However, the utilization of mannans as sustainable resources has been less advanced in sustainable biofuel development. Based on a yeast cell surface-display technology that enables the immobilization of multiple enzymes on the yeast cell walls, we constructed a recombinant Saccharomyces cerevisiae strain that co-displays beta-mannanase and beta-mannosidase; this strain is expected to facilitate ethanol fermentation using mannan as a biomass source. Results: Parental yeast S. cerevisiae assimilated mannose and glucose as monomeric sugars, producing ethanol from mannose. We constructed yeast strains that express tethered beta-mannanase and beta-mannosidase; co-display of the two enzymes on the cell surface was confirmed by immunofluorescence staining and enzyme activity assays. The constructed yeast cells successfully hydrolyzed 1,4-beta-D-mannan and produced ethanol by assimilating the resulting mannose without external addition of enzymes. Furthermore, the constructed strain produced ethanol from 1,4-beta-D-mannan continually during the third batch of repeated fermentation. Additionally, the constructed strain produced ethanol from ivory nut mannan; ethanol yield was improved by NaOH pretreatment of the substrate. Conclusions: We successfully displayed beta-mannanase and beta-mannosidase on the yeast cell surface. Our results clearly demonstrate the utility of the strain co-displaying beta-mannanase and beta-mannosidase for ethanol fermentation from mannan biomass. Thus, co-tethering beta-mannanase and beta-mannosidase on the yeast cell surface provides a powerful platform technology for yeast fermentation toward the production of bioethanol and other biochemicals from lignocellulosic materials containing mannan components.