Expression of Calvin-cycle enzymes enables anaerobic co-fermentation of glucose and sorbitol by Saccharomyces cerevisiae

The alcoholic fermentation pathway in Saccharomyces cerevisiae is intensively used in industrial ethanol production and supports a theoretical maximum product yield of 2 moles of ethanol per mole of glucose. Theoretically, supply of additional electrons could support higher ethanol yields but this option is not allowed by the native yeast metabolic network. As a model for exploring metabolic engineering strategies to eliminate this constraint, we studied alcoholic fermentation of sorbitol. This six-carbon polyol cannot be fermented anaerobically by S. cerevisiae because its oxidation to pyruvate via glycolysis yields one more NADH than conversion of glucose. To enable reoxidation of this additional NADH by alcoholic fermentation pathway, sorbitol metabolism was studied and engineered in S. cerevisiae strains expressing the Calvin-cycle enzymes ribulose-1,5-bisphosphate carboxylase (RuBisCO) and phosphoribulokinase (PRK).