Metabolic engineering of threonine catabolism enables Saccharomyces cerevisiae to produce propionate under aerobic conditions

Background: Propionate is widely used as a preservative in the food and animal feed industries. Propionate is currently produced by petrochemical processes, and fermentative production of propionate remains challenging. Methods and Results: In this study, a synthetic propionate pathway was constructed in the budding yeast Saccharomyces cerevisiae, for propionate production under aerobic conditions. Through expression of tdcB and aldH from Escherichia coli and kivD from Lactococcus lactis, L-threonine was converted to propionate via 2-ketobutyrate and propionaldehyde. The resulting yeast aerobically produced 0.21 g L^-1 propionate from glucose in a shake flask. Subsequent overexpression of pathway genes and elimination of competing pathways increased propionate production to 0.37 g L^-1. To further increase propionate production, carbon flux was pulled into the propionate pathway by weakened expression of pyruvate kinase (PYK1), together with overexpression of phosphoenolpyruvate carboxylase (ppc). The final propionate production reached 1.05 g L^-1 during fed-batch fermentation in a fermenter. Conclusions and Implications: In this work, a yeast cell factory was constructed using synthetic biology and metabolic engineering strategies to enable propionate production under aerobic conditions. Our study demonstrates engineered S. cerevisiae as a promising alternative for the production of propionate and its derivatives.