Engineering of unconventional yeast Yarrowia lipolytica for efficient succinic acid production from glycerol at low pH

Yarrowia lipolytica is considered as a potential candidate for succinic acid production because of its innate ability to accumulate citric acid cycle intermediates and its tolerance to acidic pH. Previously, a succinate-production strain was obtained through the deletion of succinate dehydrogenase subunit encoding gene Ylsdh5. However, the accumulation of by-product acetate limited further improvement of succinate production. Meanwhile, additional pH adjustment procedure increased the downstream cost in industrial application. In this study, we identified for the first time that acetic acid overflow is caused by CoA-transfer reaction from acetyl-CoA to succinate in mitochondria rather than pyruvate decarboxylation reaction in SDH negative Y. lipolytica. The deletion of CoA-transferase gene Ylach eliminated acetic acid formation and improved succinic acid production and the cell growth. We then analyzed the effect of overexpressing the key enzymes of oxidative TCA, reductive carboxylation and glyoxylate bypass on succinic acid yield and by-products formation. The best strain with phosphoenolpyruvate carboxykinase (ScPCK) from Saccharomyces cerevisiae and endogenous succinyl-CoA synthase beta subunit (YlSCS2) overexpression improved succinic acid titer by 4.3-fold. In fed-batch fermentation, this strain produced 110.7 g/L succinic acid with a yield of 0.53 g/g glycerol without pH control. This is the highest succinic acid titer achieved at low pH by yeast reported worldwide, to date, using defined media. This study not only revealed the mechanism of acetic acid overflow in SDH negative Y. lipolytica, but it also reported the development of an efficient succinic acid production strain with great industrial prospects.