Engineering cellular redox balance in Saccharomyces cerevisiae for improved production of L-lactic acid.

Owing to the growing market for the biodegradable and renewable polymer, polylactic acid, world demand for lactic acid is rapidly increasing. However, the very high concentrations desired for industrial production of the free lactic acid create toxicity and low pH concerns for manufacturers. Saccharomyces cerevisiae is the most well characterized eukaryote, a preferred microbial cell factory for the largest industrial biotechnology product (bioethanol), and a robust, commercially compatible workhorse to be exploited for the production of diverse chemicals. S. cerevisiae has also been explored as a host for lactic acid production because of its high acid tolerance. Here, we constructed an L-lactic acid-overproducing S. cerevisiae by redirecting cellular metabolic fluxes to the production of L-lactic acid. To this end, we deleted the S. cerevisiae genes encoding pyruvate decarboxylase 1 (PDC1), L-lactate cytochrome-c oxidoreductase (CYB2), and glycerol-3-phosphate dehydrogenase (GPD1), replacing them with a heterologous L-lactate dehydrogenase (LDH) gene. Two new target genes encoding isoenzymes of the external NADH dehydrogenase (NDE1 and NDE2), were also deleted from the genome to re-engineer the intracellular redox balance. The resulting strain was found to produce L-lactic acid more efficiently (32.6% increase in final L-lactic acid titer). When tested in a bioreactor in fed-batch mode, this engineered strain produced 117 g/L of L-lactic acid under low pH conditions. This result demonstrates that the redox balance engineering should be coupled with the metabolic engineering in the construction of L-lactic acid-overproducing S. cerevisiae.