Lactic acid production from lignocellulosic sugars by engineered Saccharomyces cerevisiae

As global climate change concerns escalate, development of carbon-neutral methods to produce industrially-relevant chemicals from non-food, non-fossil fuel sources has become increasingly important. Lactic acid, primarily used for synthesis of the biodegradable polylactic acid (PLA) or as a feedstock for 3D printing, is often industrially-produced by microbial fermentation using glucose as a substrate. As an alternative to glucose, cellobiose which is obtainable from lignocellulosic biomass under mild pretreatment conditions, can be utilized for producing chemicals. To produce lactic acid from cellobiose, we introduced a cellodextrin transporter (cdt1-1) and an intracellular β-glucosidase (gh1-1) both from Neurospora crassa as well as a lactate dehydrogenase from Rhizopus oryzae into Saccharomyces cerevisiae. It is well-known that deletion of genes (PDC1, PDC5, and PDC6) coding for pyruvate decarboxylase was necessary for producing lactic acid without ethanol production from glucose. However, the cellobiose-utilizing engineered S. cerevisiae was able to produce lactic acid with negligible amounts of ethanol production even though PDC1, PDC5, and PDC6 were not deleted. As a result, the engineered strain produced lactic acid at high titers from cellobiose in either minimal or complex media. Notably, the engineered S. cerevisiae produced negligible amounts of non-ethanol byproducts while exhibiting high yields of lactic acid production from cellobiose (up to 0.70 g lactic acid/g cellobiose). These results suggest that further studies involving scale-up of these experiments and improvement of tolerance will allow for usage of renewable lignocellulosic feedstocks for efficient and sustainable production of lactic acid.