Lactic Acid Production from Xylose 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 industrially-produced by microbial fermentation using glucose as a substrate. As an alternative to glucose, xylose which is easily obtainable from lignocellulosic biomass under mild pretreatment conditions without excessive use of cellulase enzymes can be utilized for producing chemicals. In order to produce lactic acid from xylose, we introduced both a xylose-utilizing pathway from Scheffersomyces stipitis and 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 xylose-utilizing engineered S. cerevisiae was able to produce lactic acid with trace 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 xylose between pH 4.0 and 5.0, in either minimal or complex media. Notably, the engineered S. cerevisiae produced negligible amounts of xylitol and exhibited high yields of lactic acid production from xylose (up to 0.7 g lactate/g xylose). These results suggest that further studies involving scale-up of these results and improvement of tolerance will allow for usage of renewable lignocellulosic feedstocks for efficient and sustainable production of lactic acid.