Lactic acid production from xylose by engineered Saccharomyces cerevisiae without PDC or ADH deletion

Abstract

Production of lactic acid from renewable sugars has received growing attention as lactic acid can be used for making renewable and bio-based plastics. However, most prior studies have focused on production of lactic acid from glucose despite that cellulosic hydrolysates contain xylose as well as glucose. Microbial strains capable of fermenting both glucose and xylose into lactic acid are needed for sustainable and economic lactic acid production. In this study, we introduced a lactic acid-producing pathway into an engineered Saccharomyces cerevisiae capable of fermenting xylose. Specifically, ldhA from the fungi Rhizopus oryzae was overexpressed under the control of the PGK1 promoter through integration of the expression cassette in the chromosome. The resulting strain exhibited a high lactate dehydrogenase activity and produced lactic acid from glucose or xylose. Interestingly, we observed that the engineered strain exhibited substrate-dependent product formation. When the engineered yeast was cultured on glucose, the major fermentation product was ethanol while lactic acid was a minor product. In contrast, the engineered yeast produced lactic acid almost exclusively when cultured on xylose under oxygen-limited conditions. The yields of ethanol and lactic acid from glucose were 0.31 g ethanol/g glucose and 0.22 g lactic acid/g glucose, respectively. On xylose, the yields of ethanol and lactic acid were <0.01 g ethanol/g xylose and 0.69 g lactic acid/g xylose, respectively. These results demonstrate that lactic acid can be produced from xylose with a high yield by S. cerevisiae without deleting pyruvate decarboxylase, and the formation patterns of fermentations can be altered by substrates.

Xylose assimilation enhances the production of isobutanol in engineered Saccharomyces cerevisiae

“…Considering that the isobutanol production pathway in SR8-Iso is compartmentalized in the mitochondria, it is likely that increased mitochondrial biogenesis is a major contributor to the enhanced isobutanol yields from xylose we observe. However, other physiological changes induced by xylose that could divert flux from ethanol to isobutanol production may still be taking place, which would be consistent with several examples where engineered biosynthetic pathways are enhanced by xylose utilization without involving their compartmentalization in mitochondria (S. K. Kim, Jo, Park, Jin, & Seo, 2017;Koivistoinen et al, 2013;Kwak, Kim et al, 2017;Salusjärvi et al, 2017;Turner et al, 2015). Future research to elucidate what physiological changes brought by xylose are responsible for this product biosynthesis enhancement may provide new insights to engineer strains with improved bioconversion of glucose into isobutanol and other nonethanol products.…”

Production of fuels and chemicals from xylose by engineered Saccharomyces cerevisiae: a review and perspective

“…Instead of genetic perturbations for diminishing ethanol production, Turner et al introduced LDH into an efficient xylose-fermenting S. cerevisiae . They demonstrated that the engineered S. cerevisiae efficiently produces lactic acid without detectable ethanol accumulation on xylose (Y lactate/xylose  = 0.69 g/g xylose, Y ethanol/xylose  < 0.01 g/g xylose), while the engineered yeast produced lactic acid and ethanol simultaneously at 2:3 ratios on glucose (Y lactate/xylose  = 0.22 g/g glucose, Y ethanol/xylose  = 0.31 g/g glucose) [12]. Weaker metabolic activity of glycolysis on xylose [126–128] is a probable interpretation of this phenomenon.…”

“…LDH competes with PDC for pyruvate which is the final product of glycolysis as well as the branch point metabolite between lactate and ethanol pathways. As LDH has smaller K M value than PDC [12], the inefficient glycolytic metabolism under xylose culture conditions would allow LDH to direct more metabolic flux from pyruvate than endogenous PDC. Additionally, the expression level of JEN1 , a lactate-proton symporter coding gene, is upregulated under xylose through dysregulation of glucose-dependent repression [130].…”

“…In this review, therefore, we focus more on the utilization of xylose as a sole carbon source by engineered yeast, and intend to discuss recent advances in metabolic engineering studies to overcome limitations on xylose fermentation by engineered yeast. Additionally, we demonstrate distinct features in metabolic physiology during xylose fermentation by engineered yeast, and potential benefits of xylose as a carbon source to produce other valuable fuels and chemicals instead of ethanol [11, 12]. …”

Lactobacillus pentosus CECT 4023 T co‐utilizes glucose and xylose to produce lactic acid from wheat straw hydrolysate: Anaerobiosis as a key factor

“…For instance, in the case of LA production, the isolated strain Enterococcus mundtii QU 25 was engineered to produce LA from glucose: xylose mixtures . An efficient LA‐producing S. cerevisiae strain was also developed when the LA pathway was introduced in a strain able to metabolize xylose . Furthermore, by the introduction of a xylose‐metabolism pathway and further adaptive evolution, an efficient LA producer Pediococcus strain was obtained …”