Toward “homolactic” fermentation of glucose and xylose by engineered Saccharomyces cerevisiae harboring a kinetically efficient l‐lactate dehydrogenase within pdc1‐pdc5 deletion background

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ABSTRACT l ‐Lactic acid is an important platform chemical and its production from the lignocellulosic sugars glucose and xylose is, therefore, of high interest. Tolerance to low pH and a generally high robustness make Saccharomyces cerevisiae a promising host for l ‐lactic acid fermentation but strain development for effective utilization of both sugars is an unsolved problem. The herein used S. cerevisiae strain IBB10B05 incorporates a NADH‐dependent pathway for oxidoreductive xylose assimilation within CEN.PK113‐7D background and was additionally evolved for accelerated xylose‐to‐ethanol fermentation. Selecting the Plasmodium falciparum l ‐lactate dehydrogenase ( pf LDH) for its high kinetic efficiency, strain IBB14LA1 was derived from IBB10B05 by placing the pfldh gene at the pdc1 locus under control of the pdc1 promotor. Strain IBB14LA1_5 additionally had the pdc5 gene disrupted. With both strains, continued l ‐lactic acid formation from glucose or xylose, each at 50 g/L, necessitated stabilization of pH. Using calcium carbonate (11 g/L), anaerobic shaken bottle fermentations at pH ≥ 5 resulted in l ‐lactic acid yields ( Y LA ) of 0.67 g/g glucose and 0.80 g/g xylose for strain IBB14LA1_5. Only little xylitol was formed (≤0.08 g/g) and no ethanol. In pH stabilized aerobic conversions of glucose, strain IBB14LA1_5 further showed excellent l ‐lactic acid productivities (1.8 g/L/h) without losses in Y LA (0.69 g/g glucose). In strain IBB14LA1, the Y LA was lower (≤0.18 g/g glucose; ≤0.27 g/g xylose) due to ethanol as well as xylitol formation. Therefore, this study shows that a S. cerevisiae strain originally optimized for xylose‐to‐ethanol fermentation was useful to implement l ‐lactic acid production from glucose and xylose; and with the metabolic engineering strategy applied, advance toward homolactic fermentation of both sugars was made. Biotechnol. Bioeng. 2017;114: 163–171.