Efficient production of L-lactic acid from xylose by metabolically engineered yeast Pichia stipitis

Pichia stipitis, a yeast which ferments naturally xylose, was genetically engineered for L-(+)-lactate production. A P. stipitis strain expressing the L-lactate dehydrogenase (LDH) from Lactobacillus helveticus under the control of the P. stipitis fermentative ADH1 promoter was constructed. Either xylose or glucose was used as the carbon source for lactate production under oxygen restricted conditions. Remarkably, the constructed P. stipitis strain produced a higher lactate concentration and yield on xylose than on glucose. Lactate accumulated as the main product on xylose-containing medium: 58 g/l lactate was produced from 100 g/l xylose. Relatively efficient lactate production was also observed on glucose medium, 41 g/l lactate was produced at a yield of 0.44 g/g glucose. Lactate was produced at the expense of ethanol production which was decreased to approximately 20% of the wild type levels on xylose-containing medium and to 75% on glucose-containing medium. Thus, LDH competed efficiently with the ethanol pathway for pyruvate, even though the pathway from pyruvate to ethanol was intact.