Search

Interest in the production of L-(+)-lactic acid is presently growing in relation to its applications in the synthesis of biodegradable polymer materials. With the aim of obtaining efficient production and high productivity, we introduced the bovine L-lactate dehydrogenase gene (LDH) into a wild-type Kluyveromyces lactis yeast strain. The observed lactic acid production was not satisfactory due to the continued coproduction of ethanol. A further restructuring of the cellular metabolism was obtained by introducing the LDH gene into a K. lactis strain in which the unique pyruvate decarboxylase gene had been deleted. With this modified strain, in which lactic fermentation substituted completely for the pathway leading to the production of ethanol, we obtained concentrations, productivities, and yields of lactic acid as high as 109 g liter(-1), 0.91 g liter(-1) h(-1), and 1.19 mol per mole of glucose consumed, respectively. The organic acid was also produced at pH levels lower than those usual for bacterial processes.

Introduction of the Lactobacillus casei lactate dehydrogenase (LDH) gene into Saccharomyces cerevisiae under the control of the TPI1 promoter yielded high LDH levels in batch and chemostat cultures. LDH expression did not affect the dilution rate above which respiro-fermentative metabolism occurred (Dc) in aerobic, glucose-limited chemostats. Above Dc, the LDH-expressing strain produced both ethanol and lactate, but its overall fermentation rate was the same as in wild-type cultures. Exposure of respiring, LDH-expressing cultures to glucose excess triggered simultaneous ethanol and lactate production. However, the specific glucose consumption rate was not affected, indicating that NADH reoxidation does not control glycolytic flux under these conditions.

Interesting challenges from metabolically engineered Saccharomyces cerevisiae cells arise from the opportunity to obtain yeast strains useful for the production of chemical(s). In this paper, we describe the accumulation of lactic acid in the culture medium of growing, engineered yeast cells expressing a mammalian lactate dehydrogenase gene (LDH-A). High and reproducible productions (20 g/L) and productivities (up to 11 g/L/h) of lactic acid have been obtained by modulating the physiological growth conditions. Since yeast cells are acid tolerant and survive at very low pH values, the production of lactate can be avoided. In perspective, the approaches described could be useful for the production of lactic acid, outflanking the problems related to the synthesis from bacteria cells. In fact, during industrial productions, there is an inhibitory effect on the metabolic activities of the growing bacteria (i.e., Lactobacillus spp.) caused by the acid produced and by the low pH value. Thus, strategies to prevent the lowering of pH are conventional operations. These processes allow the production of lactate(s) and require the purification of the acid from its salt. The biotechnological implications of this study are also discussed.