l-(+)-Lactic acid production from the heterolactic PM6-7A(pEPL2) transformed strain. Batch fermentation was carried out on yeast nitrogen base medium containing 50 g of glucose liter⁻¹ buffered at T = 0 to pH 5.5, as described in Materials and Methods. Production of d-(−)-lactic acid was not detected. The LDH-specific activity was 4 to 5 U (mg of total cell proteins)⁻¹ during the entire experiment. Final pH value was 3. ■, cells per milliliter; ○, ethanol (ET) production, grams per liter; ●, l-(+)-lactic acid (LA) production, grams per liter; and ▵, percent (wt/vol) residual glucose (Glu).

l-(+)-Lactic acid production from the homolactic PMI/C1(pEPL2) transformed strain in a bioreactor. The fermentation pH was controlled at 4.5 throughout the fermentation. Production of lactic acid (LA) (●, grams per liter), residual glucose (Glu) (▵, grams per liter), and optical density of the biomass at 660 nm (OD₆₆₀) (○) are shown.

Schematic representation of the main pyruvate dissimilation pathways in K. lactis. EMP, Embden-Meyerhof pathway; HMP, hexose-monophosphate pathway; TCA, tricarboxylic acid cycle. Key enzymatic reactions at the pyruvate branch point are catalyzed by the following enzymes: ➊, pyruvate decarboxylase (EC 4.1.1.1; ⊗ indicates that this activity is absent in the K. lactic strain PMI/C1); ➋, alcohol dehydrogenase (EC 1.1.1.1); ➌, acetaldehyde dehydrogenase (EC 1.2.1.4 or EC 1.2.1.5); ➍, acetyl coenzyme A (CoA) synthetase (EC 6.2.1.1); ➎, acetyl CoA shuttle from the cytosol to mitochondria; ❻, acetyl CoA shuttle from mitochondria to the cytosol; and ❼, heterologous l-(+)-lactate dehydrogenase (EC 1.1.1.27). Enzymatic reactions involved in anaplerotic syntheses have been omitted. Black arrows indicate the metabolic pathway leading to the production of lactic acid from glucose in heterolactic and homolactic strains PM6-7A(pEPL2) and PMI/C1(pEPL2), respectively. A schematic representation of the expression cassette [KlPDC1 promoter and l-(+) LDH gene] on the plasmid pEPL2 is also shown.

l-(+)-Lactic acid production from the homolactic PMI/C1(pEPL2) transformed strain. Batch fermentation was carried out as described in Fig. 2. The LDH-specific activity was 55 to 60 U (mg of total cell proteins)⁻¹ during the entire experiment. Final pH value was 2.95. ■, cells per milliliter; ○, ethanol (ET) production, grams per liter; ●, l-(+)-lactic acid (LA) production, grams per liter; and ▵, percent (wt/vol) residual glucose (Glu).