Tuesday, May 3, 2011
In order to achieve efficient fermentation of optically pure D-lactic acid from pentose, we carried out redirection of the phosphoketolase pathway (PK pathway) to the pentose phosphate pathway (PP pathway) in L-lactate dehydrogenase gene (ldhL1)-deficient Lactobacillus plantarum (L. plantarum ΔldhL1). The PK pathway produces both lactic acid and acetic acid, but the PP pathway produces only lactic acid as a final product. A strain substituting xpk1 and xpk2 with the heterologous transketolase gene (tkt) from Lactococcus lactis IL 1403 was constructed. The resulting strain, L. plantarum ΔldhL1-xpk1::tkt-Δxpk2, was capable of growing with arabinose and predominantly produced lactic acid. After 27 h of fermentation, 38.6 g/l of lactic acid was produced from arabinose with high yield (0.82 g per g of consumed sugar) and the optical purity of D-lactic acid was 99.9%. Then a plasmid carrying the xylAB operon from Lactobacillus pentosus (pCU-PXylAB) was introduced into L. plantarum ΔldhL1-xpk1::tkt -Δxpk2. Using L. plantarum ΔldhL1-xpk1::tkt -Δxpk2 strain harboring pCU-PXylAB, successful homo D-lactic acid production was achieved and 41.2 g/l of lactic acid was produced from xylose with a high yield (0.89 g per g of consumed sugar) and an optical purity of D-lactic acid of 99.2%. Additionally, we are trying to express xylosidase and xylanase using L. plantarum ΔldhL1-xpk1::tkt -Δxpk2 as a host, for direct fermentation from xylan.