Two different pathways for D-xylose metabolism and the effect of xylose concentration on the yield coefficient of L-lactate in mixed-acid fermentation by the lactic acid bacterium Lactococcus lactis 10-1

In lactic acid bacteria, pentoses are metabolized via the phosphoketolase pathway, which catalyzes the cleavage of D-xylulose-5-phosphate to equimolar amounts of glyceraldehyde 3-phosphate and acetylphosphate. Hence the yield coefficient of lactate from pentose does not exceed 1.0 mol/mol, while that of Lactococcus lactis IO-1(JCM7638) at high D-xylose concentrations often exceeds the theoretical value. This suggests that, in addition to the phosphoketolase pathway, L. lactis IO-1 may possess another metabolic pathway that produces only lactic acid from xylose. In the present study, the metabolism of xylose in L. lactis IO-1 was deduced from the product formation and enzyme activities of L. lactis IO-1 in batch culture and continuous culture. During cultivation with xylose concentrations above ca. 50 g/l, the yield coefficient Of L-lactate exceeded 1.0 mol/mol while those of acetate, formate and ethanol were very low. At xylose concentrations less than 5 g/l, acetate, formate and ethanol were produced with yield coefficients of about 1.0 mol/mol, while L-lactate was scarcely produced. In cells grown at high xylose concentrations, a marked decrease in the specific activities of phosphoketolase and pyruvate formate lyase (PFL), and an increase in those of transketolase and transaldolase were ob-served. These results indicate that in L. lactis IO-1 xylose may be catabolized by two different pathways, the phosphoketolase pathway yielding acetate, formate and ethanol, and the pentose phosphate (PP)/glycolytic pathway which converts xylose to L-lactate only. Furthermore, it was deduced that the change in the xylose concentration in the culture medium shifts xylulose 5-phosphate metabolism between the phosphoketolase pathway and the PP/glycolytic pathway in L. lactis IO-1, and pyruvate metabolism between cleavage to acetyl-CoA and formic acid by PFL and the reduction to L-lactate by lactate dehydrogenase.