Regulation of pyruvate metabolism in Lactococcus lactis depends on the imbalance between catabolism and anabolism

Two strains of Lactococcus lactis ssp. cremoris, MG 1820 and MG 1363, which differed by the presence or absence of the lactose plasmid, respectively, were cultivated in batch-mode fermentation on lactose as carbon substrate. A correlation between the rate of sugar consumption, the growth rate, and t he type of metabolism was observed. The MG 1820 strain grew rapidly on lact ose and homolactic fermentation occurred. The major regulating factor was t he NADH/NAD(+) ratio proportional to the catabolic flux, which inhibited gl yceraldehyde-3-phosphate dehydrogenase activity. This control led to an inc rease in metabolite concentration upstream of this enzyme, glyceraldehyde-3 -phosphate and dihydroxyacetone-phosphate, and inhibition of pyruvate forma te lyase activity, while lactate dehydrogenase was strongly activated by th e high coenzyme ratio. The contrary was observed during growth of the MG 13 63 strain. Further investigation during growth of L. lactis ssp, lactis NCD O 2118 on galactose as carbon substrate and on various culture media enabli ng the growth rate to proceed at various rates demonstrated that the relati ve flux between catabolism and anabolism was the critical regulating parame ter rather than the rate of glycolysis itself. In a minimal medium, where a nabolism was strongly limited, the rate of sugar consumption was reduced to a low value to avoid carbon and energy waste. Despite this low sugar consu mption rate, the catabolic flux was in excess relative to the anabolic capa bility and the NADH/NAD(+) ratio was high, typical of a situation of nonlim iting catabolism leading to a homolactic metabolism. (C) 2001 John Wiley & Sons, Inc.