ENZYMATIC AND GENETIC-REGULATION OF GLYCOLYSIS IN LACTOBACILLUS-DELBRUECKII SUBSP BULGARICUS

In Lactobacillus bulgaricus the genes coding for phosphofructokinase a nd pyruvate kinase are clustered into one operon, although the reactio ns catalyzed by these two enzymes are seven steps apart along glycolys is. The translation of the unique messenger RNA should yield equivalen t amounts of these two proteins, but their allosteric properties allow an independent control of the levels of their enzymatic activities. T his enzymatic regulation involves a 'loop' in which the substrate of o ne enzyme is an allosteric effector of the other: fructose-6-phosphate , the substrate of phosphofructokinase, is an activator of pyruvate ki nase, and phospho-enol-pyruvate, the substrate of pyruvate kinase, is an inhibitor of phosphofructokinase. Such a metabolic control, with an ante-activation by fructose-6-phosphate and a feed-back inhibition by phospho-enolpyruvate, contributes to the damping of changes in the gl ycolytic flux. In L. bulgaricus, the two key enzymes phosphofructokina se and pyruvate kinase are therefore coupled at the level of both thei r coordinated biosynthesis and the mutual control of their activities, which suggests that they are indeed involved in regulating the yield of D-lactate in fermentation. (C) Inra/Elsevier, Paris.