Changes in the concentration of cAMP, fructose 2,6‐bisphosphate and related metabolites and enzymes in Saccharomyces cerevisiae during growth on glucose

Abstract

Changes in the concentration of several metabolites and enzymes related to carbohydrate metabolism were measured during the growth of Saccharomyces cerevisiae on a mineral medium containing glucose as the limiting nutrient. When about 50% of the original glucose was used the exponential phase ended and the culture entered a 'transition' phase before the complete exhaustion of glucose. In this transition phase several metabolic changes occurred. CAMP, that decreased along growth, reached a constant value of about 0.7 nmol/g dry weight. A pronounced drop in fructose-6-phosphate-2-kinase activity and in the concentration of fructose 2,6-bisphosphate and fructose 1,6-bisphosphate was observed accompanied by a less marked decrease in hexose monophosphates. Trehalase activity also dropped and reached a minimal value at the onset of the stationary phase when synthesis of trehalose began. Glycogen concentration and glycogen synthase activity increased sharply during the transition phase. Plasma membrane ATPase began to increase at the middle of the exponential phase and then, coincident with the glucose exhaustion, a 90% decrease in the measurable activity was observed.Growth of Saccharomyces cerevisiae on glucose elicits a well-known pattern of enzymes and metabolites [l, 21. In the last years some observations have been reported that suggest that this pattern may not be constant during growth. For example a decrease in RNA and protein synthesis is observed well in advance of glucose exhaustion [3]. Also glycogen accumulation begins when about half of the original glucose remains in the culture irrespective of the initial concentration of the sugar [4].Recently cAMP and fructose 2,6-bisphosphate [Fru(2,6)-P2] have received considerable attention as possible 'organizers' of carbohydrate metabolism in yeast [5, 61. In order to provide more experimental data to disprove or support this idea we initiated a systematic investigation of the concentrations of these compounds and some other metabolites and enzymes related to carbohydrate metabolism during growth of yeast on glucose. The results of our study show that exponential growth ceases when glucose is still present in the culture and that important metabolic changes occur in the transition phase between the exponential and stationary phase of growth. MATERIALS AND METHODS Growth conditionsSaccharomyces cerevisiae X2180 was grown at 30°C in a mineral medium [7] using NaCl instead of the original sodium citrate. The pH of the medium was adjusted to 5.5 with KOH. Glucose was added after sterilization at a final concentration of 2%. The media were inoculated with yeasts grown to the stationary phase on a medium containing 2% glucose, 1% yeast extract and 1 YO bactopeptone. The initial cell concentration was about lo5 cells/ml. Growth was followed by measuring the turbidity of the culture at 660 nm and the corresponding dry weight was calculated from a calibration curve of dry weight versus absorbance (Fig. 1). This curve was constructed in the same conditions as the subseque...

Glycogen hyperaccumulation in Saccharomyces cerevisiae ras2 mutant A biochemical study

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