1. The effect of extracellular pH upon the regulation of aerobic glucose metabolism in resting cells of Saccharomyces cerevisiae was investigated using stationary phase cells suspended in sodium phosphate buffers at pH 2.0, pH 4.5 and pH 7.5. 2. It was found that the activity of phosphofructokinase (EC 184.108.40.206) varied with the pH of the environment being higher in cells suspended at pH 2.0 and pH 4.5 than at pH 7.5. Although the possibility of changes in the intracellular pH causing this effect could not be excluded completely, the differences in the activity of phosphofructokinase, between the three pH values, could be correlated with differences in the intracellular content of adenine nucleotides and citric acid. In addition to this effect upon phosphofructokinase, at pH 2.0 the activity of the glucose transport system was reduced by extracellular pH. 3. At pH 2.0 the combination of the high activity of phosphofructokinase and low activity of the glucose transport system reduced substrate supply to the polysaccharide synthesising system, so that more glucose was degraded via glycolysis and less synthesised to polysaccharide than at pH 4.5 and pH 7.5. At pH 7.5 compared to pH 4.5, a lower activity of phosphofructokinase reduced the amount of glucose which was degraded via glycolysis, and increased the amount synthesised to polysaccharide. 4. In vivo evidence was obtained that, during glucose utilisation, the activity of pyruvate kinase (EC 220.127.116.11) may be regulated by the intracellular content of fructose-1,6-diphosphate, which in turn was regulated by the activity of phosphofructokinase. From experiments with cells suspended at pH 4.5 and pH 7.5 in vivo evidence was obtained to support the following regulatory functions of phosphofructokinase in the aerobic glucose metabolism of resting cells of Saccharomyces cerevisiae. a) That it was the rate controlling enzyme within the glycolysis system and therefore regulated the rate of glycolysis. b) That via the intracellular content of hexose monophosphate it regulated the activity of the glucose transport system and hence the rate of glucose uptake. c) That via the activating effect of fructose-1,6-diphosphate upon pyruvate kinase it co-ordinated the activity of enzymes within the glycolysis system. d) That via alterations in the activity of the glycolysis system it altered the relative activities of the glycolysis and polysaccharide synthesising systems and hence controlled the metabolic fate of incorporated glucose.
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