Growth efficiency and regulation of key enzyme activities were studied in carbon- and energy-limited chemostat cultures of Saccharomyces cerevisiae grown on mixtures of glucose and ethanol at a fixed dilution rate. Biomass yields on substrate carbon and oxygen could be adequately described as the net result of growth on the single substrates. Activities of isocitrate lyase and malate synthase were not detected in cell-free extracts of glucose-limited cultures. However, both enzymes were present when the ethanol fraction in the reservoir medium exceeded the theoretical minimum above which the glyoxylate cycle is required for anabolic reactions. Fructose-1,6-bisphosphatase activity was only detectable at high ethanol fractions in the feed, when activity of this enzyme was required for synthesis of hexose phosphates. Phospho-enol-pyruvate-carboxykinase activity was not detectable in extracts from glucose-grown cultures and increased with the ethanol fraction in the feed. It is concluded that, during carbon-limited growth of S. cerevisiae on mixtures of glucose and ethanol, biosynthetic intermediates with three or more carbon atoms are preferentially synthesized from glucose. Synthesis of the key enzymes of gluconeogenesis and the glyoxylate cycle is adapted to the cells′ requirement for these intermediates. The gluconeogenic enzymes and their physiological antagonists (pyruvate kinase, pyruvate carboxylase and phosphofructokinase) were expressed simultaneously at high ethanol fractions in the feed. If futile cycling is prevented under these conditions, this is not primarily achieved by tight control of enzyme synthesis.
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