de Jong-Gubbels P, Vanrolleghem P, Heijnen S, van Dijken J P, Pronk J T
Department of Microbiology and Enzymology, Kluyver Laboratory of Biotechnology, Delft University of Technology, The Netherlands.
Yeast. 1995 Apr 30;11(5):407-18. doi: 10.1002/yea.320110503.
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.
在固定稀释率下，以葡萄糖和乙醇混合物为碳源和能源，对酿酒酵母在碳源和能源受限的恒化器培养物中的生长效率和关键酶活性调节进行了研究。基于底物碳和氧的生物量产量可以充分描述为在单一底物上生长的净结果。在葡萄糖受限培养物的无细胞提取物中未检测到异柠檬酸裂解酶和苹果酸合酶的活性。然而，当储液器培养基中的乙醇含量超过理论最小值时，这两种酶均存在，超过该值时，乙醛酸循环是合成代谢反应所必需的。仅在进料中乙醇含量较高时，当该酶的活性对于磷酸己糖的合成是必需时，才能检测到果糖-1,6-二磷酸酶的活性。在葡萄糖生长的培养物提取物中未检测到磷酸烯醇丙酮酸羧激酶的活性，且其活性随进料中乙醇含量的增加而增加。得出的结论是，在酿酒酵母以葡萄糖和乙醇混合物为碳源受限生长期间，优先从葡萄糖合成具有三个或更多碳原子的生物合成中间体。糖异生和乙醛酸循环关键酶的合成适应了细胞对这些中间体的需求。在进料中乙醇含量较高时，糖异生酶及其生理拮抗剂（丙酮酸激酶、丙酮酸羧化酶和磷酸果糖激酶）同时表达。如果在这些条件下防止无效循环，这主要不是通过严格控制酶的合成来实现的。
