Case 27
Regulation of Sugar and Alcohol Metabolism in
Saccharomyces cerevisiae
Focus concept
The regulation of carbohydrate metabolic pathways in yeast serves as a good model for regulation of
the same pathways in multicellular organisms.
Prerequisites
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The major pathways associated with carbohydrate metabolism, including glycolysis, the citric
acid cycle, oxidative phosphorylation, pentose phosphate pathway and gluconeogenesis.
The various fates of pyruvate via alcoholic fermentation and aerobic respiration.
Background
The simple one-celled eukaryotic organism Saccharomyces cerevisiae (yeast) has been well studied
because extensive research has shown that the metabolic pathways in the yeast are subject to the same
types of controls as cells of higher organisms. Yeast are also easy to culture in a laboratory setting. The
yeast organism has been employed for literally thousands of years in human history in the production of
leavened bread and alcoholic beverages. To the biochemist, the yeast is important because of the
Buchner’s experiments which disproved the theory of vitalism by demonstrating that metabolic
pathways could still occur in cell-free yeast extracts. Their pioneering work allowed scientists to study
the metabolic pathways in yeast in great detail. Once pathways had been elucidated, scientists turned
their attention to the various mechanisms that served to regulate the pathways. These studies have been
carried out with yeast mutants in which a single enzyme associated with a specific pathway is deficient
or nonfunctional. Observing the phenotypic character of the mutants allowed scientists to pinpoint the
mutated enzyme and increased our understanding of yeast metabolic pathways. In this case, we will
focus on carbohydrate metabolism in yeast.
Questions
1. Yeast is used in the production of alcoholic beverages.
a. Describe how the sugar in the grape (mainly fructose) is converted to ethanol by the yeast cell.
b. Explain why concentrations of acetate often rise during alcoholic fermentation.
2.It has been observed that cells lacking alcohol dehydrogenase accumulate large amounts of glycerol
during anaerobic fermentation. Explain why.
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3. Yeast are unusual in that they are able to use ethanol as a gluconeogenic substrate. Ethanol is
converted to glucose using the assistance of the glyoxylate pathway. Describe how the ethanol →
glucose conversion takes place.
4. A yeast mutant was isolated that was deficient in the enzyme phosphofructokinase. The mutant yeast
was able to grow on glycerol as an energy source, but not glucose. Explain why.
5. Yeast is used as a leavening agent in making bread. Explain, in biochemical terms, why the bread
dough rises when placed in a warm place.
6. Yeast cells grown on nonfermentable substrates which are then abruptly switched to glucose exhibit
substrate-induced inactivation of several enzymes. Which enzymes would glucose cause to be
inactivated, and why?
7. During anaerobic fermentation, the majority of the available glucose is oxidized via the glycolytic
pathway and the rest enters the pentose phosphate pathway to generate NADPH and ribose. This
occurs during aerobic respiration as well, except that the percentage of glucose entering the pentose
phosphate pathway is much greater in aerobic respiration than during anaerobic fermentation.
Explain why.
Reference
Wills, C. (1990) Critical Reviews in Biochemistry and Molecular Biology, 25, pp. 245-280.
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