Because several sugars are ordinarily present in the dough (mainly glucose and maltose), and others may be added (sucrose, high fructose corn syrup), the yeast has a variety of metabolic substrates from which to choose. In addition, more maltose may be formed in the dough during the fermentation step via the successive action of endogenous a- and p-amylases present or added to the flour and that act on damaged starch granules.The order in which these different carbohydrates are fermented by S. cere-visiae is not random, but rather is based on a specific hierarchy, with glucose being the preferred sugar.
For the most part, regulation is mediated by catabolite repression, acting at early steps in various catabolic pathways.Thus, in most strains of S. cerevisiae, glucose represses genes responsible for maltose transport and hydrolysis, as well as the invertase that hy-drolyzes sucrose to glucose and fructose (Figure 8-7). Consequently, in a dough containing glucose, sucrose, and maltose, the dis-accharides will be fermented only when the glucose is consumed. Moreover, maltose represses invertase expression, so sucrose
Figure 8-6. Biochemical reactions in Saccharomyces cerevisiae. Aerobic metabolism (upper panel, reaction 1) occurs via the tricarboxylic acid cycle, resulting in CO2 and cell mass, and an ATP yield of 38 moles per mole of glucose oxidized. Under anaerobic conditions (reaction 2), the Embden-Meyerhoff-Parnas glycolytic pathway yields ethanol and CO2, an ATP yield of 2 moles of ATP per mole of glucose fermented, and little cell mass. The regeneration of oxidized NAD by acetaldehyde dehydrogenase is essential (lower panel) to maintain glycolytic flux.
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