S. cerevisiae has the genetic capacity to metabolize sugars via either the glycolytic or respiratory (i.e., TCA) pathways. Although oxygen availability affects expression of genes encoding enzymes of these two pathways and is, therefore, an important determinant of which way metabolism will occur, gene expression is also regulated by substrate availability. Although one might expect that in the presence of oxygen, metabolism would always be via the respiratory pathway, this is not the case. If the glucose concentration is sufficiently high, metabolism will be fermentative. This is because transcription of catabolic genes, including genes coding for some of the TCA enzymes, is repressed by glucose, a phenomenon known as the glucose or Crabtree effect. Thus, in the wine fermentation, where sugar concentrations are especially high, metabolism of glucose yields mostly ethanol and CO2, and oxida-tive metabolism of sugars is unlikely to occur. Only when the substrate concentration is low (less than 2 g/L), will O2 repression of glycoly-sis occur (the so-called Pasteur effect).
Since the wine fermentation, as noted above, is anaerobic, one might reasonably expect that the only end products formed from glucose and fructose would be ethanol and CO2. If that were the case, then how could one explain the appearance of glycerol, suc-cinic acid, acetaldehyde, acetic acid, and other products, that appear during the wine fermentation? In addition, some of the glucose carbon (albeit only about 1%) is used to form biomass (i.e., cell constituents). Moreover, even after accounting for evaporative effects, the theoretical 50% yield (on a weight basis) of ethanol from glucose is never reached during the fermentation, due to byproduct formation. These byproducts can account for as much as 4% of the total products formed. Synthesis of these byproducts occurs, in part, in response to demands on the cell to maintain Eh balance and to salvage ATP. For example, when the demand for NAD is high, a portion of the dihydroxyacetone phosphate formed from FDP via the aldolase reaction is reduced to glycerol-3-phosphate, and NAD is generated.The glycerol-3-phosphate is subsequently dephosphorylated to glycerol.
Aerobic metabolism, at least at the very start of the fermentation, may also result in products other than ethanol, in particular, TCA intermediates, such as succinic acid. Even when conditions are anaerobic, TCA products may still be formed to provide the cell with carbon skeletons (e.g., a-ketoglutarate) necessary for biosynthesis of amino acids.Another factor influencing sugar metabolism is SO2, added to control the indigenous yeast population. Sulfur dioxide can bind acetaldehyde, preventing its reduction to ethanol by alcohol dehydroge-nase.As a result, NADH accumulates inside the cells and is diverted to other NAD-generating reactions, forming glycerol and other non-ethanol end products. Finally, end product formation is also influenced by the yeast strains naturally present or added to the must.
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