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The effect can be easily explained; as the yeast being facultative anaerobes can produce energy using two different metabolic pathways. While the oxygen concentration is low, the product of glycolysis, pyruvate, is turned into ethanol and carbon dioxide, and the energy production efficiency is low (2 moles of ATP per mole of glucose). If the oxygen concentration grows, pyruvate is converted to acetyl CoA that can be used in the citric acid cycle, which increases the efficiency to 32 moles of ATP per mole of glucose. Therefore, about 16 times as much glucose must be consumed anaerobically as aerobically to yield the same amount of ATP.
Under anaerobic conditions, the rate of glucose metabolism is faster, but the amount of ATP produced (as already mentioned) is smaller. When exposed to aerobic conditions, the ATP and Citrate production increases and the rate of glycolysis slows, because the ATP and citrate produced act as allosteric inhibitors for phosphofructokinase 1, the third enzyme in the glycolysis pathway. The Pasteur effect will only occur if glucose concentrations are low (<2 g/L) and if other nutrients, mostly nitrogen, are limited.
So, from the standpoint of ATP production, it is advantageous for yeast to utilize the Krebs Cycle in the presence of oxygen, as more ATP is produced from less glucose; however, Boulton et. al. (1996) have maintained that yeast will follow the anaerobic, rather than aerobic, fermentative pathway if glucose is not limited, in that respiration, although capable of producing more ATP than glycolysis per molecule of glucose, also requires more energy in terms of enzymatic and mitochondrial requirements.
The processes used in alcohol production are commonly maintained in a low oxygen condition, under a blanket of carbon dioxide, while breeding yeast for biomass is done in aerobic conditions, the broth being aerated.
3. Boulton, R.B.; Bisson, L.F.; Singleton, V.L.; Kunkee, R.E. (1996) Principles and Practices of Winemaking. Gaithersburg, MD: Aspen Publishers, Inc. p.141
- Krebs, Hans (1972). "The Pasteur effect and the relations between respiration and fermentation". Essays in Biochemistry (8): 1–34.
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