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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 36 moles of ATP per mole of glucose. Therefore, about 18 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 production increases and the rate of glycolysis slows, because the ATP produced acts as an allosteric inhibitor for phosphofructokinase 1, the third enzyme in the glycolysis pathway.
So, from the standpoint of ATP production, it is advantageous for yeast to undergo Krebs Cycle in the presence of oxygen, as more ATP is produced from less glucose.
- Krebs, Hans (1972). "The Pasteur effect and the relations between respiration and fermentation". Essays in Biochemistry (8): 1–34.
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