Pasteur effect

The Pasteur effect is an inhibiting effect of oxygen on the fermentation process. It is a sudden change from anaerobic to aerobic process.

Discovery

The effect was discovered in 1857 by Louis Pasteur, who showed that aerating yeasted broth causes yeast cell growth to increase, while conversely, fermentation rate decreases. Louis Pasteur was selected to the Académie des Sciences in 1862. He later became the professor of geology, physics, and chemistry at the École des Beaux-Art.

Explanation

The effect can be 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 31 or 29.5 moles of ATP per mole of glucose (it depends on which shuttle is used for reducing the reducing equivalent, NADH, that is formed in the cytosol). Therefore, about 15 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.

From the standpoint of ATP production then, it is advantageous for yeast to utilize the citric acid 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.

Practical implications

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.

See also

• Ethanol fermentation
• Fermentation (biochemistry)
• Allosteric regulation
• Allosteric inhibition

References

• 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.