Question

The alanine cycle requires more ATP per glucose molecule formed chan does the Cori cycle. Why is chis?

Short answer

Short Answer: The alanine cycle requires more ATP per glucose molecule formed than the Cori cycle due to the additional transamination reactions for converting pyruvate to alanine and back, as well as the involvement of the urea cycle, which consumes additional ATP molecules. This makes the alanine cycle less energy-efficient compared to the Cori cycle.

Step-by-step solution

Understanding the Cori cycle

The Cori cycle is a metabolic pathway that occurs between the muscles and liver. When muscle cells need energy, they break down glucose into lactate through anaerobic glycolysis, generating ATP molecules. Lactate is then transported to the liver, where it is converted back into glucose through gluconeogenesis. This glucose can be transported back to the muscles to be used for energy again.

Understanding the Alanine cycle

The alanine cycle is a similar pathway that occurs between muscles and the liver. However, instead of lactate, alanine is used as the main shuttle molecule. When muscle cells need energy, they break down glucose into pyruvate through glycolysis. Some of this pyruvate is then transaminated to alanine, which can be transported to the liver. In the liver, alanine is converted back to pyruvate and used for gluconeogenesis to produce glucose.

Comparing ATP requirements

The main difference between the Cori and Alanine cycles is the conversion step between lactate and alanine. The Cori cycle involves the conversion of lactate to pyruvate in the liver, which is a reversible reaction that does not require ATP. The Alanine cycle, on the other hand, involves the conversion of pyruvate to alanine in muscle cells using the enzyme alanine aminotransferase (transamination reaction). This reaction requires the input of amino group from glutamate, forming alpha-ketoglutarate. In the liver, alanine is converted back into pyruvate in the same transamination reaction, in exchange for returning the amino group to glutamate. Importantly, the alpha-ketoglutarate ↔ glutamate conversion has additional energy costs when the net balance of nitrogen is considered. This is because in order to return nitrogen to the muscle to sustain the cycle, the liver needs to produce urea through the urea cycle, which consumes additional ATP molecules per exported nitrogen.

Conclusion

In conclusion, the alanine cycle requires more ATP per glucose molecule formed than the Cori cycle because it involves the additional transamination reactions for converting pyruvate to alanine and back, as well as the involvement of the urea cycle and its additional ATP consumption. This makes the alanine cycle less energy-efficient compared to the Cori cycle.