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Cori And Alanine Cycle
Transcript of Cori And Alanine Cycle
The breakdown of glycogen in the skeletal muscles gives energy for muscular activity. The breakdown of glycogen, a process known as glycogenolysis, releases glucose in the form of glucose-1-phosphate (G-1-P). The G-1-P is converted to G-6-P by the enzyme phosphoglucomutase. G-6-P is readily fed into glycolysis, (or can go into the pentose phosphate pathway if G-6-P concentration is high) a process that provides ATP to the muscle cells as an energy source.
During muscular activity, the store of ATP needs to be constantly replenished. When the supply of oxygen is sufficient, this energy comes from feeding pyruvate, one product of glycolysis, into the Krebs cycle.
The Cori cycle ( Lactic acid cycle) refers to the metabolic pathway in which lactate produced by anaerobic glycolysis in the muscles moves to the liver and is converted to glucose, which then returns to the muscles and is metabolized back to lactate.
The alanine cycle is less productive than the Cori cycle, which uses lactate, since a byproduct of energy production from alanine is production of urea. Removal of the urea is energy-dependent, requiring four "high-energy" phosphate bonds (3 ATP hydrolyzed to 2 ADP and one AMP), thus the net ATP produced is less than that found in the Cori cycle. However, unlike in the Cori cycle, NADH is conserved because lactate is not formed. This allows for it to be oxidized via the electron transport chain.
The alanine cycle is quite similar to the Cori cycle. When muscles produce lactate during times of decreased oxygen, they also produce alanine. This alanine is shuttled to the liver where it is used to make glucose.
This pathway requires the presence of alanine aminotransferase, which is restricted to tissues such as muscle, liver, and the intestine. Therefore, this pathway is used instead of the Cori cycle only when an aminotransferase is present and when there is a need to transfer ammonia to the liver.
Cori And Alanine Cycle
Instead of accumulating inside the muscle cells, lactate produced by anaerobic fermentation is taken up by the liver. This initiates the other half of the Cori cycle. In the liver, gluconeogenesis occurs. From an intuitive perspective, gluconeogenesis reverses both glycolysis and fermentation by converting lactate first into pyruvate, and finally back to glucose. The glucose is then supplied to the muscles through the bloodstream; it is ready to be fed into further glycolysis reactions. If muscle activity has stopped, the glucose is used to replenish the supplies of glycogen through glycogenesis.
Overall, the glycolysis part of the cycle produces 2 ATP molecules at a cost of 6 ATP molecules consumed in the gluconeogenesis part. Each iteration of the cycle must be maintained by a net consumption of 4 ATP molecules. As a result, the cycle cannot be sustained indefinitely. The intensive consumption of ATP molecules indicates that the Cori cycle shifts the metabolic burden from the muscles to the liver.
The Cori Cycle
The Alanine Cycle
When oxygen supply is insufficient, typically during intense muscular activity, energy must be released through anaerobic metabolism. Lactic acid fermentation converts pyruvate to lactate by lactate dehydrogenase. Most important, fermentation regenerates NAD+, maintaining the NAD+ concentration so that additional glycolysis reactions can occur. The fermentation step oxidizes the NADH produced by glycolysis back to NAD+, transferring two electrons from NADH to reduce pyruvate into lactate.
This cycle also serves other purposes:
• Recycles carbon skeletons between muscle and liver
• Transports ammonium to the liver and is converted into urea.