MCAT Biochemistry
The Cahill Cycle, also known as the glucose-alanine cycle, is a process that shuttles amino and carbon groups from skeletal muscle to the liver for deamination and gluconeogenesis. Inside the skeletal muscle, amino acids are deaminated to form their respective alpha-ketoacids and NH3. Alpha-ketoglutarate, an intermediate of the TCA cycle, accepts NH3 to make glutamate. Muscle alanine transaminase (ALT) transfers an amino group from glutamate to pyruvate, producing alanine and alpha-ketoglutarate. Alanine then travels to the liver where it undergoes the reverse process, ultimately leading to the production of glucose which is transported back to the skeletal muscle.
The Cori Cycle, also known as the lactic acid cycle, is a process that transports lactate from actively contracting skeletal muscles to the liver, where it undergoes gluconeogenesis to produce glucose. This glucose is then transported back to the skeletal muscle to be used for energy. During anaerobic glycolysis in skeletal muscle, lactate dehydrogenase converts pyruvate to lactate and regenerates NAD+ from NADH. The lactate is then transported to the liver, where it undergoes the reverse process and ultimately becomes glucose, which is transported back to the skeletal muscle.
Lesson Outline
<ul> <li>Introduction to the Cahill Cycle (glucose-alanine cycle) <ul> <li>Transports ammonia and amino acid products to the liver</li> <li>Ammonia comes from the deamination of amino acids in skeletal muscle</li> </ul> </li> <li>Steps of the Cahill Cycle <ul> <li>Deamination of amino acids to form alpha-ketoacids and NH3</li> <li>Alpha ketoglutarate accepts NH3 to make glutamate</li> <li>Muscle alanine transaminase (ALT) transfers an amino group from glutamate to pyruvate, producing alanine and alpha ketoglutarate </li> <li>Alanine is transported to the liver</li> <li>In the liver, ALT turns alpha ketoglutarate and alanine back to glutamate and pyruvate</li> <li>Pyruvate is turned back into glucose</li> <li>Ammonia is removed from glutamate using glutamate dehydrogenase</li> <li>Ammonia enters the urea cycle</li> </ul> </li> <li>Introduction to the Cori Cycle (lactic acid cycle) <ul> <li>Transportation of lactate from skeletal muscle to the liver</li> <li>Purpose of the cycle to produce glucose for energy</li> </ul> </li> <li>Steps of the Cori Cycle <ul> <li>Skeletal muscle tissues undergo anaerobic glycolysis when oxygen is unavailable</li> <li>Formation of lactate and ATP during anaerobic glycolysis</li> <li>Lactate dehydrogenase converts pyruvate to lactate, also regenerating NAD+ from NADH</li> <li>Lactate travels to the liver</li> <li>In the liver, lactate dehydrogenase catalyzes the same reaction, but in the opposite direction to make pyruvate and NADH </li> <li>Pyruvate becomes glucose via gluconeogenesis</li> <li>Glucose is transported back to skeletal muscle, and the cycle repeats until oxygen becomes available</li> </ul> </li> <li>Comparison of Cahill and Cori Cycles <ul> <li>Both cycles transport products from skeletal muscle to the liver</li> <li>Both cycles involve a gluconeogenesis component</li> <li>Cahill cycle primarily concerned with ammonia transport, while Cori cycle deals with lactate transport</li> </ul> </li> </ul>
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FAQs
The main purpose of the Cori cycle, also known as the lactic acid cycle, is to recycle lactate produced by skeletal muscles during anaerobic glycolysis. When oxygen supply is insufficient, skeletal muscles generate ATP through anaerobic glycolysis and produce lactate as a byproduct. Lactate is transported to the liver, where it is converted back into glucose through gluconeogenesis. This glucose is then sent back to the muscles, providing a continuous source of energy during prolonged physical activity.
The Cahill cycle, also known as the glucose-alanine cycle, contributes to gluconeogenesis by transporting nitrogen in the form of alanine from skeletal muscles to the liver. During periods of intense exercise or fasting, skeletal muscles break down proteins to release amino acids as an energy source. In this process, ammonia is produced as a toxic byproduct. To safely transport and eliminate ammonia, skeletal muscles convert it to alanine by transferring the amino group to alpha-ketoacids. The liver then takes up alanine, converts it back to pyruvate through alanine aminotransferase, and uses the pyruvate to produce glucose via gluconeogenesis. Ammonia is also converted into urea in the liver through the urea cycle and is excreted by the kidneys.
Lactate dehydrogenase is the key enzyme in the Cori cycle. It catalyzes the interconversion of lactate and pyruvate in both skeletal muscle and the liver. In the absence of sufficient oxygen supply, lactate dehydrogenase helps convert pyruvate to lactate in skeletal muscles, which is then transported to the liver. In the liver, the same enzyme converts lactate back to pyruvate, allowing glucose to be produced via gluconeogenesis. This process helps maintain glucose levels in the blood and ensures the continuous supply of energy for skeletal muscles during anaerobic activity.
The glucose-alanine cycle is closely linked to the urea cycle in the liver. As skeletal muscles break down amino acids for energy, they generate toxic ammonia. To eliminate ammonia safely, skeletal muscles convert the amino group of the amino acid to alpha-ketoacids and produce alanine. Upon reaching the liver, alanine is transformed back into pyruvate through alanine aminotransferase, releasing the excess nitrogen as ammonia. The ammonia is then incorporated into the urea cycle, where it is converted to urea. Urea, being a less toxic compound, is transported to the kidneys and excreted in urine, effectively eliminating excess nitrogen from the body.
The major differences between the Cori cycle and the Cahill cycle lie in their functions and end-products. The Cori cycle deals primarily with the recycling of lactate produced by skeletal muscles during anaerobic glycolysis. It converts lactate to glucose through gluconeogenesis in the liver, ensuring a continuous supply of energy. The Cahill cycle, on the other hand, focuses on the safe transportation and elimination of excess nitrogen from skeletal muscles in the form of alanine. While both cycles contribute to gluconeogenesis, the end-products are different: the Cori cycle produces glucose, and the Cahill cycle helps eliminate ammonia and produce urea through the urea cycle.
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