Anaerobic metabolism: Cori cycle

After completion of this learning unit, you should be able to answer the following questions.

What is the Cori cycle?

When does it work?

What does it do?

Why is it important?

The Cori cycle (also known as Lactic acid cycle), named after its discoverers, Carl Cori and Gerty Cori, 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 converted back to lactate.

The Cori cycle invloves the utilization of lactate, produced by glycolysis in non-hepatic tissues, (such as muscle and erythrocytes) as a carbon source for hepatic gluconeogenesis. In this way the liver can convert the anaerobic byproduct of glycolysis (lactate), back into more glucose for reuse by non-hepatic tissues. Note that the gluconeogenic leg of the cycle (on its own) is a net consumer of energy, costing the body 4 moles of ATP more than are produced during glycolysis. Therefore, the cycle cannot be sustained indefinitely.

Under anaerobic conditions, glucose is metabolized through glycolysis which converts it to two molecules of pyruvate. Only one oxidation step has been performed when glyceraldehyde 3-phospate is oxidized to 1,3-bisphosphoglycerate. To regenerate NAD+ so glycolysis can continue, pyruvate is reduced to lactate, catalyzed by lactate dehydrogenae.   These reactions take place in the cytoplasm of cells actively engaged in anaerobic oxidation of glucose (in yeast; in muscle cells during sprint).   Note that the enzyme is named for the reverse reaction, the oxidation of lactate by NAD+.

The cycle's importance is based on the prevention of lactic acidosis in the muscle under anaerobic conditions. However, normally before this happens the lactic acid is moved out of the muscles and into the liver. The cycle is also important in producing ATP, an energy source, during muscle activity. The Cori cycle functions more efficiently when muscle activity has ceased. This allows the oxygen debt to be repaid such that the Krebs cycle and electron transport chain can produce energy at peak efficiency.

Cori Cycle (http://en.wikipedia.org/wiki/Cori_cycle)