Cori Cycle

 Cori Cycle

Definition

The Cori cycle, also known as the lactic acid cycle, is a metabolic pathway of anaerobic Glycolysis in which lactate produced in muscles is transported to the liver and converted to glucose, which then returns to the muscles and is cyclically metabolized back to lactate.

It is named after its discoverers, Carl Ferdinand Cori and Gerty Cori,

Introduction-Muscular activity requires ATP, which is provided by the breakdown of glycogen in the skeletal muscles. The breakdown of glycogen known as Glycogenolysis releases glucose in the form of glucose 1-phosphate (G1P). The G1P is converted to glucose 6-phosphate (G6P) by phosphoglucomutase. G6P is readily fed into Glycolysis, (or can go into the pentose phosphate pathway if G6P 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 private, one product of Glycolysis, into the citric acid cycle, which ultimately generates ATP through oxygen-dependent oxidative Phosphorylation.

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. This fermentation regenerates NAD⁺, maintaining its concentration so 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.

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. 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.

Significance-

1.     The Cori cycle prevents lactic acidosis during anaerobic conditions in the muscle. However, normally, before this happens, the lactic acid is moved out of the muscles and into the liver.

2.     This cycle is important in ATP production, an energy source, during muscle exertion. The end of muscle exertion allows the Cori cycle to function more effectively. This repays the oxygen debt so both the electron transport chain and citric acid cycle can produce energy at optimum effectiveness. 

3.     The Cori cycle is a much more important source of substrate for Gluconeogenesis than food. The contribution of Cori cycle lactate to overall glucose production increases with fasting duration before plateauing.