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Thursday, May 10, 2007

The Cori Cycle

Animals can synthesize glucose 6-phosphate via gluconeogenesis just like all other species. However, unlike most species, animals can convert glucose 6-phosphate to glucose, which is secreted into the circulatory system. Mammals, in particular, have a sophisticated cycle of secretion and uptake of glucose. It's called the Cori cycle after the Nobel Laureates: Carl Ferdinand Cori and Gerty Theresa Cori.

The glucose 6-phosphate molecules synthesized in the liver can either be converted to glycogen [Glycogen Synthesis] or converted to glucose and secreted into the blood stream. The glucose molecules are taken up by muscle cells where they can be stored as glucogen. During strenuous exercise the glycogen is broken down to glucose 6-phosphate [Glycogen Degradation] and oxdized via the glycolysis pathway. This pathway yields ATP that is used in muscle contraction.

If oxygen is limiting, the end product of glucose breakdown isn't CO2 but lactate. Lactate is secreted into the blood stream where it is taken up by the liver and converted to pyruvate by the enzyme lactate dehydrogenase. Pyruvate is the substrate for gluconeogenesis. The synthesis of glucose in the liver requires energy in the form of ATP and this energy is supplied by a variety of sources. The breakdown of fatty acids is the source shown in the figure.

The Cori cycle preserves carbon atoms. The six carbon molecule, glucose, is split into two 3-carbon molecules (lactate) that are then converted to another 3-carbon molecule (pyruvate). Two pyruvates are joined to make glucose.


Anonymous said...

What would happen if the interconversions of the Cori cycle occurred and remained within a single cell?

Anonymous said...

How is this cycle advantageous to the organism, inspite of its energy cost?