Glycogen Degradation/Utilization

 
Glucose is stored as the intracellular polysaccharides starch and glycogen. Starch occurs mostly in plants. Glycogen is an important storage polysaccharide in bacteria, protists, fungi and animals. Glycogen is stored in large granules. In mammals, these granules are found in muscle and liver cells. In electron micrographs, liver glycogen appears as clusters of cytosolic granules with a diameter of 100 nm—much larger than ribosomes. The enzymes required for synthesis of glycogen are found in muscle and liver cells [Glycogen Synthesis]. Those same cells contain the enzymes for glycogen degradation.

The glucose residues of starch and glycogen are released from storage polymers through the action of enzymes called polysaccharide phosphorylases: starch phosphorylase (in plants) and glycogen phosphorylase (in many other organisms). These enzymes catalyze the removal of glucose residues from the ends of starch or glycogen. As the name implies, the enzymes catalyze phosphorolysis—cleavage of a bond by group transfer to an oxygen atom of phosphate. In contrast to hydrolysis (group transfer to water), phosphorolysis produces phosphate esters. Thus, the first product of polysaccharide breakdown is α-D-glucose 1-phosphate, not free glucose.


Glucose 1-phosphate is one of the precursors required for glycogen synthesis. It is the "Cori ester" [Monday's Molecule] discovered by Carl Cori and Gerty Cori [Nobel Laureates: Carl Cori and Gerty Cori]. The Cori's also discovered and characterized glycogen phosphorylase.

In order for glucose 1-phosphate to be used in other pathways it has to be converted to glucose 6-phosphate by the enzyme phosphoglucomutase. This is the same enzyme that's used in the synthesis of glycogen from glucose 6-phosphate.

Glucose 6-phosphate can be oxidzed by the glycolysis pathway to produce ATP. This is what happens in muscle cells. Glucose is stored as glycogen during times of rest but during exercise the glycogen is broken down to glucose 6-phosphate and glycolysis is activated. The resulting ATP is used in muscle activity.

Obviously, there has to be a balance between the synthesis and degradation of glycogen and this balance is maintained by regulating the activities of the biosynthesis and degradation enzymes. This regulation occurs at many levels. Regulation by hormones is one of the classic examples of a signal transduction pathway in mammals.

[©Laurence A. Moran. Some of the text is from Principles of Biochemistry 4th ed. ©Pearson/Prentice Hall]