Glycogen

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Glycogen is a branched polysaccharide used as a readily available storage of glucose largely found in liver and muscle cells where it is kept as hydrated granules.  It consists of a chain of alpha glucose molecules joined together mostly by 1-4 glycosidic bonds but 1-6 glycosidic linkages are also present approximately every 10 glucose residues, these 1-6 linkages form the 'branches' [1].

The glycogen store in muscle is released for muscle cells and acts as a supply of glucose for when activity of the muscle tissue is high. The glycogen released by liver cells is released for other cells and plays an important role in the regulation of blood glucose level. The synthesis and degradation of glycogen both have different reaction pathways of which are controlled by a cascade of amplifying reactions. The majority of the glycogen molecule is degraded by the enzyme phosphorylase which breaks the glycosidic linkage between the C-1 terminal residue and the C-4 of the adjacent molecule forming glucose 1-phosphate. If needed this can be converted back into glucose 6-phosphate [2]. Glycogen is broken and used when cells require more ATP than they can produce from the bloodstream [3].

References

  1. Berg J., Tymoczko J. and Stryer L. (2007) Biochemistry, 6th edition, New York: WH Freeman. Page 592
  2. Stryer L. (1988) Biochemistry, 3rd edition, New York: WH Freeman. Page 450-467
  3. Alberts B., Johnson A., Lewis J., Raff M., Roberts K. and Walter P. (2008) Molecular Biology of the cell, 5th edition, New York: Garland Science.
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