Glycogen synthase

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Glycogen Synthase (GS) is an enzyme present in liver and muscle cells that catalyses the production of glycogen. It catalyses a condensation reaction between UDP-glucose and glycogen (n-residues) to form glycogen (n+1 residues) and UDP, elongating the glycogen polymer. 

Regulation of the enzyme is achieved through multiple phosphorylations at different sites by multiples kinases, including Protein Kinase B (PKB) and Glycogen Synthase Kinase 3 (GSK3). When insulin is absent from cells, GSK3 phosphorylates Glycogen Synthase, inactivating it, this maintains the level of glucose available to the cells. Upon insulin stimulation, PKB is activated and phosphorylates GSK3, which is also a repressing phosphorylation. GSK3 can now no longer inhibit Glycogen Synthase, thereby increasing the concentration of glycogen and reducing the concentration of glucose within the cells [1].

Insulin also activates a phosphatase called Protein Phosphatase-1 (PP1), which dephosphorylates GS. The pathway as to how this occurs is unclear.

References

  1. Harvey Lodish, Arnold Berk, Chris A. Kaiser, Monty Krieger, Matthew P. Scott, Anthony Bretscher and Hidde Ploegh (2008) Molecular Cell Biology, sixth edition,United States of America: W. H. Freeman and Company
 
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