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Glycogen (black granules) in spermatozoa of a flatworm; transmission electron microscopy, scale: 0.3 μm. Glycogen is a multibranched polysaccharide of glucose that serves as a form of energy storage in animals, [2] fungi, and bacteria. [3] It is the main storage form of glucose in the human body.
Glycogen synthase kinase 3 (GSK-3) is a serine/threonine protein kinase that mediates the addition of phosphate molecules onto serine and threonine amino acid residues. First discovered in 1980 as a regulatory kinase for its namesake, glycogen synthase (GS), [2] GSK-3 has since been identified as a protein kinase for over 100 different proteins in a variety of different pathways.
Once sufficient residues have been added, glycogen synthase takes over extending the chain. Glycogenin remains covalently attached to the reducing end of the glycogen molecule . Evidence accumulates that a priming protein may be a fundamental property of polysaccharide synthesis in general; the molecular details of mammalian glycogen biogenesis ...
Glycogen synthase is also regulated by protein phosphatase 1 , which activates glycogen synthase via dephosphorylation. [18] PP1 is targeted to the glycogen pellet by four targeting subunits, G M, G L, PTG and R6. These regulatory enzymes are regulated by insulin and glucagon signaling pathways.
Glycogen phosphorylase is activated by phosphorylation, whereas glycogen synthase is inhibited. Glycogen phosphorylase is converted from its less active "b" form to an active "a" form by the enzyme phosphorylase kinase. This latter enzyme is itself activated by protein kinase A and deactivated by phosphoprotein phosphatase-1.
The glycogen phosphorylase monomer is a large protein, composed of 842 amino acids with a mass of 97.434 kDa in muscle cells. While the enzyme can exist as an inactive monomer or tetramer, it is biologically active as a dimer of two identical subunits.
Glycogen synthase kinase-3 beta, (GSK-3 beta), is an enzyme that in humans is encoded by the GSK3B gene. [ 5 ] [ 6 ] In mice, the enzyme is encoded by the Gsk3b gene. [ 7 ] Abnormal regulation and expression of GSK-3 beta is associated with an increased susceptibility towards bipolar disorder .
Protein kinase A (cAMP-dependent protein kinase) can reduce the activity of PP1. The glycogen binding region, GM, becomes phosphorylated, which causes its dissociation from the catalytic PP1 unit. [12] This separation of the catalytic PP1 unit, glycogen, and other substrates causes a significant decrease in dephosphorylation.