Glycogen synthase

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glycogen (starch) synthase
GlycogenSyn Wiki.png
Identifiers
EC number 2.4.1.11
CAS number 9014-56-6
Databases
IntEnz IntEnz view
BRENDA BRENDA entry
ExPASy NiceZyme view
KEGG KEGG entry
MetaCyc metabolic pathway
PRIAM profile
PDB structures RCSB PDB PDBe PDBsum
Gene Ontology AmiGO / EGO

Glycogen synthase (UDP-glucose-glycogen glucosyltransferase) is an enzyme involved in converting glucose to glycogen. It takes short polymers of glucose and converts them into long polymers.

It is a glycosyltransferase enzyme (EC 2.4.1.11) that catalyses the reaction of UDP-glucose and (1,4-α-D-glucosyl)n to yield UDP and (1,4-α-D-glucosyl)n+1.

In other words, this enzyme converts excess glucose residues one by one into a polymeric chain for storage as glycogen. Its presence in the bloodstream is highest in the 30 to 60 minutes[2] following intense exercise. It is a key enzyme in glycogenesis.

Structure[edit]

Much research has been done on glycogen degradation through studying the structure and function of glycogen phosphorylase, the key regulatory enzyme of glycogen degradation.[3] On the other hand, much less is known about the structure of glycogen synthase, the key regulatory enzyme of glycogen synthesis. The crystal structure of glycogen synthase from Agrobacterium tumefaciens, however, has been determined at 2.3 A resolution.[4] In its asymmetric form, glycogen synthase is found as a dimer, whose monomers are composed of two Rossmann-fold domains. This structural property, among others, is shared with related enzymes, such as glycogen phosphorylase and other glycosyltransferases of the GT-B superfamily.[5]

Glycogen synthase can be classified in two general protein families. The first family (GT3), which is from mammals and yeast, is approximately 80 kDa, uses UDP-glucose as a sugar donor, and is regulated by phosphorylation and ligand binding.[6] The second family (GT5), which is from bacteria and plants, is approximately 50 kDA, uses ADP-glucose as a sugar donor, and is unregulated.[7]

Mechanism[edit]

Although the catalytic mechanisms used by glycogen synthase are not well known, structural similarities to glycogen phosphorylase at the catalytic and substrate binding site suggest that the mechanism for synthesis is similar in glycogen synthase and glycogen phosphorylase.[4]

Function[edit]

In a recent study of transgenic mice, an overexpression of glycogen synthase[8] and an overexpression of phosphatase[9] both resulted in excess glycogen storage levels. This suggests that glycogen synthase plays an important biological role in regulating glycogen/glucose levels and is activated by dephosphorylation.

Isozymes[edit]

In humans, there are two paralogous isozymes of glycogen synthase:

isozyme tissue distribution gene
glycogen synthase 1 containing carbon elements muscle and other tissues GYS1[10]
glycogen synthase 2 liver GYS2[11]

The liver enzyme expression is restricted to the liver, whereas the muscle enzyme is widely expressed. Liver glycogen serves as a storage pool to maintain the blood glucose level during fasting, whereas muscle glycogen synthesis accounts for disposal of up to 90% of ingested glucose. The role of muscle glycogen is as a reserve to provide energy during bursts of activity.[12]

glycogen synthase 1 (muscle)
Identifiers
Symbol GYS1
Entrez 2997
HUGO 4706
OMIM 138570
RefSeq NM_002103
UniProt P13807
Other data
Locus Chr. 19 q13.3
glycogen synthase 2 (liver)
Identifiers
Symbol GYS2
Entrez 2998
HUGO 4707
OMIM 138571
RefSeq NM_021957
UniProt P54840
Other data
Locus Chr. 12 p12.2-11.2

Regulation[edit]

The reaction is highly regulated by allosteric effectors such as glucose-6-phosphate, by phosphorylation reactions, and indirectly triggered by the hormone insulin, which is secreted by the pancreas. Phosphorylation of glycogen synthase decreases its activity. The enzyme also cleaves the ester bond between the C1 position of glucose and the pyrophosphate of UDP itself.

The control of glycogen synthase is a key step in regulating glycogen metabolism and glucose storage. Glycogen synthase is directly regulated by glycogen synthase kinase 3 (GSK-3), AMPK, protein kinase A (PKA), and casein kinase 2 (CK2). Each of these protein kinases lead to phosphorylated and catalytically inactive glycogen synthase. The phosphorylation sites of glycogen synthase are summarized below.

Name Phosphorylation Site Kinase Reference(s)
Site 1a PKA ,[13][14]
Site 1b PKA ,[13][14]
Site 2 Serine 7 AMPK ,[15][16]
Site 2a Serine 10 CK2
Site 3a Serine 641 GSK3 [17]
Site 3b Serine 645 GSK3 [17]
Site 3c Serine 649 GSK3 [17]
Site 3d Serine 653 GSK3 [17]
Site 4 Serine 727

For enzymes in the GT3 family, these regulatory kinases inactivate glycogen synthase by phosphorylating it at the N-terminal of the 25th residue and the C-terminal of the 120th residue.[4] Glycogen synthase is also regulated by protein phosphatase 1 (PP1), which activates glycogen synthase via dephosphorylation.[18] PP1 is targeted to the glycogen pellet by four targeting subunits, GM, GL, PTG and R6. These regulatory enzymes are regulated by insulin and glucagon signaling pathways.

Pathology[edit]

Mutations in the GYS1 gene are associated with glycogen storage disease type 0.[19] In humans, defects in the tight control of glucose uptake and utilization are also associated with diabetes and hyperglycemia. Patients with type 2 diabetes normally exhibit low glycogen storage levels because of impairments in insulin-stimulated glycogen synthesis and suppression of glycogenolysis. Insulin stimulates glycogen synthase by inhibiting glycogen synthase kinases or/and activating protein phosphatase 1 (PP1) among other mechanisms .[18]

Model organisms[edit]

Model organisms have been used in the study of GYS2 function. A conditional knockout mouse line, called Gys2tm1a(KOMP)Wtsi[25][26] was generated as part of the International Knockout Mouse Consortium program — a high-throughput mutagenesis project to generate and distribute animal models of disease to interested scientists — at the Wellcome Trust Sanger Institute.[27][28][29] Male and female animals underwent a standardized phenotypic screen to determine the effects of deletion.[23][30] Twenty six tests were carried out and two significant phenotypes were reported. Homozygous mutant male adults displayed impaired glucose tolerance, whereas females had a significant decrease in circulating glucose levels as determined by clinical chemistry.[23]

References[edit]

  1. ^ PDB 1RZU; Buschazzio A, Ugalde JE, Guerin ME, Shepard W, Ugalde RA, Alzari PM (August 2004). "Crystal structure of glycogen synthase: homologous enzymes catalyze glycogen synthesis and degradation". EMBO J. 23 (16): 3196–3205. doi:10.1038/sj.emboj.7600324. PMC 514502. PMID 15272305. ; rendered using PyMOL.
  2. ^ Jentjens R, Jeukendrup A (2003). "Determinants of post-exercise glycogen synthesis during short-term recovery". Sports Med 33 (2): 117–44. doi:10.2165/00007256-200333020-00004. PMID 12617691. 
  3. ^ Buchbinder JL, Rath VL, Fletterick RJ (2001). "Structural relationships among regulated and unregulated phosphorylases". Annu Rev Biophys Biomol Struct. 30 (1): 191–209. doi:10.1146/annurev.biophys.30.1.191. PMID 11340058. 
  4. ^ a b c Buschiazzo A, Ugalde JE, Guerin ME, Shepard W, Ugalde RA, Alzari PM (2004). "Crystal structure of glycogen synthase: homologous enzymes catalyze glycogen synthesis and degradation.". EMBO J. 23 (16): 3195–205. doi:10.1038/sj.emboj.7600324. PMC 514502. PMID 15272305. 
  5. ^ Coutinho PM, Deleury E, Davies GJ, Henrissat B (2003). "An evolving hierarchical family classification for glycosyltransferases". J. Mol. Biol. 328 (2): 307–17. doi:10.1016/S0022-2836(03)00307-3. PMID 12691742. 
  6. ^ Roach PJ (2002). "Glycogen and its Metabolism". Curr Mol Med 2 (2): 101–20. doi:10.2174/1566524024605761. PMID 11949930. 
  7. ^ Ball SG, Morell MK (2003). "From bacterial glycogen to starch: understanding the biogenesis of the plant starch granule". Annu Rev Plant Biol 54 (1): 207–33. doi:10.1146/annurev.arplant.54.031902.134927. PMID 14502990. 
  8. ^ Azpiazu I, Manchester J, Skurat AV, Roach PJ, Lawrence JC Jr (2000). "Control of glycogen synthesis is shared between glucose transport and glycogen synthase in skeletal muscle fibers". Am J Physiol Endocrinol Metab 278 (2): E234–43. PMID 10662707. 
  9. ^ Aschenbach WG, Suzuki Y, Breeden K, Prats C, Hirshman MF, Dufresne SD, Sakamoto K, Vilardo PG, Steele M, Kim JH, Jing SL, Goodyear LJ, DePaoli-Roach AA (2001). "The muscle-specific protein phosphatase PP1G/R(GL)(G(M))is essential for activation of glycogen synthase by exercise". J Biol Chem 276 (43): 39959–67. doi:10.1074/jbc.M105518200. PMID 11522787. 
  10. ^ Browner MF, Nakano K, Bang AG, Fletterick RJ (March 1989). "Human muscle glycogen synthase cDNA sequence: a negatively charged protein with an asymmetric charge distribution". Proceedings of the National Academy of Sciences of the United States of America 86 (5): 1443–7. doi:10.1073/pnas.86.5.1443. PMC 286712. PMID 2493642. 
  11. ^ Westphal SA, Nuttall FQ (February 1992). "Comparative characterization of human and rat liver glycogen synthase". Archives of biochemistry and biophysics 292 (2): 479–86. doi:10.1016/0003-9861(92)90019-S. PMID 1731614. 
  12. ^ Kollberg G, Tulinius M, Gilljam T, Ostman-Smith I, Forsander G, Jotorp P, Oldfors A, Holme E (October 2007). "Cardiomyopathy and exercise intolerance in muscle glycogen storage disease 0". The New England Journal of Medicine 357 (15): 1507–14. doi:10.1056/NEJMoa066691. PMID 17928598. 
  13. ^ a b Huang TS, Krebs EG (April 1977). "Amino acid sequence of a phosphorylation site in skeletal muscle glycogen synthetase". Biochem. Biophys. Res. Commun. 75 (3): 643–50. doi:10.1016/0006-291X(77)91521-2. PMID 405007. 
  14. ^ a b Proud CG, Rylatt DB, Yeaman SJ, Cohen P (August 1977). "Amino acid sequences at the two sites on glycogen synthetase phosphorylated by cyclic AMP-dependent protein kinase and their dephosphorylation by protein phosphatase-III". FEBS Lett. 80 (2): 435–42. doi:10.1016/0014-5793(77)80493-6. PMID 196939. 
  15. ^ Rylatt DB, Cohen P (February 1979). "Amino acid sequence at the site on rabbit skeletal muscle glycogen synthase phosphorylated by the endogenous glycogen synthase kinase-2 activity". FEBS Lett. 98 (1): 71–5. doi:10.1016/0014-5793(79)80154-4. PMID 107044. 
  16. ^ Embi N, Parker PJ, Cohen P (April 1981). "A reinvestigation of the phosphorylation of rabbit skeletal-muscle glycogen synthase by cyclic-AMP-dependent protein kinase. Identification of the third site of phosphorylation as serine-7". Eur. J. Biochem. 115 (2): 405–13. doi:10.1111/j.1432-1033.1981.tb05252.x. PMID 6263629. 
  17. ^ a b c d Rylatt DB, Aitken A, Bilham T, Condon GD, Embi N, Cohen P (June 1980). "Glycogen synthase from rabbit skeletal muscle. Amino acid sequence at the sites phosphorylated by glycogen synthase kinase-3, and extension of the N-terminal sequence containing the site phosphorylated by phosphorylase kinase". Eur. J. Biochem. 107 (2): 529–37. doi:10.1111/j.1432-1033.1980.tb06060.x. PMID 6772446. 
  18. ^ a b Saltiel AR (2001). "New perspectives into the molecular pathogenesis and treatment of type 2 diabetes". Cell 104 (4): 517–29. doi:10.1016/S0092-8674(01)00239-2. PMID 11239409. 
  19. ^ Orho M, Bosshard NU, Buist NR, Gitzelmann R, Aynsley-Green A, Blümel P, Gannon MC, Nuttall FQ, Groop LC (August 1998). "Mutations in the liver glycogen synthase gene in children with hypoglycemia due to glycogen storage disease type 0". The Journal of Clinical Investigation 102 (3): 507–15. doi:10.1172/JCI2890. PMC 508911. PMID 9691087. 
  20. ^ "Clinical chemistry data for Gys2". Wellcome Trust Sanger Institute. 
  21. ^ "Salmonella infection data for Gys2". Wellcome Trust Sanger Institute. 
  22. ^ "Citrobacter infection data for Gys2". Wellcome Trust Sanger Institute. 
  23. ^ a b c Gerdin AK (2010). "The Sanger Mouse Genetics Programme: High throughput characterisation of knockout mice". Acta Ophthalmologica 88 (S248). doi:10.1111/j.1755-3768.2010.4142.x. 
  24. ^ Mouse Resources Portal, Wellcome Trust Sanger Institute.
  25. ^ "International Knockout Mouse Consortium". 
  26. ^ "Mouse Genome Informatics". 
  27. ^ Skarnes, W. C.; Rosen, B.; West, A. P.; Koutsourakis, M.; Bushell, W.; Iyer, V.; Mujica, A. O.; Thomas, M.; Harrow, J.; Cox, T.; Jackson, D.; Severin, J.; Biggs, P.; Fu, J.; Nefedov, M.; De Jong, P. J.; Stewart, A. F.; Bradley, A. (2011). "A conditional knockout resource for the genome-wide study of mouse gene function". Nature 474 (7351): 337–342. doi:10.1038/nature10163. PMC 3572410. PMID 21677750.  edit
  28. ^ Dolgin E (June 2011). "Mouse library set to be knockout". Nature 474 (7351): 262–3. doi:10.1038/474262a. PMID 21677718. 
  29. ^ Collins FS, Rossant J, Wurst W (January 2007). "A mouse for all reasons". Cell 128 (1): 9–13. doi:10.1016/j.cell.2006.12.018. PMID 17218247. 
  30. ^ van der Weyden L, White JK, Adams DJ, Logan DW (2011). "The mouse genetics toolkit: revealing function and mechanism.". Genome Biol 12 (6): 224. doi:10.1186/gb-2011-12-6-224. PMC 3218837. PMID 21722353. 

External links[edit]