Glycogen debranching enzyme

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Function and structure of eukaryotic glycogen debranching enzyme Function and Structure of Eukaryotic Glycogen Debranching Enzyme.jpg
Function and structure of eukaryotic glycogen debranching enzyme
AGL
Identifiers
Aliases AGL , GDE, amylo-alpha-1, 6-glucosidase, 4-alpha-glucanotransferase
External IDs OMIM: 610860 MGI: 1924809 HomoloGene: 536 GeneCards: AGL
Orthologs
SpeciesHumanMouse
Entrez

178

77559

Ensembl

ENSG00000162688

ENSMUSG00000033400

UniProt

P35573

n/a

RefSeq (mRNA)

NM_001081326
NM_001362367

RefSeq (protein)

NP_000019
NP_000633
NP_000634
NP_000635
NP_000637

n/a

Location (UCSC) Chr 1: 99.85 – 99.92 Mb Chr 3: 116.53 – 116.6 Mb
PubMed search [3] [4]
Wikidata
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4-α-glucanotransferase
Identifiers
EC no. 2.4.1.25
CAS no. 9032-09-1
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 / QuickGO
Search
PMC articles
PubMed articles
NCBI proteins
amylo-α-1,6-glucosidase
Identifiers
EC no. 3.2.1.33
CAS no. 9012-47-9
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 / QuickGO
Search
PMC articles
PubMed articles
NCBI proteins

A debranching enzyme is a molecule that helps facilitate the breakdown of glycogen, which serves as a store of glucose in the body, through glucosyltransferase and glucosidase activity. Together with phosphorylases, debranching enzymes mobilize glucose reserves from glycogen deposits in the muscles and liver. This constitutes a major source of energy reserves in most organisms. Glycogen breakdown is highly regulated in the body, especially in the liver, by various hormones including insulin and glucagon, to maintain a homeostatic balance of blood-glucose levels. [5] When glycogen breakdown is compromised by mutations in the glycogen debranching enzyme, metabolic diseases such as Glycogen storage disease type III can result. [6] [7]

Contents

Glucosyltransferase and glucosidase are performed by a single enzyme in mammals, yeast, and some bacteria, but by two distinct enzymes in E. coli and other bacteria, complicating nomenclature. Proteins that catalyze both functions are referred to as glycogen debranching enzymes (GDEs). When glucosyltransferase and glucosidase are catalyzed by distinct enzymes, "glycogen debranching enzyme" usually refers to the glucosidase enzyme. In some literature, an enzyme capable only of glucosidase is referred to as a "debranching enzyme". [8]

Function

Together with phosphorylase, glycogen debranching enzymes function in glycogen breakdown and glucose mobilization. When phosphorylase has digested a glycogen branch down to four glucose residues, it will not remove further residues. Glycogen debranching enzymes assist phosphorylase, the primary enzyme involved in glycogen breakdown, in the mobilization of glycogen stores. Phosphorylase can only cleave α-1,4- glycosidic bond between adjacent glucose molecules in glycogen but branches also exist as α-1,6 linkages. When phosphorylase reaches four residues from a branching point it stops cleaving; because 1 in 10 residues is branched, cleavage by phosphorylase alone would not be sufficient in mobilizing glycogen stores. [9] [10] Before phosphorylase can resume catabolism, debranching enzymes perform two functions:

Glycosidase mechanism.png

Thus the debranching enzymes, transferase and α-1,6- glucosidase converts the branched glycogen structure into a linear one, paving the way for further cleavage by phosphorylase.

Structure and activity

Two enzymes

In E. coli and other bacteria, glucosyltransferase and glucosidase functions are performed by two distinct enzymes. In E. coli, Glucose transfer is performed by 4-alpha-glucanotransferase, a 78.5 kDa protein coded for by the gene malQ. [13] A second protein, referred to as debranching enzyme, performs α-1,6-glucose cleavage. This enzyme has a molecular mass of 73.6 kDa, and is coded for by the gene glgX. [14] Activity of the two enzymes is not always necessarily coupled. In E. coli glgX selectively catalyzes the cleavage of 4-subunit branches, without the action of glucanotransferase. The product of this cleavage, maltotetraose, is further degraded by maltodextrin phosphorylase. [6] [15]

E. coli GlgX is structurally similar to the protein isoamylase. The monomeric protein contains a central domain in which eight parallel beta-strands are surrounded by eight parallel alpha strands. Notable within this structure is a groove 26 angstroms long and 9 angstroms wide, containing aromatic residues that are thought to stabilize a four-glucose branch before cleavage. [6]

The glycogen-degrading enzyme of the archaea Sulfolobus solfataricus , treX, provides an interesting example of using a single active site for two activities: amylosidase and glucanotransferase activities. TreX is structurally similar to glgX, and has a mass of 80kD and one active site. [8] [16] Unlike either glgX, however, treX exists as a dimer and tetramer in solution. TreX's oligomeric form seems to play a significant role in altering both enzyme shape and function. Dimerization is thought to stabilize a "flexible loop" located close to the active site. This may be key to explaining why treX (and not glgX) shows glucosyltransferase activity. As a tetramer, the catalytic efficiency of treX is increased fourfold over its dimeric form. [6] [17]

One enzyme with two catalytic sites

In mammals and yeast, a single enzyme performs both debranching functions. [18] The human glycogen debranching enzyme (gene: AGL) is a monomer with a molecular weight of 175 kDa. It has been shown that the two catalytic actions of AGL can function independently of each other, demonstrating that multiple active sites are present. This idea has been reinforced with inhibitors of the active site, such as polyhydroxyamine, which were found to inhibit glucosidase activity while transferase activity was not measurably changed. [19] Glycogen debranching enzyme is the only known eukaryotic enzyme that contains multiple catalytic sites and is active as a monomer. [20] [21]

Some studies have shown that the C-terminal half of yeast GDE is associated with glucosidase activity, while the N-terminal half is associated with glucosyltransferase activity. [18] In addition to these two active sites, AGL appears to contain a third active site that allows it to bind to a glycogen polymer. [22] It is thought to bind to six glucose molecules of the chain as well as the branched glucose, thus corresponding to 7 subunits within the active site, as shown in the figure below. [23]

Hypothesized substraight binding location.png

The structure of the Candida glabrata GDE has been reported. [24] The structure revealed that distinct domains in GDE encode the glucanotransferase and glucosidase activities. Their catalyses are similar to that of alpha-amylase and glucoamylase, respectively. Their active sites are selective towards the respective substrates, ensuring proper activation of GDE. Besides the active sites GDE have additional binding sites for glycogen, which are important for its recruitment to glycogen. Mapping the disease-causing mutations onto the GDE structure provided insights into glycogen storage disease type III.

Genetic location

The official name for the gene is "amylo- α- 1,6- glucosidase, 4- α- glucanotransferase", with the official symbol AGL. AGL is an autosomal gene found on chromosome lp21. [10] The AGL gene provides instructions for making several different versions, known as isoforms, of the glycogen debranching enzyme. These isoforms vary by size and are expressed in different tissues, such as liver and muscle. This gene has been studied in great detail, because mutation at this gene is the cause of Glycogen Storage Disease Type III. [25] The gene is 85 kb long, has 35 exons and encodes for a 7.0 kb mRNA. Translation of the gene begins at exon 3,which encodes for the first 27 amino acids of the AGL gene, because the first two exons (68kb) contain the 5' untranslated region. Exons 4-35 encode the remaining 1505 amino acids of the AGL gene. [7] Studies produced by the department of pediatrics at Duke University suggest that the human AGL gene contains at minimum 2 promotor regions, sites where the transcription of the gene begins, that result in differential expression of isoform, different forms of the same protein, mRNAs in a manner that is specific for different tissues. [22] [26]

Clinical significance

When GDE activity is compromised, the body cannot effectively release stored glycogen, type III Glycogen Storage Disease (debrancher deficiency), an autosomal recessive disorder, can result. In GSD III glycogen breakdown is incomplete and there is accumulation of abnormal glycogen with short outer branches. [27]

Most patients exhibit GDE defiency in both liver and muscle (Type IIIa), although 15% of patients have retained GDE in muscle while having it absent from the liver (Type IIIb). [10] Depending on mutation location, different mutations in the AGL gene can affect different isoforms of the gene expression. For example, mutations that occur on exon 3, affect the form which affect the isoform that is primarily expressed in the liver; this would lead to GSD type III. [28]

These different manifestation produce varied symptoms, which can be nearly indistinguishable from Type I GSD, including hepatomegaly, hypoglycemia in children, short stature, myopathy, and cardiomyopathy. [7] [29] Type IIIa patients often exhibit symptoms related to liver disease and progressive muscle involvement, with variations caused by age of onset, rate of disease progression and severity. Patients with Type IIIb generally symptoms related to liver disease. [30] Type III patients be distinguished by elevated liver enzymes, with normal uric acid and blood lactate levels, differing from other forms of GSD. [28] In patients with muscle involvement, Type IIIa, the muscle weakness becomes predominant into adulthood and can lead to ventricular hypertrophy and distal muscle wasting. [28]

Related Research Articles

<span class="mw-page-title-main">Glycogenolysis</span> Breakdown of glycogen to glucose-1-phosphate and glycogen

Glycogenolysis is the breakdown of glycogen (n) to glucose-1-phosphate and glycogen (n-1). Glycogen branches are catabolized by the sequential removal of glucose monomers via phosphorolysis, by the enzyme glycogen phosphorylase.

<span class="mw-page-title-main">Maltase</span> Enzyme

Maltase is one type of alpha-glucosidase enzymes located in the brush border of the small intestine. This enzyme catalyzes the hydrolysis of disaccharide maltose into two simple sugars of glucose. Maltase is found in plants, bacteria, yeast, humans, and other vertebrates. It is thought to be synthesized by cells of the mucous membrane lining the intestinal wall.

<span class="mw-page-title-main">Phosphoglucomutase</span>

Phosphoglucomutase is an enzyme that transfers a phosphate group on an α-D-glucose monomer from the 1 to the 6 position in the forward direction or the 6 to the 1 position in the reverse direction.

Glycogenesis is the process of glycogen synthesis, in which glucose molecules are added to chains of glycogen for storage. This process is activated during rest periods following the Cori cycle, in the liver, and also activated by insulin in response to high glucose levels.

<span class="mw-page-title-main">Glycogen phosphorylase</span> Class of enzymes

Glycogen phosphorylase is one of the phosphorylase enzymes. Glycogen phosphorylase catalyzes the rate-limiting step in glycogenolysis in animals by releasing glucose-1-phosphate from the terminal alpha-1,4-glycosidic bond. Glycogen phosphorylase is also studied as a model protein regulated by both reversible phosphorylation and allosteric effects.

AGL may refer to:

<span class="mw-page-title-main">Glycogenin</span> Enzyme involved in converting glucose to glycogen

Glycogenin is an enzyme involved in converting glucose to glycogen. It acts as a primer, by polymerizing the first few glucose molecules, after which other enzymes take over. It is a homodimer of 37-kDa subunits and is classified as a glycosyltransferase.

<span class="mw-page-title-main">Glycogen synthase</span> Enzyme class, includes all types of glycogen/starch synthases

Glycogen synthase is a key enzyme in glycogenesis, the conversion of glucose into glycogen. It is a glycosyltransferase that catalyses the reaction of UDP-glucose and n to yield UDP and n+1.

<span class="mw-page-title-main">Glycogen storage disease type III</span> Medical condition

Glycogen storage disease type III (GSD III) is an autosomal recessive metabolic disorder and inborn error of metabolism (specifically of carbohydrates) characterized by a deficiency in glycogen debranching enzymes. It is also known as Cori's disease in honor of the 1947 Nobel laureates Carl Cori and Gerty Cori. Other names include Forbes disease in honor of clinician Gilbert Burnett Forbes (1915–2003), an American physician who further described the features of the disorder, or limit dextrinosis, due to the limit dextrin-like structures in cytosol. Limit dextrin is the remaining polymer produced after hydrolysis of glycogen. Without glycogen debranching enzymes to further convert these branched glycogen polymers to glucose, limit dextrinosis abnormally accumulates in the cytoplasm.

<span class="mw-page-title-main">Glycogen branching enzyme</span> Mammalian protein involved in glycogen production

1,4-alpha-glucan-branching enzyme, also known as brancher enzyme or glycogen-branching enzyme is an enzyme that in humans is encoded by the GBE1 gene.

α-Glucosidase Enzyme

α-Glucosidase is a glucosidase located in the brush border of the small intestine that acts upon α(1→4) bonds:

<span class="mw-page-title-main">Phosphorylase kinase</span>

Phosphorylase kinase (PhK) is a serine/threonine-specific protein kinase which activates glycogen phosphorylase to release glucose-1-phosphate from glycogen. PhK phosphorylates glycogen phosphorylase at two serine residues, triggering a conformational shift which favors the more active glycogen phosphorylase “a” form over the less active glycogen phosphorylase b.

<span class="mw-page-title-main">Myophosphorylase</span> Muscle enzyme involved in glycogen breakdown

Myophosphorylase or glycogen phosphorylase, muscle associated (PYGM) is the muscle isoform of the enzyme glycogen phosphorylase and is encoded by the PYGM gene. This enzyme helps break down glycogen into glucose-1-phosphate, so it can be used within the muscle cell. Mutations in this gene are associated with McArdle disease, a glycogen storage disease of muscle.

In enzymology, a 4-alpha-glucanotransferase is an enzyme that catalyzes a chemical reaction that transfers a segment of a 1,4-alpha-D-glucan to a new position in an acceptor carbohydrate, which may be glucose or a 1,4-alpha-D-glucan.

<span class="mw-page-title-main">Phosphorylase kinase, alpha 1</span> Protein-coding gene in the species Homo sapiens

Phosphorylase b kinase regulatory subunit alpha, skeletal muscle isoform is an enzyme that in humans is encoded by the PHKA1 gene. It is the muscle isoform of Phosphorylase kinase (PhK).

<span class="mw-page-title-main">Inborn errors of carbohydrate metabolism</span> Medical condition

Inborn errors of carbohydrate metabolism are inborn error of metabolism that affect the catabolism and anabolism of carbohydrates.

<span class="mw-page-title-main">Alpha glucan</span>

α-Glucans (alpha-glucans) are polysaccharides of D-glucose monomers linked with glycosidic bonds of the alpha form. α-Glucans use cofactors in a cofactor site in order to activate a glucan phosphorylase enzyme. This enzyme causes a reaction that transfers a glucosyl portion between orthophosphate and α-I,4-glucan. The position of the cofactors to the active sites on the enzyme are critical to the overall reaction rate thus, any alteration to the cofactor site leads to the disruption of the glucan binding site.

Amylo-α-1,6-glucosidase is an enzyme with systematic name glycogen phosphorylase-limit dextrin 6-α-glucohydrolase. It catalyses the hydrolysis of unsubstituted glucose units in glycogen linked by α(1→6) bonds to α(1→4)glucose chains.

Glycogen phosphorylase, liver form (PYGL), also known as human liver glycogen phosphorylase (HLGP), is an enzyme that in humans is encoded by the PYGL gene on chromosome 14. This gene encodes a homodimeric protein that catalyses the cleavage of alpha-1,4-glucosidic bonds to release glucose-1-phosphate from liver glycogen stores. This protein switches from inactive phosphorylase B to active phosphorylase A by phosphorylation of serine residue 14. Activity of this enzyme is further regulated by multiple allosteric effectors and hormonal controls. Humans have three glycogen phosphorylase genes that encode distinct isozymes that are primarily expressed in liver, brain and muscle, respectively. The liver isozyme serves the glycemic demands of the body in general while the brain and muscle isozymes supply just those tissues. In glycogen storage disease type VI, also known as Hers disease, mutations in liver glycogen phosphorylase inhibit the conversion of glycogen to glucose and results in moderate hypoglycemia, mild ketosis, growth retardation and hepatomegaly. Alternative splicing results in multiple transcript variants encoding different isoforms [provided by RefSeq, Feb 2011].

Barbara Illingworth Brown was an American biochemist. She worked primarily at Washington University in St. Louis.

References

  1. 1 2 3 GRCh38: Ensembl release 89: ENSG00000162688 - Ensembl, May 2017
  2. 1 2 3 GRCm38: Ensembl release 89: ENSMUSG00000033400 - Ensembl, May 2017
  3. ↑ "Human PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
  4. ↑ "Mouse PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
  5. ↑ Hers HG, Verhue W, Van hoof F (October 1967). "The determination of amylo-1,6-glucosidase". Eur. J. Biochem. 2 (3): 257–64. doi:10.1111/j.1432-1033.1967.tb00133.x. PMID   6078537.
  6. 1 2 3 4 Song HN, Jung TY, Park JT, Park BC, Myung PK, Boos W, Woo EJ, Park KH (June 2010). "Structural rationale for the short branched substrate specificity of the glycogen debranching enzyme GlgX". Proteins. 78 (8): 1847–55. doi:10.1002/prot.22697. PMID   20187119. S2CID   28334066.
  7. 1 2 3 Bao Y, Dawson TL, Chen YT (December 1996). "Human glycogen debranching enzyme gene (AGL): complete structural organization and characterization of the 5' flanking region". Genomics. 38 (2): 155–65. doi:10.1006/geno.1996.0611. PMID   8954797.
  8. 1 2 Woo EJ, Lee S, Cha H, Park JT, Yoon SM, Song HN, Park KH (October 2008). "Structural insight into the bifunctional mechanism of the glycogen-debranching enzyme TreX from the archaeon Sulfolobus solfataricus". J. Biol. Chem. 283 (42): 28641–8. doi: 10.1074/jbc.M802560200 . PMC   2661413 . PMID   18703518.
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  10. 1 2 3 Hondoh H, Saburi W, Mori H, et al. (May 2008). "Substrate recognition mechanism of alpha-1,6-glucosidic linkage hydrolyzing enzyme, dextran glucosidase from Streptococcus mutans". J. Mol. Biol. 378 (4): 913–22. doi:10.1016/j.jmb.2008.03.016. PMID   18395742.
  11. ↑ Chiba S (August 1997). "Molecular mechanism in alpha-glucosidase and glucoamylase". Biosci. Biotechnol. Biochem. 61 (8): 1233–9. doi:10.1271/bbb.61.1233. PMID   9301101.
  12. ↑ McCarter JD, Withers SG (December 1994). "Mechanisms of enzymatic glycoside hydrolysis". Curr. Opin. Struct. Biol. 4 (6): 885–92. doi:10.1016/0959-440X(94)90271-2. PMID   7712292.
  13. ↑ "4-alpha-glucanotransferase - Escherichia coli (strain K12)".
  14. ↑ "Glycogen debranching enzyme - Escherichia coli O139:H28 (strain E24377A / ETEC)". UniProt.
  15. ↑ Dauvillée D, Kinderf IS, Li Z, Kosar-Hashemi B, Samuel MS, Rampling L, Ball S, Morell MK (February 2005). "Role of the Escherichia coli glgX gene in glycogen metabolism". J. Bacteriol. 187 (4): 1465–73. doi:10.1128/JB.187.4.1465-1473.2005. PMC   545640 . PMID   15687211.
  16. ↑ "TreX - Actinoplanes sp. SN223/29". UniProt.
  17. ↑ Park JT, Park HS, Kang HK, Hong JS, Cha H, Woo EJ, Kim JW, Kim MJ, Boos W, Lee S, Park KH (2008). "Oligomeric and functional properties of a debranching enzyme (TreX) from the archaeon Sulfobus solfataricus P2". Biocatalysis and Biotransformation. 26 (1–2): 76–85. doi:10.1080/10242420701806652. S2CID   83831481.
  18. 1 2 Nakayama A, Yamamoto K, Tabata S (August 2001). "Identification of the catalytic residues of bifunctional glycogen debranching enzyme". J. Biol. Chem. 276 (31): 28824–8. doi: 10.1074/jbc.M102192200 . PMID   11375985.
  19. ↑ Gillard BK, White RC, Zingaro RA, Nelson TE (September 1980). "Amylo-1,6-glucosidase/4-alpha-glucanotransferase. Reaction of rabbit muscle debranching enzyme with an active site-directed irreversible inhibitor, 1-S-dimethylarsino-1-thio-beta-D-glucopyranoside". J. Biol. Chem. 255 (18): 8451–7. doi: 10.1016/S0021-9258(18)43517-X . PMID   6447697.
  20. ↑ Chen YT, He JK, Ding JH, Brown BI (December 1987). "Glycogen debranching enzyme: purification, antibody characterization, and immunoblot analyses of type III glycogen storage disease". Am. J. Hum. Genet. 41 (6): 1002–15. PMC   1684360 . PMID   2961257.
  21. ↑ "Glycogen debranching enzyme - Homo sapiens (Human)". UniProt.
  22. 1 2 Gillard BK, Nelson TE (September 1977). "Amylo-1,6-glucosidase/4-alpha-glucanotransferase: use of reversible substrate model inhibitors to study the binding and active sites of rabbit muscle debranching enzyme". Biochemistry. 16 (18): 3978–87. doi:10.1021/bi00637a007. PMID   269742.
  23. ↑ Yamamoto E, Makino Y, Omichi K (May 2007). "Active site mapping of amylo-alpha-1,6-glucosidase in porcine liver glycogen debranching enzyme using fluorogenic 6-O-alpha-glucosyl-maltooligosaccharides". J. Biochem. 141 (5): 627–34. doi:10.1093/jb/mvm065. PMID   17317688.
  24. ↑ Zhai, Liting; Feng, Lingling; Xia, Lin; Yin, Huiyong; Xiang, Song (2016-04-18). "Crystal structure of glycogen debranching enzyme and insights into its catalysis and disease-causing mutations". Nature Communications. 7: ncomms11229. Bibcode:2016NatCo...711229Z. doi:10.1038/ncomms11229. PMC   4837477 . PMID   27088557.
  25. ↑ "Genes (Genetic Home Reference a service of U.S. National Library of Medicine" . Retrieved February 29, 2012.
  26. ↑ Ding JH, de Barsy T, Brown BI, Coleman RA, Chen YT (January 1990). "Immunoblot analyses of glycogen debranching enzyme in different subtypes of glycogen storage disease type III". J. Pediatr. 116 (1): 95–100. doi:10.1016/S0022-3476(05)81652-X. PMID   2295969.
  27. ↑ Monga SP (2010). Molecular Pathology of Liver Diseases (Molecular Pathology Library). Berlin: Springer. ISBN   978-1-4419-7106-7.
  28. 1 2 3 Shen J, Bao Y, Liu HM, Lee P, Leonard JV, Chen YT (July 1996). "Mutations in exon 3 of the glycogen debranching enzyme gene are associated with glycogen storage disease type III that is differentially expressed in liver and muscle". J. Clin. Invest. 98 (2): 352–7. doi:10.1172/JCI118799. PMC   507437 . PMID   8755644.
  29. ↑ Talente GM, Coleman RA, Alter C, Baker L, Brown BI, Cannon RA, et al. (February 1994). "Glycogen storage disease in adults". Ann. Intern. Med. 120 (3): 218–26. doi:10.7326/0003-4819-120-3-199402010-00008. PMID   8273986. S2CID   24896145.
  30. ↑ Kishnani PS, Austin SL, Arn P, Bali DS, Boney A, Case LE, et al. (July 2010). "Glycogen storage disease type III diagnosis and management guidelines". Genetics in Medicine. 12 (7): 446–63. doi: 10.1097/GIM.0b013e3181e655b6 . PMID   20631546.
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