Glycogen debranching enzyme
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Location (UCSC) | Chr 1: 99.85 – 99.92 Mb | Chr 3: 116.53 – 116.6 Mb | |||||||
PubMed search | [3] | [4] |
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The glycogen debranching enzyme, in humans, is the protein encoded by the gene AGL.[5] This enzyme is essential for the breakdown of glycogen, which serves as a store of glucose in the body. It has separate glucosyltransferase and glucosidase activities.[6][7]
Together with phosphorylases, the enzyme 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.[8] 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]
The two steps of glycogen breakdown, glucosyltransferase and glucosidase, are performed by a single
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.[10][11] Before phosphorylase can resume catabolism, debranching enzymes perform two functions:
- 4-α-D-glucanotransferase (EC 2.4.1.25), or glucosyltransferase, transfers three glucose residues from the four-residue glycogen branch to a nearby branch. This exposes a single glucose residue joined to the glucose chain through an α-1,6 glycosidic linkage[10]
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.
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Structure and activity
Two enzymes
In
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.[9][17] 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][18]
One enzyme with two catalytic sites
In mammals and yeast, a single enzyme performs both debranching functions.[19] 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.[20] Glycogen debranching enzyme is the only known eukaryotic enzyme that contains multiple catalytic sites and is active as a monomer.[21][22]
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.[19] In addition to these two active sites, AGL appears to contain a third active site that allows it to bind to a glycogen polymer.[23] 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.[24]
The structure of the Candida glabrata GDE has been reported.[25] 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 1p21.[11] 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.[5] 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.[23][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).
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]
References
- ^ a b c GRCh38: Ensembl release 89: ENSG00000162688 – Ensembl, May 2017
- ^ a b c GRCm38: Ensembl release 89: ENSMUSG00000033400 – Ensembl, May 2017
- ^ "Human PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
- ^ "Mouse PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
- ^ a b "Genes (Genetic Home Reference a service of U.S. National Library of Medicine". Retrieved February 29, 2012.
- ^ S2CID 28334066.
- ^ PMID 8954797.
- PMID 6078537.
- ^ PMID 18703518.
- ^ ISBN 978-0-7167-8724-2.
- ^ PMID 18395742.
- PMID 9301101.
- PMID 7712292.
- ^ "4-alpha-glucanotransferase - Escherichia coli (strain K12)".
- ^ "Glycogen debranching enzyme - Escherichia coli O139:H28 (strain E24377A / ETEC)". UniProt.
- PMID 15687211.
- ^ "TreX - Actinoplanes sp. SN223/29". UniProt.
- S2CID 83831481.
- ^ PMID 11375985.
- PMID 6447697.
- PMID 2961257.
- ^ "Glycogen debranching enzyme - Homo sapiens (Human)". UniProt.
- ^ PMID 269742.
- PMID 17317688.
- PMID 27088557.
- PMID 2295969.
- ISBN 978-1-4419-7106-7.
- ^ PMID 8755644.
- S2CID 24896145.
- PMID 20631546.
External links
- GeneReviews/NCBI/NIH/UW entry on Glycogen Storage Disease Type III
- OMIM entries on Glycogen Storage Disease Type III
- Glycogen+debranching+enzyme at the U.S. National Library of Medicine Medical Subject Headings (MeSH)