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Importing Wikidata short description: "Mammalian protein found in Homo sapiens" (Shortdesc helper) |
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{{enzyme
| Name = [[4-alpha-glucanotransferase|4-α-glucanotransferase]]
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| EC_number = 2.4.1.25
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| IUBMB_EC_number = 3/2/1/33
| GO_code = 0004135
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}}
A '''debranching enzyme''' is a molecule that helps facilitate the [[Glycogenolysis|breakdown]] of [[glycogen]], which serves as a store of glucose in the body, through glucosyltransferase and glucosidase activity. Together with [[phosphorylase]]s, 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.<ref name="Hers"/> When glycogen breakdown is compromised by mutations in the glycogen debranching enzyme, metabolic diseases such as [[Glycogen storage disease type III]] can result.<ref name="Song"/><ref name="Bao"/>
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Together with [[phosphorylase]], glycogen debranching enzymes function in [[glycogenolysis|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 [[Glycogenolysis|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.<ref name=Berg/><ref name=Hondoh/> Before phosphorylase can resume catabolism, debranching enzymes perform two functions:
* 4-α-D-glucanotransferase ({{EC number|2.4.1.25}}), or [[glucosyltransferase]], transfers three glucose [[residue (chemistry)|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<ref name="Berg"/>
* Amylo-α-1,6-glucosidase ({{EC number|3.2.1.33}}), or [[glucosidase]], cleaves the remaining alpha-1,6 linkage, producing glucose and a linear chain of glycogen.<ref name=Berg/> The mechanism by which the glucosidase cleaves the α -1,6-linkage is not fully known because the [[amino acids]] in the [[active site]] have not yet been identified. It is thought to proceed through a two step acid base assistance type mechanism, with an [[oxocarbenium]] ion intermediate, and retention of configuration in glucose.<ref name=Molecule/> This is a common method through which to cleave bonds, with an acid below the site of [[hydrolysis]] to lend a proton and a base above to deprotinate a water which can then act as a [[nucleophile]]. These acids and bases are amino acid side chains in the active site of the enzyme. A scheme for the mechanism is shown in the figure below.<ref name=MCCarter/>
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==External links==
{{Commons category}}
* [https://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=gene&part=gsd3 GeneReviews/NCBI/NIH/UW entry on Glycogen Storage Disease Type III]
* [https://www.ncbi.nlm.nih.gov/omim/232400,610860,232400,610860 OMIM entries on Glycogen Storage Disease Type III]
* {{MeSH name|Glycogen+debranching+enzyme}}
{{Glycogenesis and glycogenolysis enzymes}}
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