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{{Short description|Mammalian protein found in Homo sapiens}}
{{Infobox_gene}}
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"/>▼
{{infobox enzyme▼
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".<ref name=Woo/>▼
== Function ==▼
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
* [[File:Glycosidase mechanism.png|thumb|Mechanism for cleaving of alpha-1,6 linkage.]]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
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.▼
{|class=wikitable
▲|{{infobox enzyme
| Name = [[4-alpha-glucanotransferase|4-α-glucanotransferase]]
| image =
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| GO_code = 0004134
}}
|{{infobox enzyme
| Name = amylo-α-1,6-glucosidase
| EC_number = 3.2.1.33
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| caption =
}}
|}
▲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"/>
▲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".<ref name=Woo/>
▲== Function ==
▲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/>
▲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 ''[[Escherichia coli|E. coli]]'' and other bacteria, glucosyltransferase and glucosidase functions are performed by two distinct
''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.<ref name=Song/>
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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.<ref name=Nakayama/> In addition to these two [[active site]]s, AGL appears to contain a third active site that allows it to bind to a glycogen polymer.<ref name="Gillard"/> 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.<ref name = Yamamoto/>
[[File:Hypothesized substraight binding ___location.png|thumb|center|Hypothesized sidechain binding sites|600px]]
The structure of the ''Candida glabrata'' GDE has been reported.<ref>{{Cite journal|last1=Zhai|first1=Liting|last2=Feng|first2=Lingling|last3=Xia|first3=Lin|last4=Yin|first4=Huiyong|last5=Xiang|first5=Song|date=2016-04-18|title=Crystal structure of glycogen debranching enzyme and insights into its catalysis and disease-causing mutations|journal=Nature Communications|language=en|volume=7|pages=ncomms11229|doi=10.1038/ncomms11229|pmid=27088557|pmc=4837477|bibcode=2016NatCo...711229Z}}</ref> 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.
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