Glycogen debranching enzyme: Difference between revisions

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[[ImageFile:Function and Structure of Eukaryotic Glycogen Debranching Enzyme.pngjpg|thumb|Image of debranching enzyme in E. Coli]]

Function and structure of eukaryotic glycogen debranching enzyme

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{{Infobox_gene}}

{{enzyme

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''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/>

Despite these advances, the complete structure of GDE in eukaryotes has yet to be determined.<ref name="Woo"/> The glycogen-degrading enzyme of the [[archaea]] ''[[Sulfolobus solfataricus]]'', istreX, betterprovides characterizedan thaninteresting thoseexample of [[eukaryotes]].using Thea GDEsingle ofactive ''S.site solfataricus''for is knowntwo as treX. Although, like mammalian GDE, treX has bothactivities: amylosidase and glucanotransferase functions,activities. TreX is structurally similar to glgX, and hasshas a mass of 80kD and one active site.<ref name="Woo" /><ref name ="UniProt A8QX06" /> Unlike either glgX or AGL, 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.<ref name="Song" /><ref name="Park" />

=== One enzyme with two catalytic sites ===

In mammals and [[yeast]], a single enzyme performs both debranching functions.<ref name="Nakayama"/> 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.<ref name=Gillard_80/> Glycogen debranching enzyme is the only known eukaryotic enzyme that contains multiple catalytic sites and is active as a monomer.<ref name="Chen"/><ref name="UniProt P35573"/>

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"/> Though the complete structure of the GDE in Eukaryotes is yet to be determined itIt 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]]

The structure of the ''Candida glabrata'' GDE has been reported<ref>{{Cite journal|last=Zhai|first=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|url=https://www.nature.com/articles/ncomms11229|journal=Nature Communications|language=en|volume=7|pages=ncomms11229|doi=10.1038/ncomms11229}}</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.

It was seen that when glucose ‘a’, ‘b’, ‘c’ and ‘0’ in the [[active site]] was hydrolyzed the most rapidly.<ref name=Yamamoto/> This indicated that this region of the glycogen chain bond strongest to the active site because a stronger interaction between enzyme and substrate leads to a more rapid hydrolysis.

Despite these advances, the complete structure of GDE in eukaryotes has yet to be determined.<ref name="Woo"/> The glycogen-degrading enzyme of the [[archaea]] ''[[Sulfolobus solfataricus]]'' is better characterized than those of [[eukaryotes]]. The GDE of ''S. solfataricus'' is known as treX. Although, like mammalian GDE, treX has both amylosidase and glucanotransferase functions, TreX is structurally similar to glgX, and hass a mass of 80kD and one active site.<ref name=Woo/><ref name ="UniProt A8QX06"/> Unlike either glgX or AGL, 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.<ref name=Song/><ref name="Park"/>

== Genetic Location ==