Glycogen debranching enzyme: Difference between revisions

Revision as of 08:07, 23 May 2014

Revision as of 05:43, 24 July 2014

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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 [[glycogen breakdown]], mobilize glycogen stores. Phosphorylase can only cleave α-1,4- glycosidic bond between adjacent glucose molecules in glycogen but branches 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 thoughthrough 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/>