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A debranching enzyme is a molecule that helps facilitate the breakdown of glycogen, which serves as a store of glucose in the body, through glucosyltransferase and glucosidase activity. Together with phosphorylases, 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.〔 When glycogen breakdown is compromised by mutations in the glycogen debranching enzyme, metabolic diseases such as Glycogen storage disease type III can result.〔〔 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".〔 == 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, 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.〔〔 Before phosphorylase can resume catabolism, debranching enzymes perform two functions: * 4-α-D-glucanotransferase (), 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〔 * Amylo-α-1,6-glucosidase (), or glucosidase, cleaves the remaining alpha-1,6 linkage, producing glucose and a linear chain of glycogen.〔 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.〔 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.〔 File:Glycosidase mechanism.png 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「glycogen debranching enzyme」の詳細全文を読む スポンサード リンク
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