Glycogen debranching enzyme

A debranching enzyme is a molecule that helps facilitate the breakdown of glycogen 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. When glycogen breakdown is compromised by mutations in the glycogen debranching enzyme, metabolic diseases such as Glycogen storage disease type III can result.^[1][2]

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 catylized 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".^[3]

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. Before phosphorylase can resume catabolism, debranching enzymes perform two functions:

• 4-α-D-glucanotransferase (EC 2.4.1.25), or glucosyltransferase, transfers three glucose residues from the four-residue glycogen branch to a nearby branch. This leaves on the donor branch a single glucose residue, connected to the main chain by an alpha-1,6 linkage.^[4]

• Amylo-α-1,6-glucosidase (EC 3.2.1.33), or glucosidase, cleaves the remaining alpha-1,6 linkage, producing glucose and a linear chain of glycogen.^[4]

Structure and activity

Two enzymes

In E. coli and other bacteria, glucosyltransferase and glucosidase functions are performed by two distinct enzymes. In E. coli, Glucose transfer is performed by 4-alpha-glucanotransferase, a 78.5 kDa protein coded for by the gene malQ.^[5] A second protein, referred to as debranching enzyme, performs α-1,6-glucose cleavage. This enzyme has a molecular mass of 73.6 kDa, and is coded for by the gene glgX.^[6] Activity of the two enzymes is not always necessarily coupled. In E. coli glgX selectively catalyzes the cleavage of 4-subunit branches, without the action of glucanotransferase. The product of this cleavage, maltotetraose, is further degraded by maltodextrin phosphorylase.^[1][7]

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.^[1]

One enzyme with two catalytic sites

In mammals and yeast, a single enzyme performs both debranching functions.^[8] The human glycogen debranching enzyme (gene: AGL) is a monomer with a molecular weight of 175 kDa. The two catalytic actions of AGL can function independently of each other, demonstrating that multiple active sites are present. Glycogen debranching enzyme is the only known eukaryotic enzyme that contains multiple catalytic sites and is active as a monomer.^[9][10]

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.^[8] In addition to these two active sites, AGL appears to contain a third active site that allows it to bind to a glycogen polymer.^[11] Despite these advances, the complete structure of GDE in eukaryotes has yet to be determined.^[3] 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.^[3][12] 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.^[1][13]

Pathology

When GDE activity is compromised, the body cannot effectively release stored glycogen, resulting in Glycogen storage disease type III. Different types of Glycogen Storage Disease type III have been identified. All forms exhibit decreased GDE activity in the liver, but some show decreased activity in the muscles as well. These different manifestation produce varied symptoms, which can include hepatomegaly, hypoglycemia in children, short stature, myopathy, and cardiomyopathy.^[2][14]

References

External Links

• GeneReviews/NCBI/NIH/UW entry on Glycogen Storage Disease Type III
• OMIM entries on Glycogen Storage Disease Type III
• MeSH Glycogen+debranching+enzyme