Debranching enzyme, limited

FIGURE 23.15 The reactions of glycogen debranching enzyme. Transfer of a group of three o -(l 4)-linked glucose residues from a limit branch to another branch is followed by cleavage of the o -(l 6) bond of the residue... 

The glycogen phosphorylase reaction degrades glycogen to produce limit dextrins, which are further degraded by debranching enzyme, as already described. 

Type III Limit dextrinosis, Forbes or Corl s disease Absence of debranching enzyme Accumulation of a characteristic branched polysaccharide. 

Figure 6-5. Glycogenolysis. Degradation of glycogen occurs stepwise by hydrolysis of one glucosyl unit at a time from the nonreducing ends by phosphorylase. The limit dextrin occurs as indicated in the second step when there are four glucosyl units remaining to a branch point. Once debranching enzyme has resolved the limit dextrin, degradation by phosphorylase can resume.

The produced glucose from phosphorylase limit dextrin is determined as a measure of the debranching enzyme activity. This assay method is used for the detection of GSD Hid, a subtype in which only the translocase activity of the enzyme (see section 4.6.17.2) is affected [9,12, 20]. 

Amylo-l,6-Glucosidase (Debranching Enzyme, EC 3.2.1.33), Assay with Limit Dextrin... 

The Coordinated Action of the Two Glycogen Debranching Enzyme Activities on Phosphorylase Limit Dextrin... 

Figure 1. The action of the debranching enzyme on phosphorylase limit dextrin...

Figure 10. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of products of limited proteolysis of the debranching enzyme with trypsin. The molecular weights shown of the various bands were determined by the methodology described previously (26). The ratio of debrancher to trypsin was 100 to 1. The incubation was conducted for 60 minutes at 25°. The gel stain was Coomassie Brilliant Blue and the absorbance was measured at 600 nm using a Gilford gel scanner with a 0-1 O.D. chart scale (36).

Figure 15. Rate of the debranching enzyme on phosphorylase limit dextrin in the presence of the irreversible inhibitor dimethyiarsenothioglucose in the presence and absence of a reversible inhibitor (Bis-Tris). Adapted from Gillard et al.

Figure 22. Double-reciprocal plot of glycogen inhibition of debranching enzyme activity on phosphorylase limit dextrin. Adapted from Gillard and Nelson (39).

Figure 24. Dimensional representation of phosphorylase limit dextrin structure and changes produced by action of the debranching enzyme...

Figure 28. Debranched outer tier of phosphorylase limit dextrin by action of debranching enzyme. The elongated main chain (B-chain) is shown after removal of the A-chain glucosyl stub. This corresponds to stage 3 in Figure 24. The Cg-OH group of the main chain residue that formed the branch chain linkage is shown...

Fitzgerald, P. M. D., and Madsen, N. B. J. 1986. Improvement of limit of diffraction and useful X-ray lifetime of crystals of glycogen debranching enzyme. J. Cryst. Growth 76 600-606. 

GSD type III, Cori s disease Deficiency of debranching enzyme activity causes accumulation of limit dextrins. See text. 

The preparation of true, /8-limit dextrins is difficult. The extent of /3-amylolysis after 2 or 24 hr. may be identical, but, after 48 or 72 hr., there may be a 1 or 2% increase which may continue for many hours. Despite the use of the most-highly purified enzymes, it is often impossible to tell whether this small increase represents the action of a minute trace of another enzyme (for example, a-amylase or a debranching enzyme), the hydrolysis of chains which are not freely accessible (for example, buried A-chains ), or a slow action on linkages near to the branch point, for which the enzyme has a lowered affinity. Caution is clearly required in the deduction of limit-dextrin structures. 

Alpha-dextrin endo-1,6-alpha-glucosidase. Pullulanase. Pullulan 6-glucanohydrolase. Limit dextrinase. Debranching enzyme. 3.2.1.41 Starch-debranching enzyme, hydrolyses (l-6)-alpha-glucosidic linkages in pullulan and starch to form maltotriose. 

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