Glycogen branching

Glycogen Branching Occurs by Transfer of Terminal Chain Segments... 

Branching enzyme is responsible for the a-1,6-branching of the a-1,4-chain in the synthesis of glycogen. Branching enzyme enhances the rate of polysaccharide (endogenous glycogen) synthesis from glucose-1 -phosphate by phosphorylase. 

As indicated in Table 4.12, four regions which constitute the catalytic regions of amylolytic enzymes are conserved in the starch-branching isoenzymes of maize endosperm, rice seed and potato tuber, and the glycogen-branching enzymes of E. coli.286,281 It would be of interest to know whether the seven highly conserved amino acid residues of the a-amylase family listed in bold letters in Table 4.12 are also functional in branching enzyme catalysis. Further experiments, such as chemical modification and analysis of the three-dimensional structure of the BEs, would be needed to determine the nature of its catalytic residues and mechanism. 

Sequence comparisons done by a number of groups, most notably by Svensson and her colleagues (Svensson, 1994), indicate that the various starch- and glycogen-branching enzymes contain consensus sequences to the four regions that are postulated to be the catalytic regions of the a-... 

IV Glycogen-branching enzyme Andersen s disease Failure to thrive cirrhosis and death at age 5 years... 

Glycogen branching requires a single transferase activity. Glycogen debranching requires two enzyme activities a transferase and an a-1,6 glucosidase. Sequence analysis suggests that the two transferases and, perhaps, the a-1,6... 

Glycogen phosphorylase sequentially removes the glu-cosyl residues from a glycogen branch until further action is sterically hindered by a branch point. This occurs when the branch is four residues long from the branch point. Debranching enzyme, a multifunctional protein, first removes the trisaccharide stump on the branch and then removes the branch point itself. The a(l 4) glycosidic bond link-... 

Glycogen debranching enzyme removes a-(l->6) glycogen branches, thereby making additional glucose residues accessible to glycogen phosphorylase. 

In this reaction phosphorylase catalyzes the polymerization of glucose-1-phosphate in order to obtain linear polysaccharide chains with a-(l—>4) glycosidic linkages the glycogen branching enzyme is able to transfer short, a-(l—>4) linked, oligosaccharides from the nonreducing end of starch to an a-(l—>6)... 

Figure 9.10 Schematic representation of the reactions catalyzed by glycogen phosphorylase and glycogen branching enzyme of Deinococcus geothermalis (GBEDg) ...

Glycogen branching enzyme. Amylo-(l,4 to l,6)transglucosidase. Branching enzyme. Amylo-(l,4-l,6)-transglycosylase. 

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