Glycogen branching enzyme

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... 

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. 

The exact way of amylopectin biosynthesis in plants is still not known today. In our current research we are using a tandem reaction of two enzymes to synthesize artificial amylopectin or rather (hyper)branched amylose in vitro. One enzyme is responsible for building the linear (amylose) part while the other enzyme introduces die branches, phosphorylase and glycogen branching enzyme respectively. 

Both described enzymes are isolated from natural sources. Phosphorylase is isolated from potatoes whereas the glycogen branching enzyme is produced by various bacterial sources. Depending on the source the properties of the products and the reaction conditions may differ. 

Bao Y, et al. Hepatic and neuromuscular forms of glycogen storage disease type IV caused by mutations in the same glycogen-branching enzyme gene. J Clin Invest. 1996 97(4) 941-8. 

Lossos A, et al. Adult polyglucosan body disease in Ashkenazi Jewish patients carrying the Tyr329Ser mutation in the glycogen-branching enzyme gene. Ann Neurol. 1998 44(6) 867-72. 

Fig. 1-10. Glycogen metabolism. Site of action of glycogen branching enzyme and of amylo-1,6-glucosidase...

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