A-l—>6-Glucosidase

More specific hydrolysis may be achieved by the use of enzymes. Thus, the enzyme a-amylase in saliva and in the gut is able to catalyse hydrolysis of al 4 bonds throughout the starch molecule to give mainly maltose, with some glucose and maltotriose, the trisaccharide of glucose. Amylose is hydrolysed completely by this enzyme, but the al 6 bonds of amylopectin are not affected. Another digestive enzyme, a-l,6-glucosidase, is required for this reaction. Finally, pancreatic maltase completes the hydrolysis by hydrolysing maltose and maltotriose. 

Glycogen degradation requires phosphorylated (active) phosphorylase kinase to maintain the production of phosphorylated (active) phosphorylase a the a-l,6-glucosidase is required to remove branch molecules from partially degraded glycogen, and phosphorylated (active) inhibitor protein is required to inactivate phosphoprotein phosphatase. 

This free glucose molecule is phosphorylated by the glycolytic enzyme hexokinase. Thus, the transferase and a-1,6-glucosidase convert the branched structure into a linear one, which paves the way for further cleavage by phosphorylase. It is noteworthy that, in eukaryotes, the transferase and the a-l,6-glucosidase activities are present in a single 160-kd polypeptide chain, providing yet another example of a bifunctional enzyme (Section 16.2.2). Furthermore, these enzymes may have additional features in common (Section 21.4.3). 

Branch points are degraded by the concerted action of an oligosaccharide transferase and an a-l,6-glucosidase. 

Telltale products. A sample of glycogen from a patient with liver disease is incubated with orthophosphate, phosphorylase, the transferase, and the dehranching enzyme (a-l,6-glucosidase). The ratio of glucose 1-phosphate to glucose formed in this mixture is 100. What is the most likely enzymatic deficiency in this patient ... 

The remaining glucose unit that is linked a-1,6 at the branch point is released as free glucose by an a-l,6-glucosidase. 

The four units remaining at a branch are removed by the debranching enzyme, which has both glucosyl 4 4 transferase and a-l,6-glucosidase activity. 

How can the remainder of the glycogen molecule be mobilized for use as a fuel Two additional enzymes, a transferase and a l, 6-glucosidase, remodel... 

Branch points in glycogen are removed by debranching enzyme (amylo-a(l,6)-glucosidase). 

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