Human Biochemistry, Volume 1 - Murray R. 1993

Bioenergetics and Carbohydrate and Lipid Metabolism
Glycogen Metabolism
Glycogenolysis

The pathway of degradation and Cleavage at branch points (see Fig. 19.1)

The rate-limiting step of glycogenolysis is the reaction catalyzed by phosphorylase:

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The enzyme specifically catalyzes the phosphorolytic cleavage (phosphorolysis) of (1→4) glycosidic bonds in Glycogen, yielding glucose-1-phosphate as the product. Glucose residues are cleaved from the outer ends of the glycogen molecule until approximately four glucose residues remain on the branches extending from each branch point ([1→6] bond) (Fig. 19.4). Another enzyme (α-[1→4] → α-[1→4] glucan transferase) transfers a trisaccharide unit from one chain to another, exposing the (1→6) branch point. The hydrolytic cleavage of (1→6) bonds is carried out by the debranching enzyme (amylo-(1→6)-glucosidase); this appears to be a second catalytic activity of glucan transferase¹. Once the branch is removed, phosphorylase acts on the glycogen chain again. The concerted action of phosphorylase and the Other Enzymes discussed above leads to the complete breakdown of glycogen. The reaction catalyzed by phosphoglucomutase is reversible, allowing glucose-1-phosphate to be converted into glucose-6-phosphate. In The Liver and Kidneys (but not in Skeletal Muscle), a specific enzyme, glucose-6-phosphatase, removes the phosphate group from glucose-6-phosphate. The resulting free glucose then diffuses out of the Cells into the bloodstream. This is The final stage of glycogenolysis in the liver, which leads to an elevation of Blood glucose levels.

¹ Since the (1→6) bond is cleaved hydrolytically, free Glucose is formed rather than glucose-1-phosphate. As a result, blood glucose levels can rise even in the absence of glucose-6-phosphatase—as observed, for instance, in type I glycogenosis (von Gierke's disease, see below) following the administration of Glucagon or adrenaline.

Fig. 19.4. Stages of glycogenolysis.