Glycogen Breakdown Is Catalyzed by Glycogen Phosphorylase

In skeletal muscle and liver, the glucose units of the outer branches of glycogen enter the glycolytic pathway through the action of three enzymes: glycogen phos-phorylase, glycogen debranching enzyme, and phos-phoglucomutase. Glycogen phosphorylase catalyzes the reaction in which an (a 1n4) glycosidic linkage between two glucose residues at a nonreducing end of glycogen undergoes attack by inorganic phosphate (Pi), removing the terminal glucose residue as a-d-glucose 1-phos-phate (Fig. 15-3). This phosphorolysis reaction is different from the hydrolysis of glycosidic bonds by amylase during intestinal degradation of dietary glyco-gen and starch. In phosphorolysis, some of the energy of the glycosidic bond is preserved in the formation of the phosphate ester, glucose 1-phosphate.

Pyridoxal phosphate is an essential cofactor in the glycogen phosphorylase reaction; its phosphate group acts as a general acid catalyst, promoting attack by Pi on the glycosidic bond. (This is an unusual role for this cofactor; its more typical role is as a cofactor in amino acid metabolism; see Fig. 18-6.)

Glycogen phosphorylase acts repetitively on the nonreducing ends of glycogen branches until it reaches a point four glucose residues away from an (a1n6) branch point (see Fig. 7-15), where its action stops. Further degradation by glycogen phosphorylase can occur only after the debranching enzyme, formally known as oligo (a1n6) to (a1n4) glucantrans-ferase, catalyzes two successive reactions that transfer

FIGURE 15-3 Removal of a terminal glucose residue from the nonreducing end of a glycogen chain by glycogen phosphorylase. This process is repetitive; the enzyme removes successive glucose residues until it reaches the fourth glucose unit from a branch point (see Fig. 15-4).

ch2oh ch2oh

FIGURE 15-3 Removal of a terminal glucose residue from the nonreducing end of a glycogen chain by glycogen phosphorylase. This process is repetitive; the enzyme removes successive glucose residues until it reaches the fourth glucose unit from a branch point (see Fig. 15-4).

Pi glycogen phosphorylase

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Gaining Weight 101

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Responses

  • lea
    Why action of glycogen phosphorylase stop away from 4 glucose residues?
    8 years ago
  • iines
    What are the reactants and products of the reaction catalyzed by glycogen phosphorylase?
    1 year ago

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