Glycogen breakdown is controlled by the enzyme glycogen phosphorylase. Which ans

Question

Glycogen breakdown is controlled by the enzyme glycogen phosphorylase. Which answer or answers are correct? (a) Glycogen phosphorylase is subject to both allosteric control and covalent (b) Glycogen phosphorylase is a dimer that exists in two forms, the active (c) Glycogen phosphorylase a is the phosphorylated form and glycogen (d) Glycogen phosphorylase takes its active form when ATP levels are high. modification, phosphorylase b and the less active phosphorylase a configurations phosphorylase b is the nonphosphorylated form. (e) Reversible phosphoregulation of glycogen phosphorylase is mediated through phosphorylation and dephosphorylation of a tyrosine residue at position 14 in each enzyme subunit Which of the following statements about glycogen and its metabolism are/is true? I. Glycogen is a polymer of glucose linked by a(1->4) glycosidic bonds with a(1->4) II. In glycogenolysis, glucose residues are sequentially removed from the nonreducing III. UDP-glucose is produced from glycogen by the action of the enzyme phosphorylase. linked branches every 8-14 residues ends The breakdown of glycogen in skeletal muscle ultimately supplies glucose-6- IV. phosphate, which can enter glycolysis to generate ATP (a) IV only (d) II, IV (e)II, III, IV

Explanation / Answer

Ans.1 Correct statememts are (a) and (c)

Glycogen phosphorylase is subject to both allostearic and covalent modification. Glycogen phosphorylase is a dimer that exists in two forms, in which phosphorylase is more active than phosphorylase b. Glycogen phoshorylase a is the phosphorylated form and phosphorylase b is dephosphorylated form.High energy substrates (ATP, G6P, glucose) inhibit Glycogen phosphorylase, while low energy substrates (AMP, others) activate it. Phosphorylase regulation (inactivation) occurs by hydrolysis of the phosphate ester bond between serine and the phosphoryl group, which is catalyzed by protein phosphatase.

Ans.2 Answer will be option (d) II, IV

Glycogen is made up of glucose which units are linked together linearly by (14) glycosidic bonds from one glucose to the next. Branches are linked to the chains from which they are branching off by (16) glycosidic bonds between the first glucose of the new branch and a glucose on the stem chain.

Glycogen phosphorylase catalyzes the release of glucose residues from the terminal residue of a nonreducing end of a glycogen branch by means of phosphorolysis.

Glycosyltransferase is the enzyme that catalyses the reaction of UDP-glucose and (1,4--D-glucosyl)n to yield UDP .

The breakdown of glycogen in skeletal muscle ultimately supplies glucose-6-phosphate, which can enter glycolysis to generate ATP.

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