BIO C HE MIA I I – ĆW ICZ E NI A

L I S T A 6

do wykładu dr. hab. inż. P. Dobryszyckiego

M e t a b o l i z m g l i k o g e n u

1. Answer the following questions about glycogen fragment in Figure 1.

Figure 1. Fragment of glycogen (R represents the rest of the glycogen molecule).

(a) Which residues are at nonreducing ends.

(b) An α-1,6 glycosidic linkage occurs between which residues?

(c) An α-1,4 glycosidic linkage occurs between which residues?

(d) Is the glycogen fragment a substrate for phosphorylase a? Explain.

(e) Is the glycogen fragment a substrate for the debranching enzyme? Explain.

(f) Is the glycogen fragment a substrate for the branching enzyme? Explain.

2. Which of the following statements about the glycogen storage are INCORRECT?

(a) Glycogen is stored in muscles and liver.

(b) Glycogen is a major source of stored energy in the brain.

(c) Glycogen reserves are less rapidly depleted than fat reserves during starvation.

(d) Glycogen nearly fills the nucleus of cells that specialize in glycogen storage.

(e) Glycogen storage occurs in the form of dense granules in the cytoplasm of cells.

3. Match the enzymes that degrade glycogen in the left column with the appropriate properties from the right column.

(a) phosphorylase

(1) is part of a single polypeptide chain with two activities

(b) α-1,6 glucosidase

(2) cleaves α-1,4 glucosidic bonds

(c) transferase

(3) releases glucose

(4) releases glucose 1-phosphate

(5) moves three sugar residues from one chain to another

(6) requires ATP.

4. Explain why the phosphorolytic cleavage of glycogen is more energetically advantageous than its hydrolytic cleavage.

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5. Starting from a glucose residue in glycogen, how many net ATP molecules will be formed in the glycolysis of the residue to pyruvate?

(a) 1

(b) 2

(c) 3

(d) 4

(e) 5

6. Consider the diagram of the different conformational states of muscle glycogen phosphorylase in Figure 2. Answer the following questions.

Figure 2. Conformational states of phosphorylase in muscle.

(a) Which are the active forms of phosphorylase?

(b) Which form requires high levels of AMP to become acivated?

(c) Which conversion is antagonized by ATP and glucose 6-phosphate?

(d) What enzyme catalyzes conversion of C to B?

7. How does regulation of phosphorylase in the liver differ from the scheme for phosphorylase regulation in muscle shown in Figure 2?

8. Indicate which of the following substances have binding sites on phosphorylase. For those that do, give their major roles or effects.

(a) calmodulin

(b) glycogen

(c) pyridoxal phosphate

(d) Ca2+

(e) AMP

(f) Pi

(g) ATP

(h) glucose.

9. Place the following steps of the reaction cascade of glycogen metabolism in the proper sequence.

(a) phosphorylation of protein kinase

(b) formation of cyclic AMP by adenylate cyclase

(c) phosphorylation by phosphorylase b

(d) hormone binding to target cell receptors

(e) phosphorylation of glycogen synthase a and phosphorylase kinase.

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10. Which of the following are common features of both glycogen synthesis and glycogen breakdown?

(a) Both require UDP-glucose.

(b) Both involve glucose 1-phosphate.

(c) Both are driven in part by the hydrolysis of pyrophospate.

(d) Both occur on cytoplasmic glycogen granules.

(e) Both use the same enzyme for branching and debranching.

11. Which of the following statements about glycogen synthase are correct?

(a) It is activated when it is dephosphorylated.

(b) It is activated when it is phosphorylated.

(c) It is activated when it is phosphorylated and in the presence of high levels of glucose 6-phosphate.

(d) It is activated when it is phosphorylated and in the presence of high levels of AMP.

12. Is it true or false that branching in the structure of glycogen increases the rates of its synthesis and degradation. Explain.

13. Which of the following statements about the hormonal regulation glycogen synthesis and degradation are correct?

(a) Insulin increases the capacity of liver to synthesize glycogen.

(b) Insulin is secreted in response to low levels of blood glucose.

(c) Glucagon and epinephrine have opposing effects on glycogen metabolism.

(d) Glucagon simulates the breakdown of glycogen, particularly in the liver.

(e) The effects of the three regulating hormones are mediated by cyclic AMP.

14. Which of the following are effects of glucose on the metabolism of glycogen in liver?

(a) The binding of glucose to phosphorylase a converts this enzyme to the inactive T form.

(b) The T form of phosphorylase a becomes susceptible to action of phosphatase.

(c) The R form of phosphorylase b becomes susceptible to action of phosphorylase kinase.

(d) When phosphorylase a is converted to phosphorylase b, the bound phosphatase is released.

(e) The free phosphatase dephosphorylates and activates glycogen synthase.

15. If glycogen synthase can add a glucose residue to a growing glycogen molecule only if the glucose chain is at least four unit long, how does a new glycogen molecule start?

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