1. Step 1 is the release of glucose 1-phosphate from glycogen by glycogen phosphorylase. Step 2 is the formation of glucose 6-phosphate from glucose 1-phosphate, a reaction catalyzed by phosphoglucomutase. Step 3 is the remodeling of the glycogen by the transferase and the glucosidase.

2. (a) 8; (b) 3; (c) 6; (d) 5; (e) 9; (f) 2; (g) 10; (h) 1; (i) 4; (j) 7.

3. Glycogen is an important fuel reserve for several reasons. The controlled breakdown of glycogen and release of glucose increase the amount of glucose that is available between meals. Hence, glycogen serves as a buffer to maintain blood-glucose levels. Glycogen’s role in maintaining blood-glucose levels is especially important because glucose is virtually the only fuel used by the brain, except during prolonged starvation. Moreover, the glucose from glycogen is readily mobilized and is therefore a good source of energy for sudden, strenuous activity. Unlike fatty acids, the released glucose can provide energy in the absence of oxygen and can thus supply energy for anaerobic activity.

4. As an unbranched polymer, α-amylose has only one nonreducing end. Therefore, only one glycogen phosphorylase molecule could degrade each α-amylose molecule. Because glycogen is highly branched, there are many nonreducing ends per molecule. Consequently, many phosphorylase molecules can release many glucose molecules per glycogen molecule.

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5. The patient has a deficiency of the branching enzyme.

6. In muscle, the b form of phosphorylase is activated by AMP. In the liver, the a form is inhibited by glucose. The difference corresponds to the difference in the metabolic role of glycogen in each tissue. Muscle uses glycogen as a fuel for contraction, whereas the liver uses glycogen to maintain blood-glucose levels.

7. Cells maintain the [P_i]/[glucose 1-phosphate] ratio at greater than 100, substantially favoring phosphorolysis. We see here an example of how the cell can alter the free-energy change to favor a reaction taking place by altering the ratio of substrate and product.

8. The high level of glucose 6-phosphate in von Gierke disease, resulting from the absence of glucose 6-phosphatase or the transporter, shifts the allosteric equilibrium of phosphorylated glycogen synthase toward the active form.

9. The phosphoryl donor is glucose 1,6-bisphosphate, which is formed from glucose 1-phosphate and ATP in a reaction catalyzed by phosphoglucokinase.

10. The different manifestations correspond to the different roles of the liver and muscle. Liver glycogen phosphorylase plays a crucial role in the maintenance of blood-glucose levels. Recall that glucose is the primary fuel for the brain. Muscle glycogen phosphorylase provides glucose only for the muscle and, even then, only when the energy needs of the muscle are high, as during exercise. The fact that there are two different diseases suggests that there are two different isozymic forms of the glycogen phosphorylase—a liver-specific isozyme and a muscle-specific isozyme.

11. Water is excluded from the active site to prevent hydrolysis. The entry of water could lead to the formation of glucose rather than glucose 1-phosphate. A site-specific mutagenesis experiment is revealing in this regard. In phosphorylase, Tyr 573 is hydrogen bonded to the 2′-OH group of a glucose residue. The ratio of glucose 1-phosphate to glucose product is 9000 : 1 for the wild-type enzyme, and 500 : 1 for the Phe 573 mutant. Model building suggests that a water molecule occupies the site normally filled by the phenolic OH group of tyrosine and occasionally attacks the oxocarbonium ion intermediate to form glucose.

12. The amylase activity was necessary to remove all of the glycogen from the glycogenin. Recall that glycogenin synthesizes oligosaccharides of about eight glucose units, and then activity stops. Consequently, if the glucose residues are not removed by extensive amylase treatment, glycogenin will not function.

13. The substrate can be handed directly from the transferase site to the debranching site.

14. During exercise, [ATP] falls and [AMP] rises. Recall that AMP is an allosteric activator of glycogen phosphorylase b. Thus, even in the absence of covalent modification by phosphorylase kinase, glycogen is degraded.

15. Although glucose 1-phosphate is the actual product of the phosphorylase reaction, glucose 6-phosphate is a more versatile molecule with respect to metabolism. Among other fates, glucose-6-phosphate can be processed to yield energy or building blocks. In the liver, glucose 6-phosphate can be converted into glucose and released into the blood.

16. Epinephrine binds to its G-protein-coupled receptor. The resulting structural changes activate a G_α protein, which in turn activates adenylate cyclase. Adenylate cyclase synthesizes cAMP, which activates protein kinase A. Protein kinase A partly activates phosphoryl kinase, which phosphorylates and activates glycogen phosphorylase. The calcium released during muscle contraction also activates the phosphorylase kinase, leading to further stimulation of glycogen phosphorylase.

17. First, the signal-transduction pathway is shut down when the initiating hormone is no longer present. Second, the inherent GTPase activity of the G protein converts the bound GTP into inactive GDP. Third, phosphodiesterases convert cyclic AMP into AMP. Fourth, PP1 removes the phosphoryl group from glycogen phosphorylase, converting the enzyme into the usually inactive b form.

18. It prevents both from operating simultaneously, which would lead to a useless expenditure of energy.

19. All these symptoms suggest central nervous system problems. If exercise is exhaustive enough or the athlete has not prepared well enough or both, liver glycogen also can be depleted. The brain depends on glucose derived from liver glycogen. The symptoms suggest that the brain is not getting enough fuel (hypoglycemia).

20. Liver phosphorylase a is inhibited by glucose, which facilitates the R → T transition. This transition releases PP1, which inactivates glycogen breakdown and stimulates glycogen synthesis. Muscle phosphorylase is insensitive to glucose.

21. (a) 4; (b) 1; (c) 5; (d) 10; (e) 7; (f) 2; (g) 8; (h) 9; (i) 6; (j) 3.

22. Phosphoglucomutase, UDP-glucose pyrophosphorylase, pyrophosphatase, glycogenin, glycogen synthase, and branching enzyme.

23. The enzyme pyrophosphatase converts the pyrophosphate into two molecules of inorganic phosphate. This conversion renders the overall reaction irreversible.

24. The presence of high concentrations of glucose 6-phosphate indicates that glucose is abundant and that it is not being used by glycolysis. Therefore, this valuable resource is saved by incorporation into glycogen.

25. Free glucose must be phosphorylated at the expense of a molecule of ATP. Glucose 6-phosphate derived from glycogen is formed by phosphorolytic cleavage, sparing one molecule of ATP. Thus, the net yield of ATP when glycogen-derived glucose is processed to pyruvate is three molecules of ATP compared with two molecules of ATP from free glucose.

26. Breakdown: Phosphoglucomutase converts glucose 1-phosphate, liberated from glycogen breakdown, into glucose 6-phosphate, which can be released as free glucose (liver) or processed in glycolysis (muscle and liver). Synthesis: Converts glucose 6-phosphate into glucose 1-phosphate, which reacts with UTP to form UDP-glucose, the substrate for glycogen synthase.

27.

28. In principle, having glycogen be the only primer for the further synthesis of glycogen should be a successful strategy. However, if the glycogen granules were not evenly divided between daughter cells, glycogen stores for future generations of cells might be compromised. Glycogenin synthesizes the primer for glycogen synthase.

29. Insulin binds to its receptor and activates the tyrosine kinase activity of the receptor, which in turn triggers a pathway that activates protein kinases. The kinases phosphorylate and inactivate glycogen synthase kinase. Protein phosphatase 1 then removes the phosphate from glycogen synthase and thereby activates the synthase.

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30.

31. In the liver, glucagon stimulates the cAMP-dependent pathway that activates protein kinase A. Epinephrine binds to a 7TM α-adrenergic receptor in the liver plasma membrane, which activates phospholipase C and the phosphoinositide cascade. This activation causes calcium ions to be released from the endoplasmic reticulum, which bind to calmodulin, and further stimulates phosphorylase kinase and glycogen breakdown.

32.

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33. Phosphorylase, transferase, glucosidase, phosphoglucomutase, and glucose 6-phosphatase.

34. Glucose is an allosteric inhibitor of phosphorylase a. Hence, crystals grown in its presence are in the T state. The addition of glucose 1-phosphate, a substrate, shifts the R-to-T equilibrium toward the R state. The conformational differences between these states are sufficiently large that the crystal shatters unless it is stabilized by chemical cross-links.

35. Galactose is converted into UDP-galactose to eventually form glucose 6-phosphate.

36. This disease can also be produced by a mutation in the gene that encodes the glucose 6-phosphate transporter. Recall that glucose 6-phosphate must be transported into the lumen of the endoplasmic reticulum to be hydrolyzed by phosphatase. Mutations in the other three essential proteins of this system can likewise lead to von Gierke disease.

37. (a) Apparently, the glutamate, with its negatively charged R group, can mimic to some extent the presence of a phosphoryl group on serine. That the stimulation is not as great is not surprising in that the carboxyl group is smaller and not as charged as the phosphate. (b) Substitution of aspartate would give some stimulation, but because it is smaller than the glutamate, the stimulation would be smaller.

38. (a) Glycogen was too large to enter the gel and, because analysis was by western blot with the use of an antibody specific to glycogenin, we would not expect to see background proteins.

(b) α-Amylase degrades glycogen, releasing the protein glycogenin, which can be visualized by a western blot.

(c) Glycogen phosphorylase, glycogen synthase, and protein phosphatase 1. These proteins might be visible if the gel were stained for protein, but a western analysis reveals the presence of glycogenin only.

39. (a) The smear was due to molecules of glycogenin with increasingly large amounts of glycogen attached to them.

(b) In the absence of glucose in the medium, glycogen is metabolized, resulting in a loss of the high-molecular-weight material.

(c) Glycogen could have been resynthesized and added to the glycogenin when the cells were fed glucose again.

(d) No difference between lanes 3 and 4 suggests that, by 1 hour, the glycogen molecules had attained maximum size in this cell line. Prolonged incubation does not apparently increase the amount of glycogen.

(e) α-Amylase removes essentially all of the glycogen, and so only the glycogenin remains.