Chapter 24

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. Complete the interactive matching exercise to see answers.

3. Because muscle maintains glucose for its own use, whereas the liver maintains glucose homeostasis for the whole organism

4. The active site is partly blocked in the T state.

5. Phosphorylase kinase is maximally active when calcium is bound, and it is subsequently phosphorylated.

6. 1. 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 concentration. 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.

   2. 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.

7. 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 concentration.

8. Although glucose 1-phosphate is the actual product of the phosphorylase reaction, glucose 6-phosphate, generated from glucose 1-phosphate by phosphoglucomutase, 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.

9. Glycogen phosphorylase b in the T state. AMP acts as an allosteric activator to stabilize the active R state of glycogen phosphorylase b.

10. Two signals account for the activation of muscle phosphorylase. First, the calcium released during muscle contraction activates the phosphorylase kinase, and makes it a substrate for protein kinase A. Second, epinephrine binds to its G-protein-coupled receptor. The resulting structural changes activate a G_αs protein, which in turn activates adenylate cyclase. Adenylate cyclase synthesizes cAMP, which activates protein kinase A. Protein kinase A phosphorylates phosphoryl kinase, completing the activation phosphorylase kinase. Phosphorylase kinase phosphorylates and activates glycogen phosphorylase.

11. 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.

    C25

12. 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 cAMP into AMP. Fourth, the enzyme protein phosphatase 1 (PP1) removes the phosphoryl groups from phosphorylase kinase and glycogen phosphorylase, converting the enzymes into their inactive forms.

13. Glycogen is an important fuel reserve for several reasons. The controlled breakdown of glycogen and the release of glucose increase the amount of glucose that is available between meals. Hence, glycogen serves as a buffer to maintain blood-glucose concentration. This role of glycogen 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.

14. 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.

15. Glucose 1-arsenate would be formed by the phosphorylase and would spontaneously hydrolyze to glucose and arsenate. Glucose liberated by phosphorylase in the presence of arsenate would have to be phosphorylated by hexokinase at the expense of a molecule of ATP.

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

17. Liver phosphorylase a is inhibited by glucose, which facilitates the R → T transition. Muscle phosphorylase is insensitive to glucose.

18. As an unbranched polymer, amylose has only one nonreducing end. Therefore, only one glycogen phosphorylase molecule could degrade each 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.

19. (a) B and D; (b) phosphorylase kinase; (c) C to D; (d) B to A; (e) D to C; (f) protein phosphatase 1

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

21. 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.

22. 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.

23. Gluconeogenesis

24. 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.

25. 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.

26. Water is excluded from the active site of phosphorylase 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 wild-type glycogen 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 OH group of tyrosine and occasionally attacks the oxocarbonium ion intermediate to form glucose.

27. As we examined previously and will see again in Chapter 25, one of the consequences of insulin resistance is failure to appropriately inhibit gluconeogenesis and glycogen breakdown. Consequently, the inhibition of liver glycogen phosphorylase would help to ameliorate the high blood-glucose concentration. The danger is that muscle phosphorylase, which does not contribute glucose to the blood, would also be inhibited.

28. 1. 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.

    2. Substitution of aspartate would give some stimulation, but being that it is smaller than the glutamate, the simulation would be smaller.