Chapter 24 - Glycogen Degradation Flashcards
The immediate product of glycogen phosphorylase is
glucose 1-phosphate
Glycogen phosphorylase degrades glycogen by a(n) _______________ reaction.
phosphorolysis
What are the three steps in glycogen degradation, and what enzymes catalyze each step?
Step 1 - The release of glucose 1-phosphate from glycogen by glycogen phosphorylase
Step 2 - The formation of glucose 6-phosphate from glucose 1-phosphate. Catalyzed by phosphoglucomutase
Step 3 - The remodeling of the glycogen by the transferase and the glucosidase
Why is the control of glycogen different in muscle and liver?
Because muscle maintains glucose for its own use, whereas the liver maintains glucose homeostasis for the whole organism.
What structural difference accounts for the fact that the T state of phosphorylase kinase is less active than the R state?
The active site is partly blocked in the T state
Compare the allosteric regulation of phosphorylase in the liver and in the muscle and explain the significance of the difference.
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.
What is the predominant form of glycogen phosphorylase in resting muscle?
Glycogen Phosphorylase b in the T state
Immediately after exercise begins, the resting glycogen phosphorylase is activated. How does this activation take place?
AMP acts as an allosteric activator to stabilize the active R site of glycogen phosphorylase b
Outline the signal-transduction cascade for glycogen degradation in muscle
Two signals account for the activation of a muscle phosphorylase.
1 - the calcium released during muscle contraction activates the phosphorylase kinase by binding to calmodulin
2 - Epinephrine binds to its G-protein-coupled receptor.
The resulting structural changes activate a G protein –> activates adenylate cyclase –> synthesizes cAMP –> activates protein kinase A –> Phosphorylates phosphoryl kinase –> phosphorylates and activates glycogen phosphorylase
There must be a way to shut down glycogen breakdown quickly to prevent the wasteful depletion of glycogen after energy needs have been met. What mechanisms are employed to turn off glycogen breakdown?
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 (PP1) removes the phosphoryl groups from phosphorylase kinase and glycogen phosphorylase, converting the enzymes into their inactive forms.
Glycogen is not as reduced as fatty acids are and consequently not as energy-rich. Why do animals store any energy as glycogen? why not convert all excess fuel into fatty acids?
The controlled breakdown of glycogen and the release of glucose increase the amount of glucose that is available between meals.
Glycogen serves as a buffer to maintain blood-glucose concentration.
the glucose from glycogen is readily mobilized and is a good source of energy for sudden, strenuous activity
the released glucose can provide energy in the absence of oxygen and can supply energy for anaerobic activity.
GLycogen depletion resulting from intense, extensive exercise can lead to exhaustion and the inability to continue exercising. Come people also experience dizziness, an inability to concentrate and a loss of muscle control. Account for these symptoms.
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.
One of the liver’s key roles is the maintenance of blood-glucose concentrations when and organism is fasting, such as during a night’s sleep. Mobilizing liver glycogen requires enzymatic teamwork. Identify the enzymes that are required for the liver to release glucose into the blood.
Phophorylase, transferase, alpha-1,6-glucosidase, phosphoglucomutase, and glucose 6-phosphatase.
What accounts for the fact that liver phosphorylase is a glucose sensor whereas muscle phosphorylase is not?
liver phosporylase a is inhibited by glucose, which facilitates the R –> T transition. Muscle phosphorylase is insensitive to glucose.
Amylose is an unbranched glucose polymer. Why would this polymer not be as effective a storage form of glucose as glycogen?
the 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.
