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Questions and Answers
What is the primary function of glycogen phosphorylase in glycogen breakdown?
What is the primary function of glycogen phosphorylase in glycogen breakdown?
Which enzyme is responsible for converting glucose 1-phosphate into glucose 6-phosphate?
Which enzyme is responsible for converting glucose 1-phosphate into glucose 6-phosphate?
What advantage does phosphorolytic cleavage have over hydrolytic cleavage in glycogen breakdown?
What advantage does phosphorolytic cleavage have over hydrolytic cleavage in glycogen breakdown?
Why can glycogen phosphorylase only partially degrade glycogen?
Why can glycogen phosphorylase only partially degrade glycogen?
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How does the charged state of glucose 1-phosphate affect its transport in muscle cells?
How does the charged state of glucose 1-phosphate affect its transport in muscle cells?
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Which statement accurately describes the process of phosphorolysis?
Which statement accurately describes the process of phosphorolysis?
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What is the fate of glucose released from hydrolytic cleavage of glycogen?
What is the fate of glucose released from hydrolytic cleavage of glycogen?
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What hinders glycogen phosphorylase from completely degrading glycogen molecules?
What hinders glycogen phosphorylase from completely degrading glycogen molecules?
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What is the primary energy source for type I muscle fibers during endurance activities?
What is the primary energy source for type I muscle fibers during endurance activities?
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Why is liver phosphorylase insensitive to regulation by AMP?
Why is liver phosphorylase insensitive to regulation by AMP?
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The conversion of phosphorylase b to phosphorylase a in both liver and muscle requires which process?
The conversion of phosphorylase b to phosphorylase a in both liver and muscle requires which process?
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In skeletal muscle, what is the role of phosphorylase kinase?
In skeletal muscle, what is the role of phosphorylase kinase?
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Which type of muscle fiber is primarily responsible for quick ATP generation in the absence of oxygen?
Which type of muscle fiber is primarily responsible for quick ATP generation in the absence of oxygen?
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What initiates the activation of phosphorylase kinase in muscle tissues?
What initiates the activation of phosphorylase kinase in muscle tissues?
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What is the role of glucagon and epinephrine in glycogen metabolism?
What is the role of glucagon and epinephrine in glycogen metabolism?
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Which structure contains the active site for phosphorylase kinase?
Which structure contains the active site for phosphorylase kinase?
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What differentiates phosphorylase a from phosphorylase b?
What differentiates phosphorylase a from phosphorylase b?
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Which fiber type has low levels of glycogen phosphorylase due to its energy metabolism reliance?
Which fiber type has low levels of glycogen phosphorylase due to its energy metabolism reliance?
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Which enzyme hydrolyzes the alpha-1,6 linkage during glycogen degradation?
Which enzyme hydrolyzes the alpha-1,6 linkage during glycogen degradation?
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What role does phosphoglucomutase play in glycogen metabolism?
What role does phosphoglucomutase play in glycogen metabolism?
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What is the default state of liver phosphorylase?
What is the default state of liver phosphorylase?
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What inhibits muscle phosphorylase b?
What inhibits muscle phosphorylase b?
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What is the action of the transferase enzyme during glycogen degradation?
What is the action of the transferase enzyme during glycogen degradation?
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Which enzyme allows glucose to exit the liver into the blood?
Which enzyme allows glucose to exit the liver into the blood?
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How does the presence of glucose affect liver phosphorylase?
How does the presence of glucose affect liver phosphorylase?
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What molecule stabilizes the active state of muscle phosphorylase b?
What molecule stabilizes the active state of muscle phosphorylase b?
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What is formed when glucose 1-phosphate undergoes action by phosphoglucomutase?
What is formed when glucose 1-phosphate undergoes action by phosphoglucomutase?
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Which of these is characteristic of glycogen phosphorylase regulation?
Which of these is characteristic of glycogen phosphorylase regulation?
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What musculoskeletal function primarily requires activation of muscle phosphorylase?
What musculoskeletal function primarily requires activation of muscle phosphorylase?
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What occurs during the equilibrium shift of phosphorylase a?
What occurs during the equilibrium shift of phosphorylase a?
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How does ATP function in muscle phosphorylase regulation?
How does ATP function in muscle phosphorylase regulation?
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What is a function of glycogen degradation in the liver?
What is a function of glycogen degradation in the liver?
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Study Notes
Glycogen Breakdown
- Glycogen breakdown requires four enzymes: one for glycogen degradation, two for remodeling, and one for product conversion.
- Glycogen phosphorylase is the key regulatory enzyme. It cleaves glycogen by adding orthophosphate, a process called phosphorolysis, to produce glucose 1-phosphate.
- Phosphorolysis is energetically favorable because the glucose is already phosphorylated.
- Phosphorylase sequentially removes glucosyl residues from the non-reducing ends of glycogen.
- Glucose 1-phosphate is readily converted to glucose 6-phosphate by phosphoglucomutase.
- Phosphorylase stops breaking down glycogen when it encounters branch points. Alpha-1,6 bonds are not cleaved.
Debranching Enzyme
- Alpha-1,6 bonds at branch points require further action.
- A transferase shifts three glucosyl units to another branch.
- A debranching enzyme (alpha-1,6 glucosidase) hydrolyzes the remaining alpha-1,6 bond, releasing a free glucose molecule.
- The free glucose is phosphorylated to glucose 6-phosphate by hexokinase.
- This whole process transforms the branched structure into a linear one, allowing phosphorylase to continue.
- In eukaryotes, these transferase and debranching enzyme activity occur in a single polypeptide.
Glucose-6-Phosphate
- Glucose 1-phosphate, produced by glycogen breakdown, must be converted to glucose 6-phosphate.
- Phosphoglucomutase facilitates this conversion by shifting a phosphoryl group.
- The enzyme uses a phosphorylated serine residue in its active site to transfer the phosphoryl group from C-1 to C-6, then back to the serine, yielding the product glucose 6-phosphate.
Liver Function
- The liver maintains blood glucose levels.
- During activity or between meals, the liver releases glucose into the blood.
- The liver contains glucose 6-phosphatase that cleaves glucose 6-phosphate, releasing free glucose that can be transported out of the liver into the blood.
- This phosphatase is also involved in gluconeogenesis.
- Glucose 6-phosphatase is on the smooth endoplasmic reticulum.
Glycogen Phosphorylase Regulation
- Glycogen phosphorylase is regulated by energy state signals and hormones.
- Phosphorylase exists in two forms: phosphorylase a (active) and phosphorylase b (inactive).
- Phosphorylase a is generally more active. This form is in equilibrium between an active relaxed (R) state and a less active tense (T) state.
- Muscle phosphorylase is primarily in the b form unless stimulated during muscle contraction.
- Liver phosphorylase is usually primed for activity as the liver is responsible for maintaining glucose levels.
- Muscle phosphorylase b is activated by AMP, which stabilizes the R state.
- ATP and glucose 6-phosphate inhibit the b form through feedback inhibition.
- The liver version shows a prominent response triggered by the binding of glucose to the active site, shifting from the active R to the inactive T state. This allows for modulation depending on blood glucose levels.
- Phosphorylase b converts to phosphorylase a by phosphorylation of a serine residue.
Phosphorylase Kinase Regulation
- Phosphorylase kinase is a large enzyme crucial for activating phosphorylase b.
- Phosphorylase kinase activation directly depends on the binding of calcium ions to calmodulin, a calcium sensor.
- This activation is paramount in the muscle, as calcium release triggers contraction. Epinephrine triggers this activation.
- Additional activation requires the phosphorylation of the protein kinase A targets of the kinase.
Muscle Fiber Types and Glycogen Metabolism
- Skeletal muscle has three fiber types: type I (slow-twitch), type IIb (fast-twitch), and type IIa (intermediate).
- Type I fibers use cellular respiration, rely less on glycogen and glycogen phosphorylase.
- Type IIb fibers rely on glycogen for energy; glycogen and phosphorylase are abundant. Contain high amounts of glycolytic enzymes.
- Type IIa are trainable, and can interconvert their glycogen metabolism capacity.
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Description
This quiz covers the key aspects of glycogen breakdown, including the enzymes involved and their functions. Understand the roles of glycogen phosphorylase and the debranching enzyme in the context of carbohydrate metabolism.
