GLYCOGEN METABOLISM

Which enzyme is the rate-limiting step in glycogenolysis?

• A. Glycogen synthase

• B. Glycogen phosphorylase

• C. Phosphoglucomutase

• D. UDP-glucose pyrophosphorylase

• Answer: B. Glycogen phosphorylase
  Explanation: Glycogen phosphorylase initiates glycogen breakdown by cleaving α-1,4 glycosidic bonds.

What is the primary function of liver glycogen?

• A. Produce ATP during contraction

• B. Maintain blood glucose levels

• C. Serve as a long-term energy reserve

• D. Provide glucose-6-phosphate for glycolysis

• Answer: B. Maintain blood glucose levels
  Explanation: Liver glycogen is mobilized to prevent hypoglycemia during fasting.

Which enzyme catalyzes branch formation in glycogen synthesis?

• A. Glycogen phosphorylase

• B. Glycogen synthase

• C. UDP-glucose pyrophosphorylase

• D. Branching enzyme

• Answer: D. Branching enzyme
  Explanation: The branching enzyme forms α-1,6 glycosidic bonds to create glycogen branches.

What is the fate of glucose-6-phosphate in muscle cells?

• A. Converted to free glucose

• B. Released into the bloodstream

• C. Utilized in glycolysis

• D. Converted back to glycogen

Answer: C. Utilized in glycolysis
Explanation: Muscle cells lack glucose-6-phosphatase, so glucose-6-phosphate enters glycolysis for ATP production.

Which molecule activates glycogen synthase?

• A. Glucagon

• B. AMP

• C. Insulin

• D. Epinephrine

Answer: C. Insulin
Explanation: Insulin promotes glycogen synthesis by activating glycogen synthase in response to hyperglycemia.

• Which enzyme catalyzes the conversion of glucose-6-phosphate to glucose-1-phosphate in glycogenesis?

  • A. Phosphoglucomutase

  • B. Glycogen synthase

  • C. UDP-glucose pyrophosphorylase

  • D. Branching enzyme
    

• Answer: A. Phosphoglucomutase
  Explanation: Phosphoglucomutase interconverts glucose-1-phosphate and glucose-6-phosphate during glycogen synthesis and degradation.

What is the role of glycogenin in glycogenesis?

• A. Extends glycogen chains

• B. Acts as a primer for glycogen synthesis

• C. Breaks down glycogen

• D. Transfers glucose units to glycogen

Answer: B. Acts as a primer for glycogen synthesis
Explanation: Glycogenin initiates glycogen synthesis by adding the first glucose molecules to itself.

Which molecule directly inhibits glycogen phosphorylase?

• A. Glucose

• B. ATP

• C. Glucagon

• D. Calcium

• Answer: A. Glucose
  Explanation: In the liver, glucose binds glycogen phosphorylase, causing its inactivation to stop glycogen breakdown.

What is the primary stimulus for glycogenolysis in muscle during exercise?

• A. Insulin

• B. Glucagon

• C. Epinephrine and AMP

• D. NADH

Answer: C. Epinephrine and AMP
Explanation: Epinephrine and AMP indicate energy demand and activate glycogenolysis for ATP production.

What prevents free glucose from being released in muscle cells?

• A. Lack of glycogen phosphorylase

• B. Lack of glucose-6-phosphatase

• C. Low ATP levels

• D. High glucose levels

Answer: B. Lack of glucose-6-phosphatase
Explanation: Muscle cells lack glucose-6-phosphatase, so glucose-6-phosphate enters glycolysis rather than being released into the bloodstream.

Which enzyme converts glucose-1-phosphate to glucose-6-phosphate?

Answer: Phosphoglucomutase.
Explanation: This enzyme catalyzes the interconversion of glucose-1-phosphate and glucose-6-phosphate.

Name the key regulatory enzyme for glycogen synthesis.

Answer: Glycogen synthase.
Explanation: Glycogen synthase catalyzes the addition of glucose units to glycogen chains.

What is the main symptom of Von Gierke’s disease?

Answer: Hypoglycemia.
Explanation: Impaired glucose release from the liver causes dangerously low blood sugar levels.

What activates glycogen phosphorylase in muscles?

Answer: AMP, Ca²⁺, and epinephrine.
Explanation: These signals indicate energy demand and stimulate glycogen breakdown.

Which enzyme creates branches in glycogen?

Answer: Branching enzyme.
Explanation: This enzyme forms α-1,6 glycosidic bonds, enhancing glycogen solubility and storage efficiency.

Branching in glycogen increases its solubility and accessibility.

• Answer: True
  Explanation: The branching enzyme increases glycogen’s solubility and creates multiple terminal points for enzymatic access.

Liver glycogen levels remain constant throughout the day.

Answer: False
Explanation: Liver glycogen fluctuates based on feeding and fasting states, depleting during fasting and replenishing after meals.

Muscle glycogen stores are primarily used for systemic glucose regulation.

Answer: False
Explanation: Muscle glycogen is used locally for ATP production during contraction, not for maintaining blood glucose levels.

Glycogen synthase is active when it is phosphorylated.

Answer: False
Explanation: Glycogen synthase is active in its dephosphorylated state, which is promoted by insulin

Glycogen phosphorylase is activated by calcium during muscle contraction.

6. Answer: True
   Explanation: Calcium released during muscle contraction activates phosphorylase kinase, which in turn activates glycogen phosphorylase to provide glucose-6-phosphate.

Which enzyme converts glucose-1-phosphate to glucose-6-phosphate?

Answer: Phosphoglucomutase.
Explanation: This enzyme catalyzes the interconversion of glucose-1-phosphate and glucose-6-phosphate.

Why is glycogen highly branched, and what advantage does this provide?

Answer: Glycogen's branching increases its solubility and provides multiple sites for enzymatic action during glycogenesis and glycogenolysis, enabling faster synthesis and breakdown.

What are the two key enzymes involved in glycogen degradation, and what roles do they play?
Answer:

• Glycogen Phosphorylase: Removes glucose units from glycogen as glucose-1-phosphate by cleaving α-1,4 glycosidic bonds.

• Debranching Enzyme: Resolves α-1,6 branches by transferring glucose residues and hydrolyzing branch points.

Describe the role of UDP-glucose in glycogen synthesis.

Answer: UDP-glucose is the activated form of glucose. It acts as the immediate donor of glucose units for glycogen elongation, facilitated by glycogen synthase.

What is the difference between the function of liver glycogen and muscle glycogen?

Answer:

• Liver Glycogen: Maintains blood glucose levels during fasting by releasing glucose into the bloodstream.

• Muscle Glycogen: Provides glucose-6-phosphate for ATP production during muscle contraction, without contributing to blood glucose.