Glycogen Metabolism: Synthesis and Breakdown Indian Medical PG Practice Questions and MCQs
Practice Indian Medical PG questions for Glycogen Metabolism: Synthesis and Breakdown. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Glycogen Metabolism: Synthesis and Breakdown Indian Medical PG Question 1: Which glycogen storage disease does not affect muscles?
- A. Type 4 (Andersen disease)
- B. Type 2 (Pompe disease)
- C. Type 1 (Von Gierke disease) (Correct Answer)
- D. Type 3 (Cori disease)
Glycogen Metabolism: Synthesis and Breakdown Explanation: ***Type 1 (Von Gierke disease)***
- This is caused by a deficiency in **glucose-6-phosphatase**, an enzyme found primarily in the **liver** and **kidneys** but **NOT in muscle tissue**.
- Since muscles do not express glucose-6-phosphatase and rely on glycogen phosphorylase for energy mobilization, this disease **does not affect muscle function**.
- Clinical features include severe **hypoglycemia**, **lactic acidosis**, **hepatomegaly**, and **growth retardation**, but patients have **normal muscle strength and function**.
*Type 2 (Pompe disease)*
- Also known as **acid maltase deficiency**, this disease severely affects **all muscle types** including cardiac, skeletal, and smooth muscles.
- It is a **lysosomal storage disease** causing progressive **muscle weakness**, **cardiomyopathy**, and **respiratory failure**.
- This is the most significant muscle-affecting GSD.
*Type 3 (Cori disease)*
- Caused by **debranching enzyme (amylo-1,6-glucosidase) deficiency**, affecting both **liver and muscles**.
- Patients develop **hepatomegaly**, **hypoglycemia**, and progressive **myopathy** with muscle weakness.
- Muscle involvement is common and clinically significant.
*Type 4 (Andersen disease)*
- Due to **branching enzyme deficiency**, primarily causing **liver cirrhosis** and **hepatosplenomegaly**.
- While mainly hepatic, this disease **can involve muscles** causing hypotonia and muscle weakness in some patients.
- The abnormal, unbranched glycogen ("amylopectin-like") accumulates in multiple tissues including muscle.
Glycogen Metabolism: Synthesis and Breakdown Indian Medical PG Question 2: Most important carbohydrate store for maintaining blood glucose homeostasis -
- A. Blood glucose
- B. Glycogen in adipose tissue
- C. Hepatic glycogen (Correct Answer)
- D. None of the options
Glycogen Metabolism: Synthesis and Breakdown Explanation: ***Hepatic glycogen***
- The liver contains **100-120g of glycogen**, which is the most crucial carbohydrate store for **maintaining blood glucose homeostasis**.
- **Hepatic glycogen** can be mobilized and released as glucose into the bloodstream to supply all body tissues, especially during fasting.
- Although muscle glycogen is quantitatively larger (~400-500g), it cannot contribute to blood glucose as muscle lacks glucose-6-phosphatase.
- The liver's unique ability to release free glucose makes hepatic glycogen the **most metabolically important** carbohydrate store.
*Blood glucose*
- **Blood glucose** (~5g total in circulation) represents carbohydrates available for immediate energy, not a storage form.
- This is far too small to be considered a major carbohydrate reserve.
*Glycogen in adipose tissue*
- **Adipose tissue** primarily stores **fat (triglycerides)**, with negligible glycogen content.
- Adipose tissue plays virtually no role in carbohydrate storage.
*None of the options*
- This is incorrect because **hepatic glycogen** is indeed the most important carbohydrate store for glucose homeostasis.
Glycogen Metabolism: Synthesis and Breakdown Indian Medical PG Question 3: An 8-month-old infant is brought in with poor feeding, lethargy, hypotonia, and hepatomegaly. Labs reveal hypoglycemia and metabolic acidosis. Which condition is most likely?
- A. Hereditary fructose intolerance
- B. Galactosemia
- C. Pompe disease
- D. Von Gierke disease (Correct Answer)
- E. Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency
Glycogen Metabolism: Synthesis and Breakdown Explanation: ***Von Gierke disease***
- **Type I glycogen storage disease** (GSD I) typically presents in infancy with **hypoglycemia** (due to impaired glucose release from glycogen), **hepatomegaly** (due to glycogen accumulation), and **lactic acidosis**.
- Other common findings include **hyperlipidemia** and **hyperuricemia**, while **hypotonia** and **poor feeding** are generalized symptoms stemming from metabolic derangements.
*Hereditary fructose intolerance*
- This condition presents when **fructose** is introduced into the diet, typically after 4-6 months of age, with symptoms like **nausea, vomiting, abdominal pain**, and **hepatomegaly**.
- While it can cause **hypoglycemia** and **metabolic acidosis**, the profound **hypotonia** and general metabolic collapse described in an 8-month-old on a typical diet makes GSD I more likely initially.
*Galactosemia*
- Symptoms usually appear within days or weeks of birth upon the initiation of **milk feeding**, including **vomiting, lethargy, poor feeding, jaundice, hepatomegaly**, and **cataracts**.
- While it causes **hypoglycemia** and can lead to acidosis and hypotonia, the age of presentation and lack of specific mention of jaundice or cataracts makes it a less precise fit.
*Pompe disease*
- Also known as **glycogen storage disease type II**, it is characterized by the accumulation of glycogen in **lysosomes**, primarily affecting muscles.
- The infantile form presents with severe **cardiomyopathy**, **muscle weakness**, and **hypotonia**, but **hypoglycemia** and **hepatomegaly** are not its primary or most prominent features.
*Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency*
- A **fatty acid oxidation disorder** that presents with episodic **hypoglycemia** (particularly during fasting or illness), **lethargy**, and **hepatomegaly**.
- Key distinguishing features include **hypoketotic hypoglycemia** and elevated **dicarboxylic acids** on urine organic acids, but the **lactic acidosis** and overall metabolic profile are more consistent with GSD I.
Glycogen Metabolism: Synthesis and Breakdown Indian Medical PG Question 4: Which is branching enzyme?
- A. Glycogen synthase
- B. Amylo-1, 4-1, 6-transglycolase (Correct Answer)
- C. Glycogen Phosphorylase
- D. Glucose-6 phosphatase
Glycogen Metabolism: Synthesis and Breakdown Explanation: ***Amylo-1, 4-1, 6-transglycolase***
- This enzyme is also known as **glycogen branching enzyme**.
- It catalyzes the formation of **α-1,6-glycosidic bonds** by transferring a segment of four to six glucosyl residues from the non-reducing end of a growing glycogen chain to another chain.
*Glycogen synthase*
- This enzyme is responsible for the **elongation of glycogen chains** by forming **α-1,4-glycosidic bonds**.
- It adds glucose units to the non-reducing end of a pre-existing glycogen primer.
*Glycogen Phosphorylase*
- This enzyme is involved in **glycogen degradation**.
- It catalyzes the **phosphorolytic cleavage** of α-1,4-glycosidic bonds, releasing glucose-1-phosphate.
*Glucose-6 phosphatase*
- This enzyme is primarily found in the **liver** and kidneys and is crucial for **gluconeogenesis** and **glycogenolysis**.
- It dephosphorylates glucose-6-phosphate to **free glucose**, allowing its release into the bloodstream.
Glycogen Metabolism: Synthesis and Breakdown Indian Medical PG Question 5: Glycogen storage disorders are primarily classified under which type of disorders?
- A. Endocrine disorders
- B. Metabolic disorders (Correct Answer)
- C. Genetic disorders
- D. Lysosomal storage disorders
Glycogen Metabolism: Synthesis and Breakdown Explanation: ***Metabolic disorders***
- Glycogen storage disorders involve defects in the enzymes responsible for **glycogen synthesis** or degradation.
- These enzymatic defects lead to abnormal accumulation or breakdown of **glycogen**, thus affecting cellular metabolism.
*Genetic disorders*
- While glycogen storage disorders are **inherited** and thus genetic, their primary classification focuses on the **metabolic pathways** affected.
- This category is too broad and refers to the origin, not the specific functional impairment.
*Lysosomal storage disorders*
- These disorders involve defective lysosomal enzymes leading to the accumulation of various **substrates within lysosomes**.
- Glycogen storage disorders primarily involve enzymes in the **cytoplasm** (or sometimes lysosomes for Pompe disease, but the general classification is metabolic).
*Endocrine disorders*
- Endocrine disorders involve dysfunction of **hormone production** or regulation.
- Glycogen storage diseases are disorders of **carbohydrate metabolism** and do not directly involve hormonal imbalance as their primary pathology.
Glycogen Metabolism: Synthesis and Breakdown Indian Medical PG Question 6: Which of the following is active in dephosphorylated state?
- A. PEPCK
- B. Pyruvate Carboxylase
- C. Glycogen Synthase (Correct Answer)
- D. Glycogen Phosphorylase
Glycogen Metabolism: Synthesis and Breakdown Explanation: ***Glycogen Synthase***
- **Glycogen synthase** is primarily active in its **dephosphorylated state**, which is promoted by insulin and signals glycogen synthesis.
- Dephosphorylation relieves the inhibitory effect of phosphorylation, allowing the enzyme to efficiently add glucose units to a **growing glycogen chain**.
*PEPCK*
- **Phosphoenolpyruvate carboxykinase (PEPCK)** activity is primarily regulated at the transcriptional level, not typically by phosphorylation state for activation.
- Its expression is induced by **glucagon** and **cortisol** during gluconeogenesis.
*Pyruvate Carboxylase*
- **Pyruvate carboxylase** is allosterically activated by **acetyl-CoA** and its activity is not directly regulated by phosphorylation/dephosphorylation in the same manner as glycogen synthase.
- This enzyme plays a key role in **gluconeogenesis** by converting pyruvate to oxaloacetate.
*Glycogen Phosphorylase*
- **Glycogen phosphorylase** is active in its **phosphorylated state**, particularly the 'a' form, which is promoted by glucagon and adrenaline for glycogen breakdown.
- Phosphorylation activates the enzyme, leading to the **breakdown of glycogen** into glucose-1-phosphate.
Glycogen Metabolism: Synthesis and Breakdown Indian Medical PG Question 7: Low insulin to glucagon ratio leads to increase in the activity of
- A. Hexokinase
- B. Glucokinase
- C. Glucose-6-phosphatase (Correct Answer)
- D. Pyruvate kinase
Glycogen Metabolism: Synthesis and Breakdown Explanation: ***Glucose-6-phosphatase***
- A low **insulin to glucagon ratio** signals a state of **low blood glucose**, leading to increased **glucagon** secretion.
- Glucagon activates **gluconeogenesis** and **glycogenolysis** in the liver, and **glucose-6-phosphatase** is a key enzyme in the final step of both pathways, releasing free glucose into the bloodstream.
*Hexokinase*
- This enzyme is responsible for the **phosphorylation of glucose in most tissues** to trap it within the cell for glycolysis.
- Its activity is generally high during periods of **high glucose and insulin levels** to promote glucose utilization.
*Glucokinase*
- This is an isoform of hexokinase found in the **liver and pancreatic beta cells**, with a higher Km for glucose, meaning it is active primarily at **high glucose concentrations**.
- Its activity is increased by **insulin**, promoting glucose uptake and utilization in times of plenty.
*Pyruvate kinase*
- This enzyme catalyzes the final step of **glycolysis**, converting phosphoenolpyruvate to pyruvate.
- Its activity is stimulated by **insulin** and inhibited by **glucagon**, reflecting its role in glucose breakdown, not production.
Glycogen Metabolism: Synthesis and Breakdown Indian Medical PG Question 8: Which glycogen storage disease also presents as a lysosomal storage disease?
- A. Von Gierke's disease
- B. McArdle's disease
- C. Andersen's disease
- D. Pompe's disease (Correct Answer)
Glycogen Metabolism: Synthesis and Breakdown Explanation: ***Pompe's disease***
- Also known as **glycogen storage disease type II**, it is caused by a deficiency of **acid alpha-glucosidase (GAA)**, a *lysosomal enzyme*.
- This deficiency leads to the accumulation of **glycogen in lysosomes**, particularly affecting muscle tissue, thereby earning its classification as both a glycogen storage disease and a lysosomal storage disease.
*Von Gierke's disease*
- This is **glycogen storage disease type I** and is due to a deficiency in **glucose-6-phosphatase**.
- It primarily affects the **liver and kidneys**, causing severe **hypoglycemia** and **lactic acidosis**, but it is not classified as a lysosomal storage disease.
*McArdle's disease*
- This is **glycogen storage disease type V**, caused by a deficiency in **muscle glycogen phosphorylase (myophosphorylase)**.
- It manifests as **exercise intolerance** and muscle pain, but it does not involve lysosomal enzyme defects or glycogen accumulation in lysosomes.
*Andersen's disease*
- This is **glycogen storage disease type IV**, caused by a deficiency in the **glycogen branching enzyme**.
- It leads to the formation of **abnormal glycogen structures**, primarily affecting the liver and causing early liver failure, but it is not a lysosomal storage disorder.
Glycogen Metabolism: Synthesis and Breakdown Indian Medical PG Question 9: When the insulin:glucagon ratio is decreased, which enzyme is active?
- A. Glucokinase
- B. Phosphofructokinase
- C. Hexokinase
- D. Glucose-6-phosphatase (Correct Answer)
Glycogen Metabolism: Synthesis and Breakdown Explanation: ***Glucose-6-phosphatase***
- A decreased insulin:glucagon ratio indicates a **fasting state** or **catabolic state**, promoting glucose production and release rather than storage.
- **Glucose-6-phosphatase** is the key enzyme that enables glucose release from the liver by removing the phosphate group from glucose-6-phosphate, producing free glucose that can exit hepatocytes.
- This enzyme is active during both **gluconeogenesis** and **glycogenolysis** and is only present in liver, kidney, and intestinal cells.
*Glucokinase*
- **Glucokinase** is active in the **fed state** when insulin levels are high and the insulin:glucagon ratio is increased.
- It phosphorylates glucose to trap it in hepatocytes for glycogen synthesis and metabolism, which is the opposite of what occurs during fasting.
*Phosphofructokinase*
- **Phosphofructokinase (PFK-1)** is the rate-limiting enzyme of **glycolysis**, active when glucose needs to be broken down for energy.
- It is stimulated by high insulin:glucagon ratios and inhibited during fasting when gluconeogenesis (the reverse pathway) is active.
*Hexokinase*
- **Hexokinase** phosphorylates glucose in peripheral tissues for intracellular utilization.
- During a low insulin:glucagon ratio, the priority is glucose **release** from the liver, not glucose **uptake** and phosphorylation in tissues.
Glycogen Metabolism: Synthesis and Breakdown Indian Medical PG Question 10: Glucose can be synthesized from all except
- A. Acetoacetate (Correct Answer)
- B. Amino acids
- C. Lactic acid
- D. Glycerol
Glycogen Metabolism: Synthesis and Breakdown Explanation: ***Acetoacetate***
- **Acetoacetate** is a **ketone body** and is metabolized to **acetyl-CoA**.
- **Acetyl-CoA** cannot be converted to pyruvate or oxaloacetate and thus cannot be used for **gluconeogenesis**.
*Amino acids*
- Many **amino acids** are **glucogenic**, meaning they can be converted into **glucose**.
- They enter the gluconeogenic pathway at various points, such as **pyruvate** or **oxaloacetate**.
*Lactic acid*
- **Lactic acid** is converted to **pyruvate** by **lactate dehydrogenase** as part of the **Cori cycle**.
- **Pyruvate** is a direct precursor for **glucose synthesis** via **gluconeogenesis**.
*Glycerol*
- **Glycerol**, derived from triglyceride breakdown, can be converted to **dihydroxyacetone phosphate** (DHAP).
- **DHAP** is an intermediate in gluconeogenesis, allowing for **glucose production**.
More Glycogen Metabolism: Synthesis and Breakdown Indian Medical PG questions available in the OnCourse app. Practice MCQs, flashcards, and get detailed explanations.
