Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 671: Glucagon acts on liver to cause:

A. Kreb's cycle
B. Glycolysis
C. Gluconeogenesis
D. Glycogenolysis (Correct Answer)

Explanation: ***Glycogenolysis*** * **Glucagon's primary role** is to increase blood glucose levels during fasting states * Glucagon binds to **glucagon receptors on hepatocytes** (liver cells) * This activates **adenylyl cyclase → cAMP → protein kinase A** cascade * PKA phosphorylates and activates **phosphorylase kinase**, which activates **glycogen phosphorylase** * Result: **Glycogen breakdown to glucose-1-phosphate → glucose-6-phosphate → free glucose** (via glucose-6-phosphatase in liver) * The free glucose is released into bloodstream to maintain blood glucose levels *Kreb's cycle* * The **Kreb's cycle (citric acid cycle)** oxidizes acetyl-CoA to produce ATP, NADH, and FADH2 * It is not directly stimulated by glucagon; its activity is regulated by **substrate availability** and **energy demands** (ATP/ADP ratio, NADH/NAD+ ratio) * Glucagon's effects are primarily on glucose homeostasis, not directly on oxidative metabolism *Glycolysis* * **Glycolysis** breaks down glucose into pyruvate, generating ATP * Glucagon **inhibits glycolysis** in the liver by decreasing fructose-2,6-bisphosphate levels * This inhibition makes sense: glucagon's role is to **increase** blood glucose, not consume it * Glucagon activates phosphodiesterase which reduces cAMP levels needed for PFK-2 activity *Gluconeogenesis* * While glucagon does **stimulate gluconeogenesis** in the liver (glucose synthesis from non-carbohydrate sources) * The question asks about the direct action, and **glycogenolysis is the primary and immediate response** to glucagon * Gluconeogenesis is a slower process that becomes more important during prolonged fasting

Question 672: Which of the following is active in dephosphorylated state?

A. PEPCK
B. Pyruvate Carboxylase
C. Glycogen Synthase (Correct Answer)
D. Glycogen Phosphorylase

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.

Question 673: Not done by insulin:

A. Glycolysis
B. Lipogenesis
C. Ketogenesis (Correct Answer)
D. Glycogen synthesis

Explanation: ***Ketogenesis*** - **Ketogenesis** is primarily a catabolic process stimulated by low insulin levels or insulin resistance, particularly during prolonged fasting or uncontrolled diabetes. - Insulin's main role is to promote anabolic processes and energy storage, thus it **inhibits ketogenesis** rather than performing it. *Glycolysis* - **Insulin** promotes **glycolysis** by increasing the expression and activity of key glycolytic enzymes, facilitating glucose breakdown for energy. - It enhances the uptake of **glucose into cells**, where it can then be metabolized via glycolysis. *Lipogenesis* - **Insulin** is a potent stimulator of **lipogenesis**, promoting the synthesis of fatty acids and triglycerides from excess glucose. - This process helps store excess energy in adipose tissue, converting carbohydrates into **fat**. *Glycogen synthesis* - **Insulin** directly stimulates **glycogen synthesis** (glycogenesis) in the liver and muscle cells. - It promotes the uptake of **glucose** and activates enzymes like **glycogen synthase**, leading to storage of glucose as glycogen.

Question 674: Which absorbs least water?

A. Cellulose (Correct Answer)
B. Mucilage
C. Pectin
D. Gums

Explanation: ***Cellulose*** - **Cellulose** is a **polysaccharide** with strong **intermolecular hydrogen bonding** between its linear chains. - These strong bonds form a highly ordered, crystalline structure that makes it **insoluble in water** and resistant to water absorption. *Mucilage* - **Mucilage** consists of **polysaccharides** that have a high capacity to absorb water, forming a slimy, gelatinous mass. - This property is due to its highly branched structure and abundance of **hydroxyl groups**, which readily form hydrogen bonds with water. *Pectin* - **Pectin** is a complex **polysaccharide** found in plant cell walls, known for its ability to absorb significant amounts of water. - It forms **gels** with water, a property widely utilized in food production. *Gums* - **Gums** are a diverse group of **polysaccharides** that are highly soluble in water and have an excellent capacity for water absorption. - They tend to form **viscous solutions** or gels when mixed with water.

Question 675: A person after consuming raw eggs presents with weakness, fatigue & hypoglycemia. Doctor gave him biotin supplements. Which biotin-dependent enzyme's reduced activity is most likely causing hypoglycemia in this patient:

A. Pyruvate carboxylase (Correct Answer)
B. Glucose 6 phosphatase
C. Glycogen phosphorylase
D. Phosphoenol pyruvate carboxykinase

Explanation: ***Pyruvate carboxylase*** - This enzyme is crucial for **gluconeogenesis**, converting **pyruvate to oxaloacetate**. Biotin deficiency, often caused by consuming **avidin** in raw eggs, impairs its activity, leading to reduced glucose production and **hypoglycemia**. - **Avidin** present in raw egg whites binds irreversibly to biotin, preventing its absorption and utilization, thereby affecting biotin-dependent enzymes. *Glucose 6 phosphatase* - This enzyme is involved in the final step of **gluconeogenesis** and **glycogenolysis**, releasing free glucose into the bloodstream. While its dysfunction can cause hypoglycemia, it is **not a biotin-dependent enzyme**. - Deficiencies in this enzyme are typically associated with **Von Gierke disease** (glycogen storage disease type I), which has distinct clinical features and is not related to raw egg consumption. *Glycogen phosphorylase* - This enzyme is responsible for the breakdown of **glycogen into glucose-1-phosphate** (**glycogenolysis**). Its inhibition would impair glycogen breakdown and could lead to hypoglycemia, but it is **not biotin-dependent**. - Deficiencies often present as **McArdle disease** (glycogen storage disease type V), characterized by exercise intolerance and muscle pain, which is not the primary presentation here. *Phosphoenol pyruvate carboxykinase* - This enzyme functions in **gluconeogenesis**, converting **oxaloacetate to phosphoenolpyruvate**. While essential for glucose production, it is **not a biotin-dependent enzyme**. - Its activity is regulated by hormones like glucagon and insulin, and its deficiency would impair gluconeogenesis, but the specific link to raw egg consumption and biotin is absent.

Question 676: In an infant presenting with doll-like facies, which enzyme is deficient in Von Gierke disease?

A. Fructose 1,6 bisphosphatase
B. Debranching enzyme
C. Glucose 6 phosphatase (Correct Answer)
D. Phosphorylase

Explanation: ***Glucose 6 phosphatase*** - **Von Gierke disease (Type I glycogen storage disease)** is characterized by a deficiency of **glucose-6-phosphatase**, an enzyme crucial for the final step of gluconeogenesis and glycogenolysis. - This enzyme's deficiency leads to the inability to release free glucose from the liver and kidneys, resulting in **hypoglycemia**, hepatomegaly, and the characteristic **doll-like facies** due to fat deposits. *Fructose 1,6 bisphosphatase* - This enzyme is involved in **gluconeogenesis**, catalyzing the conversion of fructose-1,6-bisphosphate to fructose-6-phosphate. - **Fructose-1,6-bisphosphatase deficiency** is a distinct metabolic disorder causing hypoglycemia, lactic acidosis, and hepatomegaly, but it does not present with the characteristic features of Von Gierke disease. *Debranching enzyme* - A deficiency in the **debranching enzyme** (**amylo-1,6-glucosidase**) is characteristic of **Cori's disease (GSD III)**. - While it also causes hepatomegaly and hypoglycemia, it typically presents with milder symptoms and a different metabolic profile than Von Gierke disease. *Phosphorylase* - **Glycogen phosphorylase** deficiency is associated with **McArdle's disease (GSD V)** in muscle and **Hers' disease (GSD VI)** in the liver. - These conditions primarily cause muscle weakness and cramping (McArdle's) or mild hypoglycemia and hepatomegaly (Hers'), but not the severe hypoglycemia and characteristic findings of Von Gierke disease.

Question 677: Congenital lactic acidosis is primarily due to a defect in which enzyme?

A. Branched chain alpha-ketoacid dehydrogenase
B. Isocitrate dehydrogenase (IDH)
C. Transketolase
D. Pyruvate dehydrogenase (Correct Answer)

Explanation: ***Pyruvate dehydrogenase*** - A defect in **pyruvate dehydrogenase (PDH)** complex prevents the conversion of **pyruvate to acetyl-CoA**, shunting pyruvate to **lactate production**. - This leads to an accumulation of **lactic acid** in the body, causing **congenital lactic acidosis**. *Branched chain alpha-ketoacid dehydrogenase* - A defect in **branched-chain alpha-ketoacid dehydrogenase** is responsible for **Maple Syrup Urine Disease**, not congenital lactic acidosis. - This enzyme is crucial for the metabolism of **branched-chain amino acids** (leucine, isoleucine, and valine). *Isocitrate dehydrogenase (IDH)* - **Isocitrate dehydrogenase (IDH)** is an enzyme in the **Krebs cycle** that converts isocitrate to alpha-ketoglutarate. - Defects or mutations in IDH enzymes are associated with certain **cancers**, but not primarily with congenital lactic acidosis. *Transketolase* - **Transketolase** is an enzyme involved in the **pentose phosphate pathway**, which generates NADPH and C5 sugars. - A deficiency in transketolase is associated with **Wernicke-Korsakoff syndrome** due to thiamine deficiency, not congenital lactic acidosis.

Question 678: Which coenzyme is essential for the function of pyruvate dehydrogenase?

A. Coenzyme A
B. FAD
C. Thiamine pyrophosphate (Correct Answer)
D. NAD+

Explanation: ***Thiamine pyrophosphate*** - **Thiamine pyrophosphate (TPP)**, derived from vitamin B1 (thiamine), is the coenzyme for the **E1 subunit (pyruvate dehydrogenase)** of the pyruvate dehydrogenase complex. - TPP is essential for the **decarboxylation of pyruvate**, the rate-limiting and defining step of the complex, forming a hydroxyethyl-TPP intermediate. - Its deficiency leads to impaired pyruvate metabolism, lactate accumulation, and neurological disorders like **Wernicke-Korsakoff syndrome** and beriberi. - While all five coenzymes (TPP, lipoic acid, CoA, FAD, NAD+) are essential for the PDH complex, **TPP is the most specific answer** as it catalyzes the signature decarboxylation reaction. *Coenzyme A* - **Coenzyme A (CoA)** is essential for the E2 subunit (dihydrolipoyl transacetylase), accepting the acetyl group from lipoamide to form **acetyl-CoA**. - While absolutely required for the overall reaction, it functions downstream of the initial decarboxylation step. *FAD* - **Flavin adenine dinucleotide (FAD)** is essential for the E3 subunit (dihydrolipoyl dehydrogenase), serving as an electron acceptor. - It regenerates oxidized lipoamide and transfers electrons to NAD+, but is not involved in the decarboxylation step. *NAD+* - **NAD+** is the final electron acceptor in the E3 subunit, being reduced to NADH. - Essential for overall complex function, but not specific to the pyruvate decarboxylation reaction that defines pyruvate dehydrogenase activity.

Question 679: A 22-year-old athlete has episodes of rhabdomyolysis after intense exercise. Which metabolic disorder should be suspected?

A. Pompe disease
B. Cori disease
C. Von Gierke disease
D. McArdle disease (Correct Answer)

Explanation: ***McArdle disease*** - This condition, also known as **glycogen storage disease type V**, is caused by a deficiency in **myophosphorylase** (muscle glycogen phosphorylase). - This enzyme defect prevents the breakdown of **glycogen in muscle** during exercise, leading to energy depletion, muscle pain, cramps, and **rhabdomyolysis**. *Pompe disease* - Caused by a deficiency in **lysosomal α-1,4-glucosidase** (acid maltase), leading to glycogen accumulation in lysosomes. - Presents with a wide range of symptoms including **cardiomyopathy**, hypotonia (in infants), and respiratory problems, but less commonly exercise-induced rhabdomyolysis in adults. *Cori disease* - Also known as glycogen storage disease type III, characterized by a deficiency in **glycogen debranching enzyme**. - Mainly affects the **liver and muscles**, causing hepatomegaly, hypoglycemia, and muscle weakness, but exercise-induced rhabdomyolysis is not its primary presentation. *Von Gierke disease* - This is **glycogen storage disease type I**, caused by a deficiency in **glucose-6-phosphatase**. - Primarily affects the **liver and kidneys**, leading to severe fasting hypoglycemia, hepatomegaly, lactic acidosis, and hyperlipidemia, but not typically rhabdomyolysis.

Question 680: What enzyme converts pyruvate to oxaloacetate in gluconeogenesis?

A. Pyruvate carboxylase (Correct Answer)
B. Phosphoenolpyruvate carboxykinase
C. Pyruvate dehydrogenase
D. Lactate dehydrogenase

Explanation: ***Pyruvate carboxylase*** - This enzyme catalyzes the **ATP-dependent carboxylation of pyruvate** to form **oxaloacetate**, a crucial step in gluconeogenesis. - It is a **biotin-requiring enzyme** found in the **mitochondria**, where it acts as the first bypass enzyme in gluconeogenesis. *Phosphoenolpyruvate carboxykinase* - This enzyme converts **oxaloacetate to phosphoenolpyruvate (PEP)**, the next step in gluconeogenesis after pyruvate carboxylase. - It uses **GTP as an energy source** and can be found in both the cytosol and mitochondria. *Pyruvate dehydrogenase* - This enzyme complex catalyzes the **oxidative decarboxylation of pyruvate to acetyl-CoA**, linking glycolysis to the citric acid cycle. - It effectively removes pyruvate from the gluconeogenic pathway by converting it into a molecule that cannot be directly converted back to glucose. *Lactate dehydrogenase* - This enzyme catalyzes the **interconversion of pyruvate and lactate**, often in the context of anaerobic glycolysis. - Its primary role is to regenerate NAD+ for glycolysis during intense muscular activity, not to produce oxaloacetate for gluconeogenesis.

Practice by Chapter

Carbohydrate Chemistry and Classification

Practice Questions

Glycolysis: Reactions and Regulation

Practice Questions

Gluconeogenesis: Reactions and Regulation

Practice Questions

Glycogen Metabolism: Synthesis and Breakdown

Practice Questions

Glycogen Storage Diseases

Practice Questions

Pentose Phosphate Pathway

Practice Questions

Metabolism of Fructose and Galactose

Practice Questions

Disorders of Fructose and Galactose Metabolism

Practice Questions

Blood Glucose Regulation

Practice Questions

Diabetes Mellitus: Biochemical Aspects

Practice Questions

Glycosylation and Glycoproteins

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Lactose Intolerance and Galactosemia

Practice Questions

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