Carbohydrate Metabolism Practice Questions

Q: Deficiency of glucose-6-phosphate dehydrogenase may cause which of the following?

Haemolytic anemia. **Explanation:** **Glucose-6-Phosphate Dehydrogenase (G6PD)** is the rate-limiting enzyme of the **Hexose Monophosphate (HMP) Shunt**. Its primary role is the production of **NADPH**. 1. **Why Haemolytic Anemia is correct:** In red blood cells (RBCs), NADPH is essential to maintain a pool of **reduced glutathione**. Reduced glutathione acts as an antioxidant that neutralizes Reactive Oxygen Species (ROS) like hydrogen peroxide. In G6PD deficiency, NADPH levels drop, leading to the accumulation of oxidized glutathione. This causes oxidative damage to hemoglobin, which denatures and precipitates as **Heinz bodies**. These damaged RBCs are destroyed in the spleen (hemolysis), especially after exposure to oxidative stress (e.g., Fava beans, Primaquine, or infections). 2. **Why other options are incorrect:** * **Diabetes Mellitus:** Caused by insulin deficiency or resistance, not HMP shunt defects. * **Wernicke-Korsakoff Syndrome:** Caused by **Thiamine (Vitamin B1)** deficiency, affecting enzymes like Transketolase and Pyruvate Dehydrogenase. * **Porphyria:** A group of disorders caused by defects in the **Heme synthesis pathway** (e.g., deficiency of Ferrochelatase or PBG deaminase). **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** G6PD deficiency is an **X-linked recessive** disorder. * **Morphology:** Look for **Heinz bodies** (supravital stain) and **Bite cells** (degmacytes) on a peripheral smear. * **Protective Effect:** G6PD deficiency provides a selective advantage against *Plasmodium falciparum* malaria. * **Contraindicated Drugs:** Sulfa drugs, Antimalarials (Primaquine), and Nitrofurantoin.

Q: Deficiency of lysosomal alpha-1,4 and alpha-1,6 glucosidase is seen in which condition?

Pompe disease. ### Explanation The correct answer is **Pompe disease (GSD Type II)**. **1. Why Pompe Disease is Correct:** Pompe disease is unique among Glycogen Storage Diseases (GSDs) because it is a **lysosomal storage disorder**. The deficient enzyme is **Lysosomal acid alpha-1,4-glucosidase** (also known as **Acid Maltase**). This enzyme is responsible for breaking down glycogen within lysosomes. While most glycogenolysis occurs in the cytosol, about 1-3% of glycogen is degraded in lysosomes. A deficiency leads to massive accumulation of glycogen in the lysosomes of all organs, most significantly affecting the **heart and skeletal muscles**. **2. Analysis of Incorrect Options:** * **A. Von Gierke Disease (GSD Type I):** Caused by a deficiency of **Glucose-6-Phosphatase**. It primarily affects the liver and kidneys, presenting with severe fasting hypoglycemia and hepatomegaly. * **B. Cori Disease (GSD Type III):** Caused by a deficiency of the **Debranching enzyme** (α-1,6-glucosidase in the cytosol). It results in the accumulation of "limit dextrin" (abnormally short outer chains). * **C. Tarui Disease (GSD Type VII):** Caused by a deficiency of **Phosphofructokinase (PFK)** in muscles and RBCs, leading to exercise intolerance and hemolytic anemia. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** "Pompe trashes the **Pump** (Heart)." * **Clinical Triad:** Cardiomegaly (massive), Hypotonia ("Floppy baby"), and early death due to heart failure. * **Key Feature:** It is the only GSD that is also a Lysosomal Storage Disease. * **Biochemical Marker:** Normal blood glucose levels (unlike Type I) because cytosolic glycogenolysis remains intact. * **Diagnosis:** PAS-positive material in lysosomes on muscle biopsy.

Q: Which of the following is NOT a positive signal for glycogen breakdown?

Blood glucose. **Explanation:** Glycogenolysis (glycogen breakdown) is regulated by the enzyme **Glycogen Phosphorylase**. This process is activated when the body requires energy or needs to maintain blood glucose levels. **Why Blood Glucose is the correct answer:** High levels of **blood glucose** act as a negative signal (inhibitor) for glycogenolysis. In the liver, glucose binds to glycogen phosphorylase $a$, inducing a conformational change that makes it susceptible to inactivation by protein phosphatase-1. This is a feedback mechanism: if blood glucose is already high, the body does not need to break down glycogen. **Analysis of Incorrect Options:** * **Cyclic AMP (cAMP):** This is a classic positive signal. Glucagon (in liver) or Epinephrine (in muscle) binds to G-protein coupled receptors, increasing cAMP. This activates Protein Kinase A, which eventually activates glycogen phosphorylase. * **Epinephrine:** Known as the "fight or flight" hormone, it stimulates glycogen breakdown in both liver and muscle to provide immediate fuel. * **Ca²⁺:** In contracting muscles, calcium is released from the sarcoplasmic reticulum. It binds to the calmodulin subunit of **Phosphorylase Kinase**, activating it even without phosphorylation. This ensures glycogen is broken down to support muscle contraction. **High-Yield NEET-PG Pearls:** * **Rate-limiting enzyme:** Glycogen Phosphorylase. * **Allosteric Activators:** AMP (specifically in muscle during exercise) and Ca²⁺. * **Allosteric Inhibitors:** ATP, Glucose-6-Phosphate, and Free Glucose. * **Hormonal Control:** Glucagon and Epinephrine stimulate breakdown (via phosphorylation); Insulin inhibits breakdown (via dephosphorylation).

Q: Which of the following is NOT a component enzyme of the Pyruvate Dehydrogenase complex?

Pyruvate Decarboxylase. **Explanation:** The **Pyruvate Dehydrogenase Complex (PDH)** is a multi-enzyme mitochondrial complex that catalyzes the oxidative decarboxylation of pyruvate into Acetyl-CoA, linking glycolysis to the TCA cycle. **Why Option D is correct:** **Pyruvate Decarboxylase** is an enzyme involved in **ethanol fermentation** (found in yeast and some bacteria), which converts pyruvate into acetaldehyde. It is **not** part of the human PDH complex. While the first subunit of PDH (E1) does perform decarboxylation, its formal name is Pyruvate Dehydrogenase or Pyruvate Decarboxylase *component*, but "Pyruvate Decarboxylase" as a standalone term refers to the fermentation enzyme. **Why the other options are incorrect:** The PDH complex consists of three distinct catalytic subunits: * **Option A (E1): Pyruvate Dehydrogenase.** It uses Thiamine Pyrophosphate (TPP) as a cofactor to decarboxylate pyruvate. * **Option B (E2): Dihydrolipoyl Transacetylase.** It transfers the acetyl group to Coenzyme A and utilizes Lipoic acid. * **Option C (E3): Dihydrolipoyl Dehydrogenase.** It regenerates the oxidized form of lipoic acid using FAD and NAD+. **High-Yield Clinical Pearls for NEET-PG:** * **Cofactors:** PDH requires five cofactors: **T**hiamine (B1), **R**iboflavin (B2/FAD), **N**iacin (B3/NAD), **P**antothenic acid (B5/CoA), and **L**ipoic acid (Mnemonic: **T**ender **R**eversed **N**eck **P**ads **L**oose). * **Arsenic Poisoning:** Arsenite inhibits the PDH complex by binding to the -SH groups of **Lipoic acid**, leading to lactic acidosis and garlic breath. * **Deficiency:** PDH deficiency is a common cause of congenital lactic acidosis; management involves a **ketogenic diet** (high fat, low carb).

Q: Which disaccharide is not digested in the intestine?

Sucralose. **Explanation:** The correct answer is **Sucralose**. **1. Why Sucralose is the correct answer:** Sucralose is an artificial, non-caloric sweetener. Chemically, it is a **trichlorinated derivative of sucrose** (1,6-dichloro-1,6-dideoxy-β-D-fructofuranosyl-4-chloro-4-deoxy-α-D-galactopyranoside). The substitution of three hydroxyl groups with chlorine atoms creates a molecule that the human body cannot recognize as a carbohydrate. Consequently, there are no specific enzymes (disaccharidases) in the human small intestine capable of breaking the bonds in sucralose. It passes through the digestive tract largely unchanged and is excreted, providing zero calories. **2. Why the other options are incorrect:** * **Sucrose (A):** A natural disaccharide (Glucose + Fructose) digested by the enzyme **Sucrase** located in the brush border of the small intestine. * **Isomaltose (B):** An isomer of maltose with an α(1→6) glycosidic bond. It is a product of starch digestion and is hydrolyzed by the enzyme **Isomaltase** (part of the Sucrase-Isomaltase complex). * **Trehalose (C):** A disaccharide found in mushrooms and yeast (Glucose + Glucose with an α1→1 linkage). It is digested by the brush border enzyme **Trehalase**. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Sucrase-Isomaltase Deficiency:** A congenital condition leading to osmotic diarrhea and abdominal distension upon consuming sugar or starch. * **Trehalase Deficiency:** Rare, but presents with symptoms similar to lactose intolerance after eating mushrooms. * **Lactose:** The only major dietary disaccharide with a **β(1→4) linkage**; digested by Lactase. * **Sucralose** is approximately 600 times sweeter than sucrose and is heat-stable, making it popular for baking.

Q: Strenuous exercise is not indicated in which of the following glycogen storage diseases?

McArdle disease. **Explanation:** **McArdle Disease (GSD Type V)** is caused by a deficiency of **muscle glycogen phosphorylase (myophosphorylase)**. This enzyme is essential for glycogenolysis in skeletal muscle, converting glycogen into glucose-1-phosphate to fuel muscle contraction. During strenuous exercise, muscles rely heavily on glycogen for rapid ATP production. In McArdle disease, the inability to mobilize glycogen leads to an energy crisis, resulting in severe muscle cramps, exercise intolerance, and **myoglobinuria** (due to rhabdomyolysis). A classic clinical feature is the **"second wind phenomenon,"** where patients improve after a brief period of exercise as the body switches to using free fatty acids and blood glucose. **Why other options are incorrect:** * **Anderson Disease (Type IV):** Caused by a branching enzyme deficiency. It primarily affects the liver, leading to cirrhosis and hepatosplenomegaly in infancy; it is not primarily characterized by exercise-induced muscle crises. * **Pompe Disease (Type II):** Caused by lysosomal acid alpha-glucosidase deficiency. It leads to generalized glycogen accumulation (especially in the heart), causing hypertrophic cardiomyopathy and respiratory failure, rather than acute exercise-induced cramps. * **Von Gierke Disease (Type I):** Caused by glucose-6-phosphatase deficiency. It affects the liver and kidneys, presenting with severe fasting hypoglycemia, lactic acidosis, and hepatomegaly, but does not directly impair muscle glycogenolysis. **High-Yield Facts for NEET-PG:** * **Ischemic Forearm Exercise Test:** In McArdle disease, there is a failure of blood lactate to rise, but a significant rise in ammonia levels. * **Biopsy finding:** Subsarcolemmal deposits of glycogen in muscle fibers. * **Management:** Patients are advised to consume simple sugars before exercise and avoid sudden, strenuous bursts of activity.

Q: Which enzyme is inhibited by insulin?

Glycogen phosphorylase. **Explanation:** The regulation of carbohydrate metabolism by insulin is a high-yield topic for NEET-PG. Insulin is an **anabolic hormone** secreted by the pancreatic beta cells in response to high blood glucose levels. Its primary goal is to lower blood glucose by promoting glucose utilization (glycolysis), storage (glycogenesis), and inhibiting glucose production. **1. Why Glycogen Phosphorylase is the Correct Answer:** Glycogen phosphorylase is the rate-limiting enzyme of **glycogenolysis** (breakdown of glycogen into glucose). Insulin triggers a signaling cascade that activates **Protein Phosphatase-1 (PP1)**. This phosphatase dephosphorylates glycogen phosphorylase, converting it from its active form (Phosphorylase *a*) to its inactive form (Phosphorylase *b*). By inhibiting this enzyme, insulin prevents the mobilization of glucose stores. **2. Analysis of Incorrect Options:** * **Glucokinase (Option A):** This enzyme facilitates glucose uptake in the liver. Insulin **induces** the synthesis of Glucokinase to promote glucose utilization. * **PFK-1 (Option B):** Phosphofructokinase-1 is the rate-limiting enzyme of glycolysis. Insulin stimulates PFK-1 indirectly by increasing levels of Fructose-2,6-bisphosphate, its most potent allosteric activator. * **Glycogen Synthase (Option D):** This is the rate-limiting enzyme for glycogen synthesis. Insulin **activates** this enzyme via dephosphorylation (by PP1), promoting the storage of glucose as glycogen. **Clinical Pearls for NEET-PG:** * **Rule of Thumb:** Insulin generally **dephosphorylates** enzymes. In most cases, dephosphorylation **activates** anabolic enzymes (e.g., Glycogen synthase) and **inhibits** catabolic enzymes (e.g., Glycogen phosphorylase, Hormone-sensitive lipase). * **Glucagon/Epinephrine** act as antagonists to insulin by increasing cAMP, which activates Protein Kinase A (PKA), leading to the phosphorylation (activation) of glycogen phosphorylase.

Q: All of the following substrates are glucogenic except:

Acetyl CoA. **Explanation:** The core concept tested here is the **irreversibility of the Pyruvate Dehydrogenase (PDH) complex**. In humans, the conversion of Pyruvate to Acetyl CoA is a one-way reaction. **1. Why Acetyl CoA is the Correct Answer:** Acetyl CoA cannot be used for the net synthesis of glucose (gluconeogenesis). When Acetyl CoA enters the TCA cycle, it condenses with Oxaloacetate (OAA) to form Citrate. During the cycle, two carbons are lost as $CO_2$ before OAA is regenerated. Therefore, there is **no net gain of carbon atoms** to be diverted toward glucose synthesis. Furthermore, the PDH reaction is irreversible, meaning Acetyl CoA cannot be converted back into Pyruvate. **2. Why the other options are Glucogenic:** * **Pyruvate:** It is the primary substrate for gluconeogenesis. It is converted to OAA by *Pyruvate Carboxylase*, bypassing the irreversible step of glycolysis. * **Glycerol:** Derived from triacylglycerol breakdown in adipose tissue, it is phosphorylated to Glycerol-3-Phosphate and then oxidized to Dihydroxyacetone phosphate (DHAP), an intermediate of gluconeogenesis. * **Lactate:** Produced via anaerobic glycolysis (e.g., in RBCs or exercising muscle), it is converted back to Pyruvate by *Lactate Dehydrogenase* in the liver (**Cori Cycle**) to form glucose. **High-Yield NEET-PG Pearls:** * **Odd-chain fatty acids** are glucogenic because their terminal product, **Propionyl CoA**, enters the TCA cycle as Succinyl CoA. * **Even-chain fatty acids** are strictly non-glucogenic (they produce only Acetyl CoA). * **Leucine and Lysine** are the only two amino acids that are purely ketogenic (non-glucogenic). * **Key Regulatory Enzyme:** Pyruvate Carboxylase (requires Biotin and is activated by Acetyl CoA).

Q: Which of the following metabolic pathways occurs in two cellular compartments?

Gluconeogenesis. **Explanation:** The correct answer is **Gluconeogenesis**. In biochemistry, metabolic pathways are often compartmentalized to ensure efficient regulation. While many pathways occur entirely in the cytoplasm, gluconeogenesis is a classic example of a "dual-compartment" pathway. **1. Why Gluconeogenesis is correct:** Gluconeogenesis begins in the **mitochondria** and ends in the **cytosol**. * **Mitochondria:** Pyruvate is converted to Oxaloacetate (OAA) by *Pyruvate Carboxylase*. Since OAA cannot cross the mitochondrial membrane, it is reduced to Malate, exported to the cytosol, and then re-oxidized back to OAA. * **Cytosol:** The remaining steps, starting from the conversion of OAA to Phosphoenolpyruvate (PEP) by *PEPCK*, occur in the cytoplasm. * *Note:* The final step (Glucose-6-Phosphate to Glucose) occurs in the **Endoplasmic Reticulum** of liver and kidney cells. **2. Why other options are incorrect:** * **Glycolysis:** Occurs entirely in the **cytosol** of all cells. * **Glycogenesis (Glycogen synthesis):** Occurs entirely in the **cytosol**, primarily in the liver and skeletal muscle. * **Glycogenolysis (Glycogen breakdown):** Occurs in the **cytosol**. (Small amounts of glycogen are degraded in lysosomes by acid maltase, but the primary metabolic pathway is cytosolic). **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Dual-Compartment Pathways:** Remember **"HUG"** — **H**eme synthesis, **U**rea cycle, and **G**luconeogenesis. * **Rate-limiting enzyme of Gluconeogenesis:** Fructose-1,6-bisphosphatase. * **Key Organelle:** The enzyme **Glucose-6-Phosphatase** is located on the luminal surface of the **Smooth ER**. Its deficiency leads to Von Gierke’s Disease (GSD Type I).

Question 1

Deficiency of glucose-6-phosphate dehydrogenase may cause which of the following?

Accepted Answer

Haemolytic anemia

Answer

Diabetes mellitus

Answer

Wernicke-Korsakoff syndrome

Answer

Porphyria

Question 2

Deficiency of lysosomal alpha-1,4 and alpha-1,6 glucosidase is seen in which condition?

Accepted Answer

Pompe disease

Answer

Von Gierke disease

Answer

Cori disease

Answer

Tarui disease

Question 3

Which of the following is NOT a positive signal for glycogen breakdown?

Accepted Answer

Blood glucose

Answer

Cyclic AMP

Answer

Epinephrine

Answer

Ca2+

Question 4

Which is the rate-limiting step in the Hexose Monophosphate (HMP) shunt?

Accepted Answer

Formation of 6-phosphogluconolactone from glucose-6-phosphate

Answer

Formation of 6-phosphogluconate from 6-phosphogluconolactone

Answer

Formation of ribulose-5-phosphate from 6-phosphogluconate

Answer

Formation of xylulose-5-phosphate from ribulose-5-phosphate

Question 5

Which of the following is NOT a component enzyme of the Pyruvate Dehydrogenase complex?

Accepted Answer

Pyruvate Decarboxylase

Answer

Pyruvate Dehydrogenase

Answer

Dihydrolipoyl Transacetylase

Answer

Dihydrolipoyl Dehydrogenase

Question 6

Which disaccharide is not digested in the intestine?

Accepted Answer

Sucralose

Answer

Sucrose

Answer

Isomaltose

Answer

Trehalose

Question 7

Strenuous exercise is not indicated in which of the following glycogen storage diseases?

Accepted Answer

McArdle disease

Answer

Anderson disease

Answer

Pompe disease

Answer

Von Gierke disease

Question 8

Which enzyme is inhibited by insulin?

Accepted Answer

Glycogen phosphorylase

Answer

Glucokinase

Answer

PFK-1

Answer

Glycogen synthase

Question 9

All of the following substrates are glucogenic except:

Accepted Answer

Acetyl CoA

Answer

Pyruvate

Answer

Glycerol

Answer

Lactate

Question 10

Which of the following metabolic pathways occurs in two cellular compartments?

Accepted Answer

Gluconeogenesis

Answer

Glycolysis

Answer

Glycogenesis

Answer

Glycogenolysis

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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