Carbohydrate Metabolism Practice Questions

Q: Which of the following is a reversible enzyme?

Lactate dehydrogenase. **Explanation:** In metabolic pathways, enzymes are classified as either **reversible** (catalyzing reactions in both directions depending on substrate concentration) or **irreversible** (acting as metabolic "checkpoints" or rate-limiting steps). **Correct Option: C. Lactate Dehydrogenase (LDH)** Lactate dehydrogenase catalyzes the interconversion of **Pyruvate to Lactate** (and vice versa) with the coupled interconversion of NADH and NAD+. This reaction is a classic example of a reversible reaction in anaerobic glycolysis and the Cori cycle. The direction depends on the NAD+/NADH ratio and the tissue type (e.g., LDH-1 in the heart favors pyruvate formation, while LDH-5 in the muscle favors lactate). **Incorrect Options:** * **A. Pyruvate Kinase:** This is the final irreversible step of glycolysis, converting Phosphoenolpyruvate (PEP) to Pyruvate. It is a key regulatory point. * **B. Pyruvate Dehydrogenase (PDH):** This multienzyme complex converts Pyruvate to Acetyl-CoA. This reaction is strictly irreversible in humans, preventing the net synthesis of glucose from acetyl-CoA (fatty acids). * **D. Hexokinase:** This is the first irreversible "priming" step of glycolysis, trapping glucose inside the cell by phosphorylating it to Glucose-6-Phosphate. **NEET-PG High-Yield Pearls:** 1. **Irreversible Steps of Glycolysis:** Remember the "Big Three"—**Hexokinase/Glucokinase, Phosphofructokinase-1 (PFK-1), and Pyruvate Kinase.** These must be bypassed by different enzymes during Gluconeogenesis. 2. **Clinical Correlation:** LDH is a tetramer with five isoenzymes. An elevation in **LDH-1 > LDH-2** (flipped pattern) is a classic (though older) marker for Myocardial Infarction. 3. **Metabolic Logic:** Most "Dehydrogenases" in the TCA cycle are irreversible, but LDH and Malate Dehydrogenase are notable reversible exceptions.

Q: G6PD helps in maintaining the integrity of RBC by:

Controlling oxidative stress on RBC. **Explanation:** **Glucose-6-Phosphate Dehydrogenase (G6PD)** is the rate-limiting enzyme of the **Hexose Monophosphate (HMP) Shunt**. Its primary function in Red Blood Cells (RBCs) is the production of **NADPH**. 1. **Why Option A is Correct:** RBCs are constantly exposed to Reactive Oxygen Species (ROS) like hydrogen peroxide ($H_2O_2$). To neutralize these, the cell uses **Reduced Glutathione**. During this process, glutathione becomes oxidized. To regenerate reduced glutathione, the enzyme **Glutathione Reductase** requires **NADPH** (produced by G6PD) as a cofactor. Without G6PD, NADPH levels drop, oxidized glutathione accumulates, and oxidative stress causes hemoglobin to denature, forming **Heinz bodies** and leading to hemolysis. 2. **Why Other Options are Incorrect:** * **Option B:** "Reduction stress" is not a physiological challenge for RBCs; the threat is always oxidative damage from free radicals. * **Option C:** While G6PD deficiency eventually leads to membrane damage (via Heinz body removal by splenic macrophages), membrane flexibility is primarily maintained by cytoskeletal proteins like **Spectrin and Ankyrin** (defective in Hereditary Spherocytosis). * **Option D:** The Electron Transport Chain (ETC) is located in the mitochondria. Mature RBCs lack mitochondria and rely solely on glycolysis for energy. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** G6PD deficiency is an **X-linked Recessive** disorder. * **Triggers:** Hemolysis is triggered by **Fava beans**, infections, or drugs (e.g., **Primaquine**, Sulfa drugs, Nitrofurantoin). * **Morphology:** Look for **Heinz Bodies** (denatured Hb) and **Bite Cells** (deformed RBCs after splenic macrophages "bite" out the Heinz bodies). * **Protection:** G6PD deficiency offers a selective advantage against **Falciparum malaria**.

Q: A hypotonic baby shows an increased ratio of Pyruvate to Acetyl CoA. The baby also exhibits features of lactic acidosis. The underlying defect prevents pyruvate from forming Acetyl CoA in fibroblasts. Which of the following can revert this situation?

Thiamin. ### Explanation **Correct Option: D (Thiamin)** The clinical presentation of hypotonia, lactic acidosis, and a high Pyruvate/Acetyl CoA ratio indicates a defect in the **Pyruvate Dehydrogenase (PDH) Complex**. This multienzyme complex is responsible for the oxidative decarboxylation of pyruvate into Acetyl CoA, bridging glycolysis and the TCA cycle. The PDH complex requires five essential cofactors: **T**hiamine pyrophosphate (TPP/B1), **R**iboflavin (FAD/B2), **N**iacin (NAD/B3), **P**antothenic acid (CoA/B5), and **L**ipoic acid (Mnemonic: **T**ender **R**oving **N**ights **P**lease **L**uck). In many cases of PDH deficiency, the E1 subunit (pyruvate decarboxylase) has a reduced affinity for its cofactor, **Thiamin (TPP)**. Administering high doses of Thiamin can stabilize the enzyme, increase its catalytic activity, and effectively lower lactate levels by diverting pyruvate toward Acetyl CoA formation. **Analysis of Incorrect Options:** * **A. Biotin:** A cofactor for carboxylases (e.g., Pyruvate Carboxylase). Deficiency would impair gluconeogenesis, not the conversion of pyruvate to Acetyl CoA. * **B. Pyridoxine (B6):** Primarily involved in transamination (ALT/AST) and heme synthesis. It does not play a role in the PDH complex. * **C. Free fatty acids:** These are metabolized into Acetyl CoA via beta-oxidation. While they provide an alternative fuel source, they do not fix the enzymatic defect or reduce the accumulation of pyruvate/lactate. **Clinical Pearls for NEET-PG:** * **PDH Deficiency:** The most common cause of congenital lactic acidosis. It is often an **X-linked dominant** inheritance (E1-alpha subunit mutation). * **Dietary Management:** Patients are often placed on a **Ketogenic Diet** (high fat, low carb) to provide energy via ketones/Acetyl CoA, bypassing the PDH block. * **Leucine and Lysine:** These are purely ketogenic amino acids and are preferred in PDH deficiency as they do not contribute to pyruvate formation.

Q: Which of the following enzymes is present inside the given organelle?

Glucose-6-phosphatase. ***Glucose-6-phosphatase*** - This enzyme is specifically localized to the **endoplasmic reticulum (ER)** membrane and plays a crucial role in **gluconeogenesis** and **glycogenolysis**. - It catalyzes the final step in glucose production by converting **glucose-6-phosphate** to free glucose, which can then be released into the bloodstream. *Glucokinase* - This enzyme is primarily found in the **cytosol** of hepatocytes and pancreatic beta cells, not in the ER. - It functions as a **glucose sensor** and catalyzes the phosphorylation of glucose to glucose-6-phosphate during **glycolysis**. *Isocitrate dehydrogenase* - This enzyme is located in the **mitochondrial matrix** as part of the **citric acid cycle (Krebs cycle)**. - It catalyzes the oxidative decarboxylation of **isocitrate** to **α-ketoglutarate**, generating **NADH** and **CO₂**. *Fructose 2, 6 bisphosphatase* - This enzyme is found in the **cytosol** and is involved in regulating **glycolysis** and **gluconeogenesis**. - It hydrolyzes **fructose-2,6-bisphosphate** to **fructose-6-phosphate**, acting as a key regulatory enzyme in glucose metabolism.

Q: Which of the following is NOT a product of the Hexose Monophosphate (HMP) shunt?

Glycerol-3-phosphate. ### Explanation The **Hexose Monophosphate (HMP) Shunt** (Pentose Phosphate Pathway) occurs in the cytosol and is divided into an irreversible oxidative phase and a reversible non-oxidative phase. **Why Glycerol-3-phosphate is the correct answer:** Glycerol-3-phosphate is **not** a product of the HMP shunt. It is an intermediate of **Glycolysis** (derived from Dihydroxyacetone phosphate via Glycerol-3-phosphate dehydrogenase) or lipid metabolism. While the HMP shunt provides precursors for various pathways, it does not directly generate Glycerol-3-phosphate. **Analysis of Incorrect Options:** * **Glyceraldehyde 3-phosphate (A):** This is a key product of the **non-oxidative phase**. Transketolase and transaldolase enzymes recycle pentose sugars back into glycolytic intermediates like Glyceraldehyde 3-phosphate and Fructose 6-phosphate. * **3CO₂ (C):** In the **oxidative phase**, for every 3 molecules of Glucose 6-phosphate entering the shunt, 3 molecules of CO₂ are released during the decarboxylation of 6-phosphogluconate to Ribulose 5-phosphate. * **6NADPH (D):** The primary purpose of the oxidative phase is the generation of reducing equivalents. For 3 molecules of Glucose 6-phosphate, **6 molecules of NADPH** are produced (2 per glucose molecule). **High-Yield NEET-PG Pearls:** * **Rate-limiting enzyme:** Glucose-6-phosphate dehydrogenase (G6PD). * **Tissues involved:** Highly active in the liver, lactating mammary glands, adrenal cortex, and RBCs (where NADPH maintains reduced glutathione to prevent oxidative stress). * **Clinical Correlation:** G6PD deficiency leads to hemolytic anemia due to the inability to neutralize free radicals, characterized by **Heinz bodies** and **Bite cells** on blood smears. * **Thiamine (B1) Connection:** Transketolase requires Thiamine pyrophosphate (TPP) as a cofactor; measuring erythrocyte transketolase activity is a diagnostic test for Thiamine deficiency.

Q: Which of the following is NOT used in gluconeogenesis?

Oleate. **Explanation:** The core concept in gluconeogenesis is that a substrate must be capable of a net conversion into **Oxaloacetate (OAA)** or other intermediates of the TCA cycle to enter the gluconeogenic pathway. **Why Oleate is the correct answer:** Oleate is a long-chain **fatty acid** (C18:1). In humans, the oxidation of even-chain fatty acids produces **Acetyl-CoA**. Acetyl-CoA cannot be used for the net synthesis of glucose because: 1. The **Pyruvate Dehydrogenase (PDH) reaction** is irreversible; Acetyl-CoA cannot be converted back to Pyruvate. 2. In the TCA cycle, the two carbons of Acetyl-CoA are lost as $CO_2$ before reaching Oxaloacetate, resulting in **no net gain** of carbon atoms for gluconeogenesis. **Analysis of Incorrect Options:** * **Succinate:** This is a TCA cycle intermediate. It is oxidized to Malate and then to Oxaloacetate, which enters gluconeogenesis via PEP carboxykinase. * **Glutamate:** A glucogenic amino acid. It is converted to **$\alpha$-ketoglutarate** (via transamination or glutamate dehydrogenase), which enters the TCA cycle to form Oxaloacetate. * **Aspartate:** A glucogenic amino acid. It undergoes transamination to directly form **Oxaloacetate**. **High-Yield Clinical Pearls for NEET-PG:** * **Odd-chain fatty acids** ARE gluconeogenic because their terminal metabolism yields **Propionyl-CoA**, which enters the TCA cycle as Succinyl-CoA. * **Leucine and Lysine** are the only purely ketogenic amino acids (cannot form glucose). * **Glycerol** (from triacylglycerol breakdown) is gluconeogenic as it enters the pathway at the level of Dihydroxyacetone phosphate (DHAP). * **Acetyl-CoA** is an allosteric **activator of Pyruvate Carboxylase**, thus promoting gluconeogenesis while inhibiting the PDH complex.

Q: Which of the following glucose transporters is primarily utilized by adipocytes?

GLUT4. **Explanation:** The correct answer is **GLUT4**. Glucose transporters (GLUTs) are transmembrane proteins that facilitate the movement of glucose across cell membranes via facilitated diffusion. **Why GLUT4 is correct:** GLUT4 is the only **insulin-dependent** glucose transporter. It is primarily expressed in **adipocytes (fat cells)** and **skeletal muscle**. In the fasting state, GLUT4 remains sequestered in intracellular vesicles. Upon insulin secretion (post-prandial state), these vesicles translocate and fuse with the plasma membrane, allowing for rapid glucose uptake. This mechanism is crucial for lowering post-prandial blood glucose levels. **Why the other options are incorrect:** * **GLUT1:** This is a basal glucose transporter found in almost all tissues. It is highly expressed in **Red Blood Cells (RBCs)** and the **Blood-Brain Barrier**. It ensures a steady, insulin-independent supply of glucose. * **GLUT2:** A high-capacity, low-affinity transporter found in the **Liver, Pancreas (beta cells), Kidney, and Small Intestine**. It acts as a "glucose sensor" in the pancreas and allows for bidirectional glucose flux in the liver. * **GLUT3:** A high-affinity transporter primarily found in **Neurons**. It ensures that the brain receives adequate glucose even when blood sugar levels are low. **High-Yield Clinical Pearls for NEET-PG:** * **GLUT4 & Exercise:** Muscle contraction can trigger GLUT4 translocation to the cell membrane independently of insulin, which is why exercise helps manage blood sugar in Type 2 Diabetes. * **GLUT5:** This is unique because it is primarily a **Fructose** transporter, located in the small intestine and spermatozoa. * **SGLT1/2:** Unlike GLUTs, Sodium-Glucose Linked Transporters (SGLTs) utilize **active transport** (secondary) and are found in the intestinal mucosa and renal tubules. SGLT2 inhibitors (e.g., Dapagliflozin) are now major drugs for Diabetes and Heart Failure.

Q: What are the enzymes required for the formation of Phosphoenolpyruvate from Pyruvate?

Pyruvate carboxylase and Phosphoenolpyruvate Carboxykinase. ### Explanation The conversion of Pyruvate to Phosphoenolpyruvate (PEP) is the first major hurdle in **Gluconeogenesis**. In Glycolysis, the conversion of PEP to Pyruvate by *Pyruvate Kinase* is irreversible. To bypass this, the body employs a two-step "bypass" mechanism: 1. **Pyruvate Carboxylase (PC):** Located in the mitochondria, this enzyme converts Pyruvate into Oxaloacetate (OAA). This reaction requires **Biotin** as a cofactor and ATP. 2. **Phosphoenolpyruvate Carboxykinase (PEPCK):** This enzyme converts OAA into PEP. This step occurs in the cytosol (or mitochondria) and requires **GTP**. **Analysis of Incorrect Options:** * **Pyruvate Dehydrogenase (PDH):** This enzyme converts Pyruvate to Acetyl-CoA. This is an irreversible step that leads into the TCA cycle, not gluconeogenesis. "Acetyl-CoA cannot be converted back to glucose." * **Lactate Dehydrogenase (LDH):** This catalyzes the reversible conversion of Pyruvate to Lactate (anaerobic glycolysis). It does not participate in the bypass of Pyruvate Kinase. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** Pyruvate Carboxylase is a **mitochondrial** enzyme, while PEPCK is found in both the mitochondria and cytosol. * **Obligatory Activator:** Pyruvate Carboxylase is allosterically activated by **Acetyl-CoA**. High levels of Acetyl-CoA signal that the cell has enough energy, shunting pyruvate toward gluconeogenesis. * **Cofactor:** Remember the mnemonic **ABC** for carboxylases: **A**TP, **B**iotin, and **C**O₂ are required. * **Rate-limiting step:** PEPCK is often considered a key regulatory point in gluconeogenesis.

Q: Which of the following enzymes becomes deficient following an episode of severe infectious gastroenteritis?

Lactase. **Explanation:** The correct answer is **Lactase**. This question tests the concept of **Secondary Lactose Intolerance** following mucosal injury. **Why Lactase is the correct answer:** Lactase is a brush-border disaccharidase located at the **tips of the intestinal villi**. In cases of severe infectious gastroenteritis (viral, bacterial, or protozoal like Giardia), the inflammatory process leads to villous atrophy and sloughing of the enterocytes. Because lactase is located most superficially on the villi compared to other disaccharidases (like sucrase or maltase), it is the first enzyme to be lost and the last to recover during mucosal healing. This leads to a temporary inability to digest lactose, resulting in osmotic diarrhea, bloating, and flatulence after milk ingestion. **Why the other options are incorrect:** * **Amylase, Trypsin, and Lipase:** These are **pancreatic enzymes** secreted by the acinar cells of the pancreas into the duodenal lumen. Since infectious gastroenteritis primarily affects the intestinal mucosa and not the pancreatic parenchyma, the production and secretion of these enzymes remain largely unaffected. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** Lactase is found on the apical surface of differentiated enterocytes (brush border). * **Diagnosis:** Secondary lactose intolerance is often diagnosed via a **Hydrogen Breath Test** (increased $H_2$ due to bacterial fermentation of undigested lactose) or finding a **low stool pH** (acidic) with the presence of **reducing substances**. * **Recovery:** It may take 2–4 weeks for the brush border to regenerate and lactase levels to return to normal post-infection. * **Congenital vs. Acquired:** While congenital lactase deficiency is rare, secondary (acquired) deficiency is a common post-sequela of Rotavirus infection in children.

Question 1

Which of the following is a reversible enzyme?

Accepted Answer

Lactate dehydrogenase

Answer

Pyruvate kinase

Answer

Pyruvate dehydrogenase

Answer

Hexokinase

Question 2

G6PD helps in maintaining the integrity of RBC by:

Accepted Answer

Controlling oxidative stress on RBC

Answer

Controlling reduction stress on RBC

Answer

Maintaining flexibility of cell membrane

Answer

Component of electron transport chain

Question 3

A hypotonic baby shows an increased ratio of Pyruvate to Acetyl CoA. The baby also exhibits features of lactic acidosis. The underlying defect prevents pyruvate from forming Acetyl CoA in fibroblasts. Which of the following can revert this situation?

Accepted Answer

Thiamin

Answer

Biotin

Answer

Pyridoxine

Answer

Free fatty acid

Question 4

Which of the following enzymes is present inside the given organelle?

Accepted Answer

Glucose-6-phosphatase

Answer

Glucokinase

Answer

Isocitrate dehydrogenase

Answer

Fructose 2, 6 bisphosphatase

Question 5

Which of the following is NOT a product of the Hexose Monophosphate (HMP) shunt?

Accepted Answer

Glycerol-3-phosphate

Answer

Glyceraldehyde 3-phosphate

Answer

3CO2

Answer

6NADPH

Question 6

Which of the following is NOT used in gluconeogenesis?

Accepted Answer

Oleate

Answer

Succinate

Answer

Glutamate

Answer

Aspartate

Question 7

Which of the following glucose transporters is primarily utilized by adipocytes?

Accepted Answer

GLUT4

Answer

GLUT1

Answer

GLUT2

Answer

GLUT3

Question 8

What are the enzymes required for the formation of Phosphoenolpyruvate from Pyruvate?

Accepted Answer

Pyruvate carboxylase and Phosphoenolpyruvate Carboxykinase

Answer

Pyruvate dehydrogenase and Pyruvate carboxylase

Answer

Lactate Dehydrogenase and Pyruvate Dehydrogenase

Answer

Phosphoenolpyruvate carboxykinase and Pyruvate Dehydrogenase

Question 9

All of the following are true about changes in brain metabolism after traumatic brain injury, except:

Accepted Answer

Increased CSF lactate is associated with good prognosis

Answer

Shut down of pyruvate dehydrogenase activity

Answer

Accumulation of lactate in brain

Answer

Increased lactate uptake from circulation

Question 10

Which of the following enzymes becomes deficient following an episode of severe infectious gastroenteritis?

Accepted Answer

Lactase

Answer

Amylase

Answer

Trypsin

Answer

Lipase

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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