Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 561: What is the Pasteur effect?

A. Inhibition of glycolysis by oxygen (Correct Answer)
B. Inhibition of glycolysis due to absence of oxygen
C. Reversible reaction of glycolysis
D. Acceleration of glycolysis by oxygen

Explanation: **Explanation:** The **Pasteur Effect** refers to the observation that the rate of glycolysis is significantly decreased in the presence of oxygen compared to anaerobic conditions. **Why Option A is Correct:** In the presence of oxygen (aerobic conditions), cells can utilize the **Electron Transport Chain (ETC)** and oxidative phosphorylation. This process is far more efficient, yielding **30–32 ATP** per glucose molecule, compared to only **2 ATP** produced during anaerobic glycolysis. Because the cell’s energy requirements are met more efficiently, high levels of ATP and citrate act as allosteric inhibitors of **Phosphofructokinase-1 (PFK-1)**, the rate-limiting enzyme of glycolysis. Thus, oxygen "inhibits" the rapid consumption of glucose. **Why Other Options are Incorrect:** * **Option B:** The absence of oxygen actually *stimulates* glycolysis (the opposite of the Pasteur effect) to compensate for low ATP yields. * **Option C:** Glycolysis contains three irreversible steps (Hexokinase, PFK-1, and Pyruvate Kinase); the Pasteur effect is a regulatory phenomenon, not a description of reversibility. * **Option D:** Oxygen decelerates glycolysis; the *acceleration* of glycolysis in the presence of oxygen is known as the **Warburg Effect**, typically seen in cancer cells. **NEET-PG High-Yield Pearls:** * **Key Enzyme:** The Pasteur effect primarily targets **PFK-1**. * **Warburg Effect:** This is the "clinical opposite" where cancer cells prefer glycolysis even when oxygen is plentiful (aerobic glycolysis), allowing them to use glycolytic intermediates for cell growth. * **Crabtree Effect:** The inhibition of oxygen consumption by high concentrations of glucose (seen in yeast and some tumor cells).

Question 562: Muscle glycogen cannot contribute to blood glucose due to the absence of which enzyme?

A. Phosphoglutamase
B. Branching enzyme
C. Debranching enzyme
D. Glucose-6-phosphatase (Correct Answer)

Explanation: **Explanation:** The correct answer is **Glucose-6-phosphatase**. **1. Why Glucose-6-phosphatase is the correct answer:** Glycogenolysis in both the liver and muscle produces Glucose-1-phosphate, which is converted to Glucose-6-phosphate (G6P). However, G6P is a charged molecule that cannot cross the cell membrane to enter the bloodstream. To become free glucose, it must be dephosphorylated by the enzyme **Glucose-6-phosphatase**. This enzyme is present in the liver and kidneys but is **absent in skeletal muscle**. Consequently, muscle glycogen is used exclusively as an internal energy source for glycolysis to generate ATP during contraction, rather than for maintaining blood glucose levels. **2. Why the other options are incorrect:** * **Phosphoglucomutase (Option A):** This enzyme is present in muscle; it catalyzes the reversible conversion of Glucose-1-phosphate to Glucose-6-phosphate. * **Branching enzyme (Option B):** Also known as glucosyl 4:6 transferase, it is required for glycogen *synthesis* (glycogenesis), not breakdown. * **Debranching enzyme (Option C):** This enzyme is present in muscle and is essential for the complete breakdown of glycogen branches. Its deficiency leads to Cori’s disease (GSD Type III). **3. NEET-PG High-Yield Clinical Pearls:** * **Von Gierke’s Disease (GSD Type I):** Caused by a deficiency of Glucose-6-phosphatase. It presents with severe fasting hypoglycemia, hepatomegaly, and hyperuricemia because the liver cannot release glucose. * **Muscle vs. Liver:** Muscle glycogen stores are larger in total mass, but liver glycogen is the primary source for blood glucose during the first 12–18 hours of fasting. * **GLUT-4:** Remember that glucose uptake in muscles is mediated by insulin-dependent GLUT-4 transporters, but glucose *release* is impossible due to the lack of the phosphatase enzyme.

Question 563: Alpha-amylase acts on which type of glycosidic bond?

A. Alpha 1-4 glycosidic bond (Correct Answer)
B. Alpha 1-6 glycosidic bond
C. Beta 1-4 glycosidic bond
D. Beta 1-6 glycosidic bond

Explanation: **Explanation:** **Alpha-amylase** (both salivary and pancreatic) is an **endo-glycosidase** that specifically hydrolyzes the internal **alpha 1-4 glycosidic bonds** of polysaccharides like starch and glycogen. By breaking these linear bonds, it converts complex carbohydrates into smaller units like maltose, maltotriose, and alpha-limit dextrins. **Analysis of Options:** * **Option A (Correct):** Alpha-amylase acts exclusively on alpha 1-4 bonds. It requires a pH of 6.7–7.0 and chloride ions ($Cl^-$) for optimal activity. * **Option B (Incorrect):** Alpha 1-6 glycosidic bonds are the "branch points" in amylopectin and glycogen. Alpha-amylase **cannot** hydrolyze these bonds. These are broken by "debranching enzymes" (like isomaltase in the gut). * **Option C (Incorrect):** Beta 1-4 bonds are found in **cellulose**. Humans lack the enzyme (cellulase) to break these bonds, which is why cellulose serves as dietary fiber. * **Option D (Incorrect):** Beta 1-6 bonds are not typically found in major dietary starches or glycogen. **High-Yield Clinical Pearls for NEET-PG:** * **Chloride Activation:** Alpha-amylase is a metalloenzyme that requires **Calcium ($Ca^{2+}$)** for stability and **Chloride ($Cl^-$)** for activation. * **Limit Dextrins:** Because amylase cannot bypass or break alpha 1-6 branch points, the remaining branched fragments are called **alpha-limit dextrins**. * **Diagnostic Marker:** Serum amylase rises in **Acute Pancreatitis** (within 2–12 hours), though lipase is considered more specific. * **Isoenzymes:** Salivary amylase (Ptyalin) is inactivated by gastric HCl, while Pancreatic amylase continues digestion in the duodenum.

Question 564: In which of the following tissues is glycogen incapable of contributing directly to blood glucose?

A. Liver
B. Muscle (Correct Answer)
C. Both
D. None

Explanation: **Explanation:** The correct answer is **Muscle** because it lacks the enzyme **Glucose-6-Phosphatase**. **1. Why Muscle is the Correct Answer:** While both the liver and skeletal muscle store glycogen, their physiological roles differ. In the liver, glycogenolysis produces Glucose-6-Phosphate (G6P), which is then converted into free glucose by the enzyme **Glucose-6-Phosphatase**. This free glucose can exit the cell via GLUT-2 transporters to maintain blood glucose levels. In contrast, skeletal muscle lacks Glucose-6-Phosphatase. Therefore, the G6P generated from muscle glycogenolysis is trapped within the muscle cell and must enter the **glycolytic pathway** to provide ATP for local muscle contraction. Consequently, muscle glycogen cannot contribute directly to blood glucose. **2. Why Other Options are Incorrect:** * **A. Liver:** The liver is the primary organ responsible for maintaining blood glucose during fasting. It contains high concentrations of Glucose-6-Phosphatase, allowing it to release glucose into the bloodstream. * **C & D:** These are incorrect based on the specific enzymatic deficiency in muscle tissue described above. **High-Yield Clinical Pearls for NEET-PG:** * **Von Gierke’s Disease (GSD Type I):** Caused by a deficiency of Glucose-6-Phosphatase. It results in severe fasting hypoglycemia because neither glycogenolysis nor gluconeogenesis can release glucose from the liver. * **Cori Cycle:** Muscle glycogen can *indirectly* contribute to blood glucose via the Cori Cycle, where muscle-derived lactate travels to the liver to be converted back into glucose. * **Key Enzyme:** Glycogen phosphorylase is the rate-limiting enzyme for glycogenolysis, but Glucose-6-Phosphatase is the "gatekeeper" for glucose release into the blood.

Question 565: Which enzyme is responsible for the complete oxidation of glucose to CO2 and water?

A. Cytosol
B. Mitochondria (Correct Answer)
C. Lysosomes
D. Endoplasmic Reticulum

Explanation: **Explanation:** The complete oxidation of glucose involves three major stages: Glycolysis, the TCA (Krebs) cycle, and the Electron Transport Chain (ETC). While glycolysis occurs in the cytosol, it only partially oxidizes glucose to pyruvate. The **Mitochondria** is the definitive site for complete oxidation because it houses the Pyruvate Dehydrogenase (PDH) complex, the TCA cycle enzymes, and the machinery for Oxidative Phosphorylation. Within the mitochondrial matrix, acetyl-CoA is oxidized to **CO2**, and the resulting reducing equivalents (NADH/FADH2) are used by the ETC on the inner mitochondrial membrane to produce **water** and ATP. **Analysis of Incorrect Options:** * **A. Cytosol:** This is the site for glycolysis (partial oxidation) and the HMP shunt. It lacks the oxygen-dependent machinery required to break down pyruvate into CO2 and water. * **C. Lysosomes:** These are "suicide bags" involved in the degradation of macromolecules (proteolysis, lipolysis) via acid hydrolases, not energy metabolism. * **D. Endoplasmic Reticulum:** The ER is primarily involved in protein synthesis (RER), lipid synthesis, and detoxification (SER). It plays a role in gluconeogenesis (Glucose-6-phosphatase) but not glucose oxidation. **High-Yield NEET-PG Pearls:** * **Mitochondria** are often called the "Powerhouse of the cell" because they generate >90% of cellular ATP. * **RBCs** lack mitochondria; therefore, they can never oxidize glucose completely and rely solely on anaerobic glycolysis (lactate production). * **Key Mitochondrial Enzymes:** Pyruvate Dehydrogenase, Citrate Synthase, and Cytochrome Oxidase. * **Site Marker:** Succinate dehydrogenase is a marker enzyme for the inner mitochondrial membrane.

Question 566: Skeletal muscle is deficient in which of the following enzymes?

A. Glucose-6-phosphatase (Correct Answer)
B. Hexokinase
C. Isomerase
D. Phosphofructokinase

Explanation: ### Explanation **1. Why Glucose-6-phosphatase is the correct answer:** Glucose-6-phosphatase is the enzyme responsible for converting Glucose-6-phosphate into free glucose. This enzyme is primarily located in the **liver** and **kidneys**. Skeletal muscle lacks this enzyme; therefore, it cannot release free glucose into the bloodstream. Instead, the glucose-6-phosphate derived from muscle glycogenolysis enters the glycolytic pathway to provide ATP locally for muscle contraction. This ensures that muscle glycogen is a "selfish" fuel source, reserved exclusively for the muscle's own energy needs. **2. Analysis of Incorrect Options:** * **Hexokinase (Option B):** This enzyme is present in skeletal muscle. It catalyzes the first step of glycolysis (Glucose → Glucose-6-phosphate). It has a low Km (high affinity) for glucose, allowing muscles to utilize glucose even at low blood concentrations. * **Isomerase (Option C):** Specifically Phosphohexose Isomerase, this enzyme is essential for the second step of glycolysis (Glucose-6-P ↔ Fructose-6-P) and is present in all tissues, including muscle. * **Phosphofructokinase (Option D):** PFK-1 is the rate-limiting enzyme of glycolysis and is highly active in skeletal muscle to support rapid energy production during exercise. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Von Gierke’s Disease (GSD Type I):** Caused by a deficiency of Glucose-6-phosphatase. It presents with severe fasting hypoglycemia because neither glycogenolysis nor gluconeogenesis can export glucose from the liver. * **Cori Cycle:** Since muscles cannot release glucose, they release **lactate** during anaerobic exercise. This lactate travels to the liver, where it is converted back to glucose via gluconeogenesis. * **Glucokinase vs. Hexokinase:** Remember that the liver has Glucokinase (high Km), while muscles have Hexokinase (low Km).

Question 567: The Rapaport-Luebering cycle causes a shift of the O2 dissociation curve to the right because of which of the following?

A. 2,3-Bisphosphoglycerate (2,3-BPG) (Correct Answer)
B. 1,3-Bisphosphoglycerate (1,3-BPG)
C. 3-Phosphoglycerate
D. Fructose 1,6-bisphosphate

Explanation: ### Explanation **1. Why Option A is Correct:** The **Rapaport-Luebering cycle** is a supplemental pathway of glycolysis occurring specifically in erythrocytes (RBCs). In this shunt, the enzyme **Bisphosphoglycerate mutase** converts 1,3-bisphosphoglycerate (1,3-BPG) into **2,3-bisphosphoglycerate (2,3-BPG)**. 2,3-BPG is a potent allosteric effector of hemoglobin. It binds to the central cavity of the deoxyhemoglobin tetramer (T-state), stabilizing it and reducing hemoglobin's affinity for oxygen. This results in a **rightward shift of the Oxygen Dissociation Curve (ODC)**, facilitating the unloading of oxygen to peripheral tissues. **2. Why Other Options are Incorrect:** * **Option B (1,3-BPG):** This is a high-energy intermediate of the standard glycolytic pathway. While it is the precursor for 2,3-BPG, it does not directly bind to hemoglobin or affect oxygen affinity. * **Option C (3-Phosphoglycerate):** This is the product formed when 2,3-BPG is hydrolyzed by phosphatase or when 1,3-BPG transfers a phosphate to ADP. It has no regulatory role in oxygen transport. * **Option D (Fructose 1,6-bisphosphate):** This is an upstream intermediate of glycolysis (cleaved by Aldolase A). It does not participate in the Rapaport-Luebering shunt. **3. High-Yield Clinical Pearls for NEET-PG:** * **Energy Trade-off:** By bypassing the Phosphoglycerate kinase step, the RBC forfeits the production of 2 ATP molecules. Thus, the Rapaport-Luebering cycle is "expensive" for the cell but essential for tissue oxygenation. * **Adaptation:** 2,3-BPG levels increase during **chronic hypoxia** and at **high altitudes** to improve oxygen delivery to tissues. * **Fetal Hemoglobin (HbF):** HbF has a lower affinity for 2,3-BPG compared to HbA (due to the substitution of Histidine with Serine in the gamma chain). This allows HbF to have a higher oxygen affinity, enabling oxygen transfer from mother to fetus. * **Storage:** 2,3-BPG levels decrease in stored blood; hence, massive transfusions of old blood can cause "oxygen trapping" (left shift).

Question 568: A 2-week-old neonate presents with complete hypotonia, convulsions, failure to thrive, and metabolic acidosis. The urine has a 'burnt sugar' odor, and a positive DNPH test. What is the enzyme deficiency in this metabolic disorder?

A. Isovaleryl CoA dehydrogenase
B. Dihydrolipoamide dehydrogenase
C. Branched chain keto acid dehydrogenase (Correct Answer)
D. Transacylase

Explanation: ### Explanation The clinical presentation of hypotonia, convulsions, failure to thrive, and metabolic acidosis in a neonate, combined with the pathognomonic **"burnt sugar" odor** of urine, points directly to **Maple Syrup Urine Disease (MSUD)**. **1. Why the Correct Answer is Right:** MSUD is caused by a deficiency in the **Branched-Chain Keto Acid Dehydrogenase (BCKAD) complex**. This multi-enzyme complex is responsible for the oxidative decarboxylation of keto acids derived from the branched-chain amino acids (BCAAs): **Leucine, Isoleucine, and Valine**. * The accumulation of **Isoleucine** (specifically its byproduct, alpha-keto-beta-methylvalerate) is responsible for the characteristic odor. * The **DNPH (2,4-Dinitrophenylhydrazine) test** is positive because it reacts with alpha-keto acids to form a yellow-white precipitate, confirming the presence of these metabolites in the urine. **2. Why Other Options are Incorrect:** * **Option A (Isovaleryl CoA dehydrogenase):** Deficiency leads to **Isovaleric Acidemia**. While it presents similarly, the characteristic odor is described as **"sweaty feet"** or "cheese-like," not burnt sugar. * **Option B & D (Dihydrolipoamide dehydrogenase & Transacylase):** These are components of the BCKAD complex (E3 and E2 subunits, respectively). While their deficiency can cause MSUD, the question asks for the primary enzyme complex deficiency associated with the classic clinical syndrome. BCKAD is the standard answer for the overall enzyme deficiency in MSUD. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for BCAAs:** "LIV" (Leucine, Isoleucine, Valine). * **Cofactors for BCKAD:** TPP (B1), FAD (B2), NAD (B3), Lipoic acid, and CoA (The same as Pyruvate Dehydrogenase). * **Treatment:** Dietary restriction of BCAAs and, in some cases, high-dose **Thiamine (Vitamin B1)** supplementation (for Thiamine-responsive variants). * **Diagnosis:** Elevated levels of BCAAs in plasma (especially Leucine) and presence of **Alloisoleucine** (diagnostic marker).

Question 569: Which of the following reactions in the TCA cycle is an example of substrate-level phosphorylation?

A. Isocitrate to oxalosuccinate
B. Alpha-ketoglutarate to succinyl CoA
C. Succinyl CoA to succinate (Correct Answer)
D. Succinate to fumarate

Explanation: ### Explanation **Correct Answer: C. Succinyl CoA to succinate** In the TCA cycle, the conversion of **Succinyl CoA to Succinate** is the only step that generates a high-energy phosphate bond directly without the involvement of the electron transport chain. This process is called **Substrate-Level Phosphorylation (SLP)**. The enzyme **Succinate thiokinase** (also known as Succinyl CoA synthetase) cleaves the high-energy thioester bond of Succinyl CoA. The energy released is used to phosphorylate GDP to **GTP** (in the liver and kidneys) or ADP to **ATP** (in heart and muscle). This is a high-yield fact because it represents the only "direct" energy gain within the cycle itself. #### Analysis of Incorrect Options: * **Option A (Isocitrate to oxalosuccinate):** This is part of the reaction catalyzed by *Isocitrate dehydrogenase*. It involves the reduction of NAD+ to **NADH**, not the direct formation of ATP/GTP. * **Option B (Alpha-ketoglutarate to succinyl CoA):** Catalyzed by the *α-ketoglutarate dehydrogenase complex*, this oxidative decarboxylation produces **NADH** and CO₂. * **Option D (Succinate to fumarate):** Catalyzed by *Succinate dehydrogenase* (Complex II of ETC), this reaction involves the reduction of FAD to **FADH₂**. #### NEET-PG High-Yield Pearls: 1. **Location:** The TCA cycle occurs in the **mitochondrial matrix**, but Succinate dehydrogenase is the only enzyme attached to the **inner mitochondrial membrane**. 2. **Arsenite Poisoning:** The α-ketoglutarate dehydrogenase complex is inhibited by Arsenite. 3. **ATP Yield:** One turn of the TCA cycle yields **10 ATP** (3 NADH = 7.5, 1 FADH₂ = 1.5, 1 GTP = 1). 4. **Regulatory Step:** The conversion of Isocitrate to α-ketoglutarate is the **rate-limiting step** of the cycle.

Question 570: What is the enzyme deficiency in McArdle syndrome?

A. Muscle phosphorylase (Correct Answer)
B. Liver phosphorylase
C. Liver debranching enzyme
D. Glycogen synthase

Explanation: **Explanation:** **McArdle Syndrome (GSD Type V)** is a glycogen storage disease characterized by the deficiency of **Muscle Phosphorylase** (also known as myophosphorylase). This enzyme is responsible for the rate-limiting step of glycogenolysis in skeletal muscle, breaking down glycogen into glucose-1-phosphate. Without it, muscles cannot mobilize glucose during exercise, leading to ATP depletion. **Why the correct answer is right:** * **Muscle Phosphorylase (Option A):** In GSD Type V, the muscle-specific isoform of glycogen phosphorylase is absent. This results in an inability to perform anaerobic glycolysis, leading to exercise intolerance, muscle cramps, and myoglobinuria. **Why the incorrect options are wrong:** * **Liver Phosphorylase (Option B):** Deficiency of this enzyme causes **Hers Disease (GSD Type VI)**, which presents with hepatomegaly and mild hypoglycemia, but no muscle symptoms. * **Liver Debranching Enzyme (Option C):** Deficiency causes **Cori Disease (GSD Type III)**. It affects both liver and muscle but is characterized by the accumulation of "limit dextrins" (abnormal glycogen structure). * **Glycogen Synthase (Option D):** Deficiency leads to **GSD Type 0**, characterized by fasting hypoglycemia and ketosis because glycogen cannot be stored at all. **High-Yield Clinical Pearls for NEET-PG:** 1. **"Second Wind" Phenomenon:** A classic sign where patients experience a decrease in heart rate and improved exercise tolerance after a few minutes of activity (due to a switch to fatty acid oxidation and increased blood flow). 2. **Ischemic Forearm Exercise Test:** Shows a **failure of blood lactate to rise** (flat lactate curve) with a significant rise in ammonia. 3. **Burgundy-colored urine:** Post-exercise myoglobinuria can lead to acute renal failure. 4. **Biopsy:** Shows subsarcolemmal deposits of normal-structured glycogen.

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