Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 381: In which one of the following tissues is glucose transport into the cell unaffected by insulin?

A. Skeletal muscle
B. Liver (Correct Answer)
C. Adipose tissue
D. Smooth muscle

Explanation: **Explanation:** The entry of glucose into cells is mediated by a family of glucose transporters (GLUT). The key to answering this question lies in distinguishing between **insulin-dependent** and **insulin-independent** transporters. **Why Liver is the correct answer:** The liver primarily utilizes **GLUT-2** for glucose transport. GLUT-2 is an insulin-independent transporter with a high $K_m$ (low affinity) and high capacity. This allows the liver to sense and uptake glucose proportionally to blood glucose levels (e.g., after a meal) without requiring insulin to trigger the translocation of transporters to the cell membrane. While insulin does influence hepatic glucose metabolism (by stimulating glycolysis and glycogenesis), the physical **transport** of glucose into the hepatocyte remains unaffected by insulin levels. **Why the other options are incorrect:** * **Skeletal Muscle & Smooth Muscle:** These tissues primarily express **GLUT-4**, which is the major insulin-responsive glucose transporter. In the resting state, GLUT-4 is sequestered in intracellular vesicles. Insulin binding to its receptor triggers the translocation of GLUT-4 to the plasma membrane, increasing glucose uptake. * **Adipose Tissue:** Like muscle, adipocytes rely on **GLUT-4** for glucose uptake. Without insulin, glucose entry into these cells is significantly restricted. **NEET-PG High-Yield Pearls:** * **GLUT-1:** Found in RBCs and the Blood-Brain Barrier (Basal uptake). * **GLUT-2:** Found in Liver, Pancreatic $\beta$-cells, Kidney, and Small Intestine (Bidirectional). * **GLUT-3:** Found in Neurons (Highest affinity/Low $K_m$). * **GLUT-4:** Found in Skeletal muscle, Cardiac muscle, and Adipose tissue (**Only insulin-dependent GLUT**). * **GLUT-5:** Primarily a Fructose transporter found in the Small Intestine and Spermatozoa.

Question 382: Which is the rate-limiting enzyme in the TCA cycle?

A. Isocitrate dehydrogenase (Correct Answer)
B. Citrate synthase
C. Alpha-ketoglutarate dehydrogenase
D. Succinate dehydrogenase

Explanation: **Explanation:** The **TCA cycle (Krebs cycle)** is the final common pathway for the oxidation of carbohydrates, lipids, and proteins. While several steps are irreversible, the primary **rate-limiting and committed step** is the conversion of isocitrate to alpha-ketoglutarate. **1. Why Isocitrate Dehydrogenase (ICDH) is correct:** ICDH catalyzes the oxidative decarboxylation of isocitrate. It is the most important regulatory checkpoint because it is strongly inhibited by high energy signals (**ATP and NADH**) and activated by low energy signals (**ADP and $Ca^{2+}$**). This enzyme dictates the overall velocity of the cycle based on the cell's energy status. **2. Analysis of Incorrect Options:** * **Citrate Synthase:** This is the first enzyme of the cycle. While it is a regulatory step (inhibited by ATP, NADH, and Succinyl-CoA), it is not considered the primary rate-limiting step because its substrate, Oxaloacetate, is often the limiting factor rather than the enzyme activity itself. * **Alpha-ketoglutarate Dehydrogenase:** This enzyme complex catalyzes an irreversible step and is a key regulatory point (inhibited by its products Succinyl-CoA and NADH). However, it functions downstream of ICDH. * **Succinate Dehydrogenase:** This enzyme (also known as Complex II of the ETC) catalyzes a reversible reaction and is not a major regulatory or rate-limiting site. **High-Yield Clinical Pearls for NEET-PG:** * **Co-factors:** Alpha-ketoglutarate dehydrogenase requires five cofactors: **T**hiamine (B1), **R**iboflavin (B2), **N**iacin (B3), **P**antothenic acid (B5), and **L**ipoic acid (Mnemonic: **T**ender **R**oving **N**ights **P**lease **L**uck). * **Inhibitor:** Fluoroacetate inhibits Aconitase, while Arsenite inhibits Alpha-ketoglutarate dehydrogenase. * **ATP Yield:** One turn of the TCA cycle produces **10 ATP** (3 NADH = 7.5, 1 $FADH_2$ = 1.5, 1 GTP = 1).

Question 383: Which metabolite is considered the dead end in glycolysis?

A. Pyruvate
B. Lactate (Correct Answer)
C. 2,3-bisphosphoglycerate
D. 3-phosphoglycerate

Explanation: ### Explanation **Correct Option: B. Lactate** In biochemistry, a "dead end" metabolite refers to a molecule that has no further metabolic pathway available to it other than being converted back into its immediate precursor. Under anaerobic conditions or in cells lacking mitochondria (like mature RBCs), pyruvate is reduced to **Lactate** by the enzyme **Lactate Dehydrogenase (LDH)**. This reaction is essential to regenerate **NAD+** from NADH, allowing glycolysis to continue. Once lactate is formed, it cannot be further metabolized within that specific cell; it must be released into the blood and transported to the liver to be converted back to glucose via the Cori Cycle (gluconeogenesis). **Analysis of Incorrect Options:** * **A. Pyruvate:** This is a central metabolic hub, not a dead end. It can be converted into Acetyl-CoA (link reaction), Oxaloacetate (gluconeogenesis), or Alanine (transamination). * **C. 2,3-bisphosphoglycerate (2,3-BPG):** This is a bypass product of the Rapoport-Luebering shunt in RBCs. It is an intermediate that can re-enter the glycolytic pathway as 3-phosphoglycerate. * **D. 3-phosphoglycerate:** This is a standard intermediate of the payoff phase of glycolysis and continues forward to eventually form pyruvate. **NEET-PG High-Yield Pearls:** * **The Cori Cycle:** The metabolic cooperation between skeletal muscle (producing lactate) and the liver (converting lactate to glucose) is a favorite exam topic. * **RBC Metabolism:** Since RBCs lack mitochondria, lactate is their obligatory end product of glycolysis. * **Lactic Acidosis:** Occurs when there is a failure in the delivery of oxygen (hypoxia) or a failure in the liver's ability to clear lactate, leading to a drop in blood pH.

Question 384: Which of the following statements is true?

A. Glucose is a ketose.
B. Glucose is a C2 epimer of fructose.
C. Glucose is a C4 epimer of galactose. (Correct Answer)
D. Ribose and fructose are epimers.

Explanation: **Explanation** **1. Why the Correct Answer is Right:** The concept of **epimerism** refers to isomers that differ in configuration around only one specific carbon atom (excluding the anomeric carbon). Glucose and galactose are both aldohexoses with the same chemical formula ($C_6H_{12}O_6$). They are identical in structure except for the orientation of the hydroxyl (-OH) group at the **C4 position**. In glucose, the -OH at C4 is on the right (Fisher projection), whereas in galactose, it is on the left. Thus, Glucose is the C4 epimer of Galactose. **2. Why the Other Options are Incorrect:** * **A. Glucose is a ketose:** Incorrect. Glucose is an **aldose** (contains an aldehyde group at C1). Fructose is the most common example of a ketose (contains a keto group at C2). * **B. Glucose is a C2 epimer of fructose:** Incorrect. Glucose and fructose are **functional isomers**, not epimers, because they belong to different chemical families (aldose vs. ketose). **Mannose** is the C2 epimer of glucose. * **D. Ribose and fructose are epimers:** Incorrect. Ribose is a 5-carbon sugar (pentose), while fructose is a 6-carbon sugar (hexose). Epimers must have the same number of carbon atoms. **3. NEET-PG High-Yield Clinical Pearls:** * **The "Big Three" Epimers:** * Glucose & Galactose (C4) * Glucose & Mannose (C2) * **Clinical Correlation:** In **Galactosemia** (deficiency of GALT enzyme), the body fails to convert galactose to glucose. * **Essential Fact:** Epimers are a subtype of diastereomers. All epimers are isomers, but not all isomers are epimers. * **Enzyme Note:** Enzymes that interconvert epimers are called **epimerases** (e.g., UDP-glucose 4-epimerase).

Question 385: Which type of glycogen storage disease predominantly involves muscle?

A. Type I
B. Type III
C. Type IV
D. Type V (Correct Answer)

Explanation: **Explanation:** **Type V Glycogen Storage Disease (GSD)**, also known as **McArdle Disease**, is caused by a deficiency of **muscle glycogen phosphorylase** (myophosphorylase). This enzyme is responsible for breaking down glycogen into glucose-1-phosphate within myocytes. Since muscle lacks glucose-6-phosphatase, it uses glycogen solely for local energy production during exercise. A deficiency leads to an inability to mobilize glucose during anaerobic exercise, resulting in exercise intolerance, muscle cramps, and myoglobinuria. **Analysis of Incorrect Options:** * **Type I (von Gierke Disease):** Involves a deficiency of Glucose-6-Phosphatase. It predominantly affects the **liver** and kidneys, presenting with severe fasting hypoglycemia and hepatomegaly. Muscle is not affected because muscle lacks this enzyme normally. * **Type III (Cori Disease):** Caused by a deficiency of the **Debranching enzyme**. While it can involve muscle (Type IIIa), it is primarily characterized by hepatomegaly and growth retardation, similar to a milder version of Type I. * **Type IV (Andersen Disease):** Caused by a deficiency of the **Branching enzyme**. It leads to the accumulation of abnormal glycogen (amylopectin-like) which triggers an immune response, primarily causing **liver cirrhosis** and failure in early childhood. **High-Yield Clinical Pearls for NEET-PG:** * **"Second Wind" Phenomenon:** A classic feature of McArdle disease where symptoms improve after a few minutes of exercise as the body switches to using fatty acids and blood glucose. * **Ischemic Forearm Test:** Patients with Type V show a **failure of blood lactate to rise** after strenuous exercise (since they cannot break down glycogen to glucose/lactate). * **Burgundy-colored urine:** Post-exercise myoglobinuria can lead to renal failure; look for elevated Creatine Kinase (CK) levels.

Question 386: Which glycogen storage disease does not affect muscles?

A. Type 1 (Correct Answer)
B. Type 2
C. Type 3
D. Type 4

Explanation: **Explanation:** The correct answer is **Type 1 (Von Gierke Disease)**. **Why Type 1 is the correct answer:** Type 1 Glycogen Storage Disease (GSD) is caused by a deficiency of **Glucose-6-Phosphatase**. This enzyme is primarily located in the **liver and kidneys**, where it is responsible for the final step of gluconeogenesis and glycogenolysis (converting Glucose-6-Phosphate into free glucose). Crucially, **skeletal muscle lacks this enzyme** even under normal physiological conditions. Therefore, the metabolic defect in Type 1 GSD is restricted to the liver and kidneys, leading to severe fasting hypoglycemia and hepatomegaly, but sparing the muscles. **Why the other options are incorrect:** * **Type 2 (Pompe Disease):** Caused by a deficiency of **Lysosomal acid alpha-glucosidase**. This enzyme is present in all tissues. Its deficiency leads to glycogen accumulation in the lysosomes of cardiac and skeletal muscles, causing massive cardiomegaly and generalized muscle weakness. * **Type 3 (Cori Disease):** Caused by a deficiency of the **Debranching enzyme**. This enzyme is active in both the liver and muscles. Consequently, patients present with both hepatomegaly and myogenic features like progressive muscle weakness. * **Type 4 (Andersen Disease):** Caused by a deficiency of the **Branching enzyme**. This leads to the formation of abnormal glycogen (polyglucosan) which triggers an immune response, affecting the liver, heart, and skeletal muscles (myopathy). **High-Yield Clinical Pearls for NEET-PG:** * **Type 1 (Von Gierke):** Characterized by "Doll-like facies," hyperuricemia (gout), lactic acidosis, and hyperlipidemia. * **Type 5 (McArdle Disease):** Affects **only** the muscles (Myophosphorylase deficiency), presenting with exercise-induced cramps and myoglobinuria. * **Mnemonic:** Remember that Glucose-6-Phosphatase is absent in muscle; hence, muscle cannot contribute to blood glucose levels.

Question 387: Which of the following is NOT a substrate for gluconeogenesis?

A. Lactate
B. Propionate
C. Alanine
D. Acetyl-CoA (Correct Answer)

Explanation: **Explanation:** Gluconeogenesis is the metabolic pathway that generates glucose from non-carbohydrate precursors. The correct answer is **Acetyl-CoA** because it cannot be converted into glucose in humans. **Why Acetyl-CoA is NOT a substrate:** The conversion of Pyruvate to Acetyl-CoA by the *Pyruvate Dehydrogenase (PDH) complex* is **irreversible**. Once Acetyl-CoA enters the TCA cycle, it condenses with oxaloacetate to form citrate. During the cycle, two carbons are lost as $CO_2$ before oxaloacetate is regenerated. Therefore, there is no net gain of carbon atoms to be diverted toward glucose synthesis. **Analysis of other options:** * **Lactate:** Produced by anaerobic glycolysis in muscles and RBCs, it is transported to the liver and converted back to pyruvate via the **Cori Cycle**, serving as a major substrate. * **Alanine:** The primary glucogenic amino acid. Through the **Glucose-Alanine Cycle**, it undergoes transamination to form pyruvate. * **Propionate:** This is the only part of a fatty acid that is glucogenic. Derived from **odd-chain fatty acids**, it enters the TCA cycle as Succinyl-CoA. **High-Yield Clinical Pearls for NEET-PG:** * **Key Regulatory Enzyme:** Fructose-1,6-bisphosphatase (inhibited by Fructose-2,6-bisphosphate). * **Location:** Occurs primarily in the **Liver** (90%) and Kidney (10%). * **Subcellular Sites:** It is a "bipartite" pathway, occurring in both the **Mitochondria** (Pyruvate carboxylase) and **Cytosol**. * **Leucine and Lysine:** These are the only two amino acids that are strictly ketogenic and cannot serve as substrates for gluconeogenesis.

Question 388: Which of the following is not a glycogen storage disorder?

A. Lesch Nyhan syndrome (Correct Answer)
B. McArdle's disease
C. Pompe's disease
D. Von-Gierke's disease

Explanation: **Explanation:** The correct answer is **Lesch-Nyhan syndrome** because it is a disorder of **purine metabolism**, not carbohydrate metabolism. It is caused by a deficiency of the enzyme **Hypoxanthine-Guanine Phosphoribosyltransferase (HGPRT)**, leading to the overproduction of uric acid. Clinically, it is characterized by hyperuricemia, intellectual disability, and a hallmark sign of **self-mutilation**. **Analysis of Incorrect Options (Glycogen Storage Diseases - GSDs):** * **Von Gierke’s Disease (GSD Type I):** Caused by a deficiency of **Glucose-6-Phosphatase**. It results in severe fasting hypoglycemia, hepatomegaly, and elevated levels of lactate and uric acid. * **Pompe’s Disease (GSD Type II):** Caused by a deficiency of **Lysosomal acid alpha-1,4-glucosidase (Acid Maltase)**. It is unique because it is a lysosomal storage disorder affecting the heart (cardiomegaly) and muscles. * **McArdle’s Disease (GSD Type V):** Caused by a deficiency of **Skeletal Muscle Glycogen Phosphorylase**. It presents with exercise-induced muscle cramps and myoglobinuria (Burgundy-colored urine). **NEET-PG High-Yield Pearls:** * **Mnemonic for GSDs:** "**V**ery **P**oor **C**arbohydrate **A**nd **M**etabolism **H**urt" (Type I: **V**on Gierke, II: **P**ompe, III: **C**ori, IV: **A**ndersen, V: **M**cArdle, VI: **H**ers). * **Lesch-Nyhan** is X-linked recessive; remember the "3 Ts": **T**ophaceous gout, **T**witching (dystonia), and **T**errible self-injury. * **Von Gierke** is the most common GSD; **Pompe** is the only GSD that is also a Lysosomal Storage Disease.

Question 389: What is the primary site of glycolysis in eukaryotic cells?

A. Cytoplasm (Correct Answer)
B. Mitochondria
C. Nucleus
D. Endoplasmic reticulum

Explanation: **Explanation:** **1. Why Cytoplasm is Correct:** Glycolysis (the Embden-Meyerhof pathway) is the universal metabolic pathway that converts glucose into pyruvate. In eukaryotic cells, all the enzymes required for this 10-step anaerobic process are located in the **cytoplasm (cytosol)**. This localization allows the cell to generate ATP and NADH rapidly without the need for oxygen or specialized organelles, making it the primary energy source for cells lacking mitochondria (like mature RBCs). **2. Why Other Options are Incorrect:** * **Mitochondria:** This is the site for aerobic metabolism, including the **TCA cycle (Krebs cycle)**, Electron Transport Chain (ETC), and Beta-oxidation of fatty acids. Pyruvate enters the mitochondria only *after* glycolysis is completed. * **Nucleus:** The nucleus is primarily involved in genetic material storage, DNA replication, and transcription; it does not house the metabolic machinery for glucose oxidation. * **Endoplasmic Reticulum (ER):** The ER is involved in protein synthesis (Rough ER) and lipid/steroid synthesis (Smooth ER). While the final step of gluconeogenesis (Glucose-6-phosphatase activity) occurs in the ER lumen, glycolysis does not. **3. NEET-PG High-Yield Clinical Pearls:** * **RBC Dependency:** Mature erythrocytes lack mitochondria; therefore, they depend **100% on cytoplasmic glycolysis** for energy. A defect in glycolytic enzymes (e.g., **Pyruvate Kinase deficiency**) leads to hereditary hemolytic anemia. * **Rapoport-Luebering Cycle:** A shunt of glycolysis occurring in RBCs that produces **2,3-BPG**, which decreases hemoglobin's affinity for oxygen, shifting the dissociation curve to the right. * **Rate-Limiting Step:** The conversion of Fructose-6-P to Fructose-1,6-bisphosphate by **Phosphofructokinase-1 (PFK-1)** is the key regulatory step of glycolysis.

Question 390: Substrate-level phosphorylation is seen in a reaction catalyzed by which of the following?

A. Succinate dehydrogenase
B. Alpha-ketoglutarate dehydrogenase
C. Succinyl CoA thiokinase (Correct Answer)
D. Malate dehydrogenase

Explanation: **Explanation:** **Substrate-level phosphorylation (SLP)** is the direct synthesis of ATP or GTP from ADP or GDP by the transfer of a high-energy phosphate group from a phosphorylated intermediate, without the involvement of the Electron Transport Chain (ETC) or molecular oxygen. In the Citric Acid Cycle (TCA cycle), the conversion of **Succinyl CoA to Succinate** is the only step that generates high-energy phosphate via SLP. This reaction is catalyzed by **Succinyl CoA thiokinase** (also known as Succinate thiokinase or Succinyl CoA synthetase). The high-energy thioester bond of Succinyl CoA is cleaved, providing the energy to phosphorylate GDP to GTP (which is subsequently converted to ATP). **Analysis of Incorrect Options:** * **A. Succinate dehydrogenase:** Catalyzes the oxidation of succinate to fumarate. It is part of Complex II of the ETC and generates FADH₂, which leads to ATP production via oxidative phosphorylation, not SLP. * **B. Alpha-ketoglutarate dehydrogenase:** An oxidative decarboxylation step that produces NADH and Succinyl CoA. While it creates a high-energy bond, it does not directly produce ATP/GTP. * **D. Malate dehydrogenase:** Catalyzes the conversion of malate to oxaloacetate, producing NADH. **High-Yield Clinical Pearls for NEET-PG:** * **Total SLP sites in Glucose Metabolism:** There are **3 sites** per molecule of glucose (under aerobic conditions): 1. 1,3-bisphosphoglycerate → 3-phosphoglycerate (Phosphoglycerate kinase) - *Glycolysis* 2. Phosphoenolpyruvate → Pyruvate (Pyruvate kinase) - *Glycolysis* 3. Succinyl CoA → Succinate (Succinyl CoA thiokinase) - *TCA Cycle* * **Arsenite Poisoning:** Inhibits the alpha-ketoglutarate dehydrogenase complex, halting the TCA cycle. * **Tissue Specificity:** In the liver and kidneys, Succinyl CoA thiokinase uses GDP (gluconeogenic tissues), while in heart and skeletal muscle, it primarily uses ADP.

Practice by Chapter

Carbohydrate Chemistry and Classification

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Glycolysis: Reactions and Regulation

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Gluconeogenesis: Reactions and Regulation

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Glycogen Metabolism: Synthesis and Breakdown

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Glycogen Storage Diseases

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Pentose Phosphate Pathway

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Metabolism of Fructose and Galactose

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Disorders of Fructose and Galactose Metabolism

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Blood Glucose Regulation

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Diabetes Mellitus: Biochemical Aspects

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Glycosylation and Glycoproteins

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Lactose Intolerance and Galactosemia

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