Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 181: Which of the following enzyme activities decreases during fasting?

A. Hormone sensitive lipase
B. Glycogen Phosphorylase
C. Acetyl CoA Carboxylase (Correct Answer)
D. CPS I

Explanation: **Explanation:** The metabolic state of **fasting** is characterized by a high **Glucagon:Insulin ratio**. This hormonal shift triggers the phosphorylation of key regulatory enzymes, leading to the activation of catabolic pathways (to generate energy) and the inhibition of anabolic pathways (to conserve energy). **Why Acetyl CoA Carboxylase (ACC) is the correct answer:** ACC is the rate-limiting enzyme for **De novo Fatty Acid Synthesis** (lipogenesis). During fasting, elevated Glucagon and Epinephrine trigger cAMP-dependent protein kinase (PKA) and AMP-activated protein kinase (AMPK), which **phosphorylate and inactivate ACC**. This prevents the synthesis of Malonyl-CoA, thereby halting fatty acid production and allowing fatty acid oxidation to proceed. **Analysis of Incorrect Options:** * **A. Hormone Sensitive Lipase (HSL):** This enzyme catalyzes lipolysis in adipose tissue. During fasting, HSL is **activated** via phosphorylation by PKA to mobilize free fatty acids for fuel. * **B. Glycogen Phosphorylase:** This is the rate-limiting enzyme of glycogenolysis. It is **activated** (phosphorylated) during fasting to maintain blood glucose levels. * **C. CPS I (Carbamoyl Phosphate Synthetase I):** This urea cycle enzyme is typically **increased** or maintained during fasting/high-protein intake to handle the nitrogen load from increased amino acid catabolism (gluconeogenesis). **High-Yield NEET-PG Pearls:** * **"Fed State" Enzymes (Dephosphorylated = Active):** Glucokinase, PFK-1, Pyruvate Kinase, Acetyl CoA Carboxylase, HMG-CoA Reductase. * **"Fasting State" Enzymes (Phosphorylated = Active):** Glycogen Phosphorylase, Fructose-2,6-Bisphosphatase, Hormone Sensitive Lipase. * **Malonyl-CoA** (produced by ACC) is a potent inhibitor of **Carnitine Palmitoyltransferase-1 (CPT-1)**; thus, when ACC is inactive, CPT-1 is active, facilitating Beta-oxidation.

Question 182: Which of the following enzymes is required for glycolysis?

A. Pyruvate kinase (Correct Answer)
B. Pyruvate carboxylase
C. Glucose-6-phosphatase
D. Glycerokinase

Explanation: ### Explanation **Correct Option: A. Pyruvate kinase** Glycolysis is the metabolic pathway that converts glucose into pyruvate. **Pyruvate kinase** is the enzyme responsible for the final step of glycolysis, where it catalyzes the irreversible transfer of a phosphate group from phosphoenolpyruvate (PEP) to ADP, yielding one molecule of pyruvate and one molecule of ATP. This is a key regulatory step and an example of substrate-level phosphorylation. **Why the other options are incorrect:** * **B. Pyruvate carboxylase:** This is a gluconeogenic enzyme that converts pyruvate to oxaloacetate. It is located in the mitochondria and requires biotin as a cofactor. * **C. Glucose-6-phosphatase:** This enzyme is involved in gluconeogenesis and glycogenolysis (found in the liver and kidneys). It converts glucose-6-phosphate back to free glucose, allowing it to enter the bloodstream. It is absent in muscle tissue. * **D. Glycerokinase:** This enzyme is involved in lipid metabolism, specifically the phosphorylation of glycerol to glycerol-3-phosphate. It is primarily found in the liver, which is why adipose tissue cannot reuse glycerol for TG synthesis. **High-Yield NEET-PG Pearls:** * **Rate-limiting step of glycolysis:** Phosphofructokinase-1 (PFK-1). * **Irreversible steps of glycolysis:** Glucokinase/Hexokinase, PFK-1, and Pyruvate Kinase (Steps 1, 3, and 10). * **Clinical Correlation:** **Pyruvate Kinase deficiency** is the second most common cause of enzyme-deficient hemolytic anemia (after G6PD deficiency). It leads to decreased ATP production, causing RBC membrane instability and "echinocytes" (burr cells) on peripheral smear. * **Regulation:** Pyruvate kinase is allosterically activated by Fructose-1,6-bisphosphate (feed-forward activation) and inhibited by ATP and Alanine.

Question 183: What is the net generation of ATP in glycolysis?

A. 5
B. 7 (Correct Answer)
C. 15
D. 20

Explanation: **Explanation:** In the context of NEET-PG and modern biochemistry (based on Harper’s Illustrated Biochemistry), the net ATP yield of glycolysis is calculated based on the **Malate-Aspartate Shuttle**, which is the predominant shuttle in the liver and heart. **Why 7 is the correct answer:** The net yield is calculated by subtracting the ATP consumed from the total ATP produced: 1. **ATP Consumed:** 2 ATP (at the Hexokinase and Phosphofructokinase-1 steps). 2. **ATP Produced (Substrate-level phosphorylation):** 4 ATP (at the Phosphoglycerate kinase and Pyruvate kinase steps). 3. **ATP from NADH (Oxidative phosphorylation):** 2 NADH are produced. Using the current oxidative phosphorylation ratios (1 NADH = 2.5 ATP), 2 NADH yield **5 ATP**. * **Calculation:** (4 + 5) - 2 = **7 ATP**. **Analysis of Incorrect Options:** * **Option A (5):** This would be the net yield if the **Glycerol-3-Phosphate Shuttle** (common in muscle/brain) were used, where 1 NADH yields only 1.5 ATP (Total: 4 + 3 - 2 = 5). However, unless specified, the higher yield is generally considered the standard "net" for the pathway. * **Option C (15) & D (20):** These values are incorrect for glycolysis alone. 15 ATP is the yield for one turn of the TCA cycle (including the PDH reaction), and 30-32 ATP is the total yield for the complete aerobic oxidation of one glucose molecule. **High-Yield Clinical Pearls for NEET-PG:** * **Anaerobic Glycolysis:** The net yield is always **2 ATP** because NADH is consumed to reduce pyruvate to lactate. * **Rate-limiting step:** Phosphofructokinase-1 (PFK-1). * **Rapoport-Luebering Cycle:** In RBCs, 2,3-BPG is produced, bypassing the first substrate-level phosphorylation, resulting in a net yield of **0 ATP**. * **Arsenic Poisoning:** Inhibits ATP production in glycolysis by competing with inorganic phosphate at the Glyceraldehyde-3-phosphate dehydrogenase step.

Question 184: Cori's cycle is a term used for which of the following pathways?

A. Lactic acid cycle (Correct Answer)
B. Citric acid cycle
C. Pentose phosphate pathway
D. None of the above

Explanation: **Explanation:** **Cori’s Cycle (Lactic Acid Cycle)** is a metabolic pathway that describes the interplay between the skeletal muscle and the liver. 1. **Why Option A is correct:** During vigorous exercise, muscular demand for ATP exceeds the oxygen supply, leading to **anaerobic glycolysis**. In this process, pyruvate is converted to **lactate** by the enzyme Lactate Dehydrogenase (LDH). This lactate is released into the bloodstream and taken up by the **liver**, where it is converted back into glucose via **gluconeogenesis**. This glucose is then returned to the muscle to be used as energy. This recycling of lactate to glucose is why it is synonymous with the Lactic acid cycle. 2. **Why other options are incorrect:** * **Option B (Citric acid cycle):** Also known as the Krebs cycle or TCA cycle, this occurs in the mitochondria and is the final common pathway for the oxidation of carbohydrates, fats, and proteins. * **Option C (Pentose phosphate pathway):** Also known as the Hexose Monophosphate (HMP) Shunt, this pathway generates NADPH and ribose-5-phosphate; it does not involve lactate recycling. **High-Yield Clinical Pearls for NEET-PG:** * **Net Energy Cost:** The Cori cycle is energy-consuming. It costs **6 ATP** in the liver to synthesize glucose, while only **2 ATP** are produced during anaerobic glycolysis in the muscle (Net loss of 4 ATP). * **Purpose:** Its primary role is to prevent **lactic acidosis** in the muscle and provide a continuous glucose supply during fasting or intense exercise. * **Glucose-Alanine Cycle (Cahill Cycle):** Often confused with Cori’s cycle, the Cahill cycle involves the transport of amino groups from muscle to liver via **Alanine** instead of lactate.

Question 185: All of the following vitamins are required in the citric acid cycle, EXCEPT:

A. Riboflavin
B. Niacin
C. Thiamin
D. Ascorbic acid (Correct Answer)

Explanation: **Explanation:** The Citric Acid Cycle (TCA cycle) is the central metabolic pathway for the oxidation of acetyl-CoA. It requires several B-complex vitamins acting as essential cofactors for its enzymatic reactions. **Why Ascorbic Acid (Vitamin C) is the correct answer:** Ascorbic acid is primarily involved in collagen synthesis (hydroxylation of proline and lysine), antioxidant defense, and iron absorption. It plays **no direct role** as a cofactor in the enzymes of the citric acid cycle. **Why the other options are incorrect:** The TCA cycle requires four specific B-vitamins to function: * **Thiamin (B1):** As Thiamin Pyrophosphate (TPP), it is a mandatory cofactor for the **α-Ketoglutarate dehydrogenase** complex. * **Riboflavin (B2):** As FAD, it acts as a prosthetic group for **Succinate dehydrogenase**. * **Niacin (B3):** As NAD+, it serves as an electron acceptor for **Isocitrate dehydrogenase**, **α-Ketoglutarate dehydrogenase**, and **Malate dehydrogenase**. * **Pantothenic acid (B5):** (Though not listed in options) It is a structural component of **Coenzyme A**, essential for the formation of Acetyl-CoA and Succinyl-CoA. **High-Yield Clinical Pearls for NEET-PG:** 1. **The "Big Four" Vitamins:** Remember that the α-Ketoglutarate dehydrogenase complex requires five cofactors: **T**hiamine (B1), **R**iboflavin (B2), **N**iacin (B3), **P**antothenic acid (B5), and **L**ipoic acid (Mnemonic: **T**ender **R**evolving **N**ew **P**ants **L**oose). 2. **Arsenite Poisoning:** Arsenite inhibits the α-Ketoglutarate dehydrogenase complex by binding to the -SH groups of Lipoic acid, leading to a clinical presentation similar to pyruvate dehydrogenase deficiency. 3. **Succinate Dehydrogenase:** This is the only enzyme of the TCA cycle that is also part of the Electron Transport Chain (Complex II) and is located on the inner mitochondrial membrane.

Question 186: For the tricarboxylic acid cycle to be continuous, what molecule requires regeneration?

A. Pyruvic acid
B. Oxaloacetic acid (Correct Answer)
C. Alpha-oxoglutaric acid
D. Malic acid

Explanation: The Tricarboxylic Acid (TCA) cycle, also known as the Krebs cycle, is a series of reactions occurring in the mitochondrial matrix. For the cycle to remain continuous, it must function as a true "closed loop." **Explanation of the Correct Answer:** **Oxaloacetic acid (OAA)** is the correct answer because it acts as the "starting material" and the "final product" of the cycle. The cycle begins when the 2-carbon Acetyl-CoA condenses with the 4-carbon **Oxaloacetate** to form Citrate (catalyzed by Citrate Synthase). Through a series of redox and decarboxylation reactions, OAA is eventually regenerated from Malate. If OAA is not regenerated, the cycle halts because there is no acceptor molecule for the incoming Acetyl-CoA. **Why the other options are incorrect:** * **Pyruvic acid:** This is a precursor to the TCA cycle (converted to Acetyl-CoA via the PDH complex) but is not a component *within* the cycle itself. * **Alpha-oxoglutaric acid (α-Ketoglutarate):** This is an intermediate produced during the cycle. While essential, it is consumed to form Succinyl-CoA and is not the specific molecule required to restart the condensation step. * **Malic acid:** This is the immediate precursor to OAA. While its oxidation is necessary, it is the regeneration of OAA specifically that allows the cycle to accept a new unit of Acetyl-CoA. **NEET-PG High-Yield Pearls:** * **Anaplerotic Reactions:** These are "filling up" reactions that replenish TCA intermediates. The most important anaplerotic reaction is the conversion of Pyruvate to OAA by **Pyruvate Carboxylase** (requires Biotin and ATP). * **Rate-Limiting Step:** Isocitrate Dehydrogenase is the key rate-limiting enzyme of the TCA cycle. * **Energy Yield:** One turn of the TCA cycle produces 3 NADH, 1 FADH₂, and 1 GTP (Total ~10 ATP equivalents).

Question 187: Glucose-6-phosphate dehydrogenase deficiency causes which of the following conditions?

A. Megaloblastic anemia
B. Hemolytic anemia (Correct Answer)
C. Sickle cell anemia
D. Microcytic anemia

Explanation: **Explanation:** **Glucose-6-phosphate dehydrogenase (G6PD) deficiency** is an X-linked recessive disorder and the most common enzyme deficiency worldwide. **Why Hemolytic Anemia is Correct:** G6PD is the rate-limiting enzyme of the **Hexose Monophosphate (HMP) Shunt**. Its primary role in red blood cells (RBCs) is to produce **NADPH**. NADPH is essential for maintaining a pool of **reduced glutathione**, which acts as an antioxidant to neutralize reactive oxygen species (like H2O2). In G6PD deficiency, the lack of NADPH leads to oxidative stress, causing hemoglobin to denature and precipitate as **Heinz bodies**. These damaged RBCs are destroyed in the spleen, resulting in **episodic hemolytic anemia**, typically triggered by infections, fava beans, or oxidant drugs (e.g., Primaquine, Sulfa drugs). **Why Other Options are Incorrect:** * **Megaloblastic Anemia:** Caused by Vitamin B12 or Folic acid deficiency, leading to impaired DNA synthesis and macrocytic RBCs. * **Sickle Cell Anemia:** A qualitative hemoglobinopathy caused by a point mutation (Glu → Val) in the beta-globin chain. * **Microcytic Anemia:** Most commonly caused by Iron deficiency or Thalassemia, characterized by small RBCs (low MCV). **High-Yield Clinical Pearls for NEET-PG:** * **Peripheral Smear:** Look for **Heinz bodies** (supravital stain) and **Bite cells** (degluticytes) formed by splenic macrophages. * **Inheritance:** X-linked recessive (primarily affects males). * **Protective Effect:** G6PD deficiency offers a survival advantage against *Plasmodium falciparum* malaria. * **Key Trigger:** Primaquine is a classic board-exam trigger for a hemolytic crisis in these patients.

Question 188: Which form of carbohydrate is present in glycoproteins?

A. Monosaccharide (Correct Answer)
B. Sugar alcohol
C. Homo polysaccharide
D. Hetero polysaccharide

Explanation: ### Explanation **Correct Answer: A. Monosaccharide** **Why it is correct:** Glycoproteins are proteins covalently bonded to short, often branched chains of carbohydrates. The fundamental building blocks attached to the polypeptide backbone are **monosaccharides** (such as glucose, galactose, mannose, N-acetylglucosamine, and sialic acid). These are typically linked via N-glycosidic bonds (to Asparagine) or O-glycosidic bonds (to Serine/Threonine). While these monosaccharides form short chains called oligosaccharides, the question asks for the *form* of carbohydrate present; since these chains are composed of individual sugar units rather than long-chain polymers, "monosaccharide" is the most accurate description of the constituent units. **Why other options are incorrect:** * **B. Sugar alcohol:** These (e.g., sorbitol, mannitol) are polyols formed by the reduction of aldoses or ketoses. They are not standard components of glycoprotein chains. * **C. Homo polysaccharide:** These consist of a single type of monosaccharide (e.g., glycogen, starch). Glycoproteins contain diverse, heterogeneous sugar units. * **D. Hetero polysaccharide:** These are long, high-molecular-weight chains (e.g., Glycosaminoglycans or GAGs). While glycoproteins contain different sugars, they are characterized by short **oligosaccharide** chains, not the long, repeating disaccharide units found in heteropolysaccharides (which characterize Proteoglycans). **High-Yield NEET-PG Pearls:** * **Glycoprotein vs. Proteoglycan:** Glycoproteins are mostly protein by weight with short, branched oligosaccharides. Proteoglycans are mostly carbohydrate (GAGs) by weight. * **Sialic Acid (NANA):** Often the terminal monosaccharide in glycoproteins, giving them a negative charge. * **I-Cell Disease:** A high-yield clinical correlation where a defect in adding a specific monosaccharide (Mannose-6-Phosphate) to glycoproteins leads to lysosomal storage issues. * **Dolichol Phosphate:** The lipid carrier required for the synthesis of N-linked glycoproteins in the ER.

Question 189: Within the red blood cell, hypoxia stimulates glycolysis via which of the following regulatory mechanisms?

A. Hypoxia stimulates pyruvate dehydrogenase by increased 2,3-DPG
B. Hypoxia inhibits hexokinase
C. Hypoxia stimulates the release of all glycolytic enzymes from band 3 or RBC membranes (Correct Answer)
D. Activation of regulatory enzymes by high pH

Explanation: **Explanation:** The regulation of glycolysis in red blood cells (RBCs) under hypoxic conditions involves a unique structural mechanism involving the RBC membrane. **1. Why Option C is Correct:** In the RBC membrane, the cytoplasmic domain of **Band 3 (Anion Exchanger 1)** acts as a docking site for several key glycolytic enzymes (including PFK, Aldolase, and GAPDH). When these enzymes are bound to Band 3, they are **enzymatically inactive**. Under hypoxic conditions, deoxyhemoglobin (deoxy-Hb) has a much higher affinity for Band 3 than oxyhemoglobin. Deoxy-Hb binds to Band 3, effectively **displacing the glycolytic enzymes** into the cytosol. Once released, these enzymes become active, thereby stimulating the glycolytic flux to meet the cell's energy needs and increase 2,3-BPG production. **2. Why Other Options are Incorrect:** * **Option A:** RBCs lack mitochondria; therefore, they do **not** contain Pyruvate Dehydrogenase (PDH). They rely solely on anaerobic glycolysis. * **Option B:** Hypoxia stimulates glycolysis to maintain ATP levels; inhibiting hexokinase (the rate-limiting step) would be counterproductive. * **Option C:** Hypoxia and increased CO2 typically lead to a **decrease** in pH (acidosis) via the Bohr effect, not a high pH. **3. High-Yield Clinical Pearls for NEET-PG:** * **Rapoport-Luebering Shunt:** A bypass of glycolysis unique to RBCs that produces **2,3-DPG (2,3-BPG)**. * **Role of 2,3-DPG:** It stabilizes the T-state (taut) of hemoglobin, shifting the oxygen-dissociation curve to the **right**, facilitating oxygen unloading to tissues. * **Mature RBC Metabolism:** Since they lack mitochondria, RBCs are entirely dependent on glucose for energy, producing lactate as the end product.

Question 190: Which compound can give rise to glucose by gluconeogenesis?

A. Acetyl CoA
B. Lactate (Correct Answer)
C. Palmitic acid
D. Fructose

Explanation: **Explanation:** **Why Lactate is Correct:** Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors. **Lactate** is a major substrate for this pathway via the **Cori Cycle**. In exercising muscle or RBCs, pyruvate is reduced to lactate, which travels to the liver. There, **Lactate Dehydrogenase (LDH)** converts it back into pyruvate, which enters the gluconeogenic pathway to eventually form glucose. **Why the Other Options are Incorrect:** * **Acetyl CoA:** This is the most important "distractor" in NEET-PG. Acetyl CoA cannot be converted to glucose because the **Pyruvate Dehydrogenase (PDH) reaction is irreversible**. Furthermore, in the TCA cycle, the two carbons of Acetyl CoA are lost as $CO_2$ before reaching oxaloacetate, resulting in no net gain of glucose. * **Palmitic Acid:** This is a long-chain fatty acid. Even-chain fatty acids undergo $\beta$-oxidation to produce Acetyl CoA, which (as stated above) cannot be used for gluconeogenesis. Only odd-chain fatty acids (producing Propionyl CoA) are glucogenic. * **Fructose:** While fructose is a carbohydrate that can enter glycolysis/gluconeogenesis pathways, it is technically a **sugar**, not a "non-carbohydrate precursor" used to define gluconeogenesis. It is metabolized into intermediates rather than being a primary substrate for the de novo synthesis of glucose from scratch. **High-Yield Clinical Pearls for NEET-PG:** * **Major Substrates:** Lactate (Cori Cycle), Glucogenic amino acids (mainly Alanine via the Glucose-Alanine cycle), and Glycerol (from TG breakdown). * **Key Enzyme:** Pyruvate Carboxylase (requires **Biotin**) converts pyruvate to oxaloacetate, bypassing the irreversible PDH step. * **Energy Requirement:** Gluconeogenesis is energy-expensive, requiring **6 ATP** per molecule of glucose synthesized. * **Odd-chain Fatty Acids:** These are the *only* lipids that are glucogenic because they yield Propionyl CoA, which enters the TCA cycle as Succinyl CoA.

Practice by Chapter

Carbohydrate Chemistry and Classification

Practice Questions

Glycolysis: Reactions and Regulation

Practice Questions

Gluconeogenesis: Reactions and Regulation

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Glycogen Metabolism: Synthesis and Breakdown

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Glycogen Storage Diseases

Practice Questions

Pentose Phosphate Pathway

Practice Questions

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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