Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 431: How many ATP molecules are generated in one cycle of the TCA cycle?

A. 2
B. 8
C. 10 (Correct Answer)
D. 11

Explanation: The TCA cycle (Krebs cycle) is the final common pathway for the oxidation of carbohydrates, fats, and proteins. The energy yield is calculated based on the production of reduced coenzymes and high-energy phosphates during one turn of the cycle. ### **Breakdown of ATP Generation (per Acetyl CoA):** 1. **Isocitrate → α-Ketoglutarate:** 1 NADH produced (**2.5 ATP**) 2. **α-Ketoglutarate → Succinyl CoA:** 1 NADH produced (**2.5 ATP**) 3. **Succinyl CoA → Succinate:** 1 GTP produced via Substrate Level Phosphorylation (**1 ATP**) 4. **Succinate → Fumarate:** 1 FADH₂ produced (**1.5 ATP**) 5. **Malate → Oxaloacetate:** 1 NADH produced (**2.5 ATP**) **Total Calculation:** (3 NADH × 2.5) + (1 FADH₂ × 1.5) + (1 GTP × 1) = **10 ATP.** *(Note: Older textbooks used 3 ATP/NADH and 2 ATP/FADH₂, totaling 12 ATP, but current NEET-PG standards follow the P:O ratios of 2.5 and 1.5).* ### **Why other options are incorrect:** * **Option A (2):** This represents the net ATP gain from anaerobic glycolysis. * **Option B (8):** This is the net ATP yield of aerobic glycolysis (per glucose molecule). * **Option D (11):** This is a common distractor; it incorrectly includes the ATP from the Pyruvate Dehydrogenase (PDH) complex, which is a link reaction, not part of the TCA cycle itself. ### **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Isocitrate Dehydrogenase. * **Substrate Level Phosphorylation:** Occurs at the step catalyzed by **Succinate Thiokinase**. * **Only membrane-bound enzyme:** Succinate Dehydrogenase (also part of Complex II of the ETC). * **Inhibitors:** Fluoroacetate (inhibits Aconitase), Arsenite (inhibits α-Ketoglutarate Dehydrogenase), and Malonate (competitive inhibitor of Succinate Dehydrogenase).

Question 432: In vivo control of the citric acid cycle is affected by which of the following?

A. Acetyl CoA
B. Coenzyme A
C. ATP (Correct Answer)
D. Citrate

Explanation: The Citric Acid Cycle (TCA cycle) is the central metabolic hub of the cell, and its rate is primarily determined by the cell's energy status, specifically the **ATP/ADP ratio** and the **NADH/NAD+ ratio**. ### Why ATP is the Correct Answer ATP acts as a potent **allosteric inhibitor** of key rate-limiting enzymes in the TCA cycle. When energy levels are high (high ATP), the cycle slows down to prevent unnecessary oxidation of fuel. Specifically: * **Isocitrate Dehydrogenase:** This is the primary rate-limiting step of the TCA cycle. It is strongly inhibited by ATP and stimulated by ADP. * **α-Ketoglutarate Dehydrogenase:** Also inhibited by high levels of ATP and NADH. * **Citrate Synthase:** Inhibited by ATP (though less significantly than Isocitrate Dehydrogenase). ### Explanation of Incorrect Options * **A & B (Acetyl CoA and Coenzyme A):** While Acetyl CoA is a substrate and its availability affects the cycle's initiation, it is not the primary *regulatory* signal for the cycle's overall flux in vivo. The ratio of Acetyl CoA to CoA regulates the Pyruvate Dehydrogenase (PDH) complex, which is *upstream* of the TCA cycle. * **D (Citrate):** Citrate is a product of the first step. While it can inhibit Citrate Synthase via product inhibition, its more significant regulatory role is as an inhibitor of **Phosphofructokinase-1 (PFK-1)** in glycolysis, rather than being the primary controller of the TCA cycle itself. ### NEET-PG High-Yield Pearls * **Rate-Limiting Enzyme:** Isocitrate Dehydrogenase. * **Most Potent Activator:** ADP (signals low energy). * **Most Potent Inhibitors:** ATP and NADH. * **Calcium Ions:** In skeletal muscle, $Ca^{2+}$ activates Isocitrate Dehydrogenase and α-Ketoglutarate Dehydrogenase to link muscle contraction with increased energy production. * **Amphibolic Nature:** The TCA cycle is both catabolic (energy production) and anabolic (provides precursors for heme, glucose, and amino acids).

Question 433: Which of the following is NOT a product of the Hexose Monophosphate (HMP) shunt?

A. Glyceraldehyde 3-phosphate
B. Glycerol-3-phosphate (Correct Answer)
C. 3CO2
D. 6NADPH

Explanation: ### 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.

Question 434: Which of the following amino acids cannot be used for glycogen synthesis?

A. Alanine
B. Threonine
C. Leucine (Correct Answer)
D. Methionine

Explanation: ### Explanation The ability of an amino acid to contribute to glycogen synthesis depends on whether its carbon skeleton can be converted into **glucose** (Gluconeogenesis). **1. Why Leucine is the Correct Answer:** Amino acids are classified as glucogenic, ketogenic, or both. **Leucine and Lysine** are the only two **purely ketogenic** amino acids. Their catabolism yields Acetyl-CoA or Acetoacetate, which enters the TCA cycle but results in the loss of two carbons as $CO_2$ before reaching Oxaloacetate. Consequently, they cannot provide a net synthesis of glucose or contribute to glycogen stores. **2. Analysis of Incorrect Options:** * **Alanine (Option A):** The most important glucogenic amino acid. It undergoes transamination to form **Pyruvate**, a direct precursor for gluconeogenesis via the Glucose-Alanine cycle. * **Threonine (Option B):** A glucogenic and ketogenic amino acid. It can be converted into **Pyruvate** or **Succinyl-CoA**, both of which can enter the gluconeogenic pathway. * **Methionine (Option D):** A purely glucogenic amino acid. Its catabolism leads to the formation of **Propionyl-CoA**, which is converted to **Succinyl-CoA**, a TCA cycle intermediate used for glucose synthesis. **3. NEET-PG High-Yield Pearls:** * **Purely Ketogenic:** Leucine and Lysine (Mnemonic: The "L"s). * **Both Glucogenic & Ketogenic:** Phenylalanine, Tyrosine, Tryptophan, Isoleucine (Mnemonic: PITTT). * **Purely Glucogenic:** All other 14 amino acids. * **Key Concept:** Acetyl-CoA cannot be converted back to Pyruvate in humans because the Pyruvate Dehydrogenase reaction is irreversible; this is why ketogenic substrates cannot form glucose.

Question 435: Which of the following enzymes requires NAD as a cofactor?

A. Citrate Synthase
B. Isocitrate Dehydrogenase
C. alpha-Ketoglutarate dehydrogenase (Correct Answer)
D. Succinate Thiokinase

Explanation: **Explanation:** The **$\alpha$-Ketoglutarate Dehydrogenase (α-KGDH) complex** is a key regulatory enzyme in the Citric Acid Cycle (TCA cycle). It catalyzes the oxidative decarboxylation of $\alpha$-ketoglutarate to succinyl-CoA. This enzyme is a multi-enzyme complex that requires **five essential cofactors**: Thiamine pyrophosphate (TPP/B1), Lipoic acid, CoA (B5), FAD (B2), and **NAD+ (B3)**. NAD+ acts as the final electron acceptor in this reaction, being reduced to NADH. **Analysis of Options:** * **Isocitrate Dehydrogenase (Option B):** While this enzyme also produces NADH in the TCA cycle, the question specifically highlights $\alpha$-KGDH as the classic "multi-enzyme complex" similar to Pyruvate Dehydrogenase. Note: In many contexts, Isocitrate Dehydrogenase is also NAD-dependent; however, $\alpha$-KGDH is the high-yield answer often tested for its specific requirement of the "five-cofactor" group. * **Citrate Synthase (Option A):** This is a condensation reaction (Acetyl-CoA + Oxaloacetate → Citrate). It does not involve redox chemistry and thus does not require NAD. * **Succinate Thiokinase (Option D):** Also known as Succinyl-CoA synthetase, this enzyme performs substrate-level phosphorylation to produce **GTP** (or ATP). It does not require NAD. **High-Yield Clinical Pearls for NEET-PG:** * **The "Tender Loving Care For Nancy" Mnemonic:** Use this to remember the five cofactors for both $\alpha$-KGDH and Pyruvate Dehydrogenase: **T**PP, **L**ipoic acid, **C**oA, **F**AD, **N**AD. * **Arsenite Poisoning:** Arsenite inhibits enzymes requiring Lipoic acid (like $\alpha$-KGDH), leading to a buildup of upstream metabolites and lactic acidosis. * **Rate-Limiting Step:** While Isocitrate Dehydrogenase is the primary rate-limiting step of the TCA cycle, $\alpha$-KGDH is a major site of inhibition by high ATP, NADH, and Succinyl-CoA.

Question 436: Which of the following enzymes is absent in muscle?

A. Glucose-1-phosphatase
B. Glucose 6 phosphatase (Correct Answer)
C. Glycogen phosphorylase
D. Thiophorase

Explanation: **Explanation:** The correct answer is **B. Glucose-6-phosphatase**. **1. Why Glucose-6-phosphatase is absent in muscle:** 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**. Its absence in skeletal muscle is a critical physiological feature: it prevents muscle from releasing glucose into the bloodstream. Instead, muscle glycogen is used exclusively for internal energy production (glycolysis) to fuel contraction. This ensures that muscle remains a "selfish" consumer of its own glucose stores during exercise. **2. Analysis of Incorrect Options:** * **A. Glucose-1-phosphatase:** This is not a major regulatory enzyme in glycogen metabolism; however, enzymes handling G-1-P (like Phosphoglucomutase) are present in muscle to shuttle intermediates between glycogen and glycolysis. * **C. Glycogen phosphorylase:** This is the rate-limiting enzyme of glycogenolysis. It is highly active in muscle to break down glycogen into Glucose-1-Phosphate during physical activity. * **D. Thiophorase (Succinyl-CoA:3-ketoacid CoA transferase):** This enzyme is essential for **ketolysis** (utilization of ketone bodies). It is present in extrahepatic tissues, including muscle and brain, but is notably **absent in the liver**. **High-Yield NEET-PG Clinical Pearls:** * **Von Gierke’s Disease (GSD Type I):** Caused by a deficiency of Glucose-6-phosphatase. It presents with severe fasting hypoglycemia and hepatomegaly because the liver cannot export glucose. * **McArdle’s Disease (GSD Type V):** Caused by a deficiency of **Muscle Glycogen Phosphorylase**, leading to exercise intolerance and cramps. * **Metabolic Logic:** The liver maintains blood glucose (has G-6-Pase); the muscle provides ATP for contraction (lacks G-6-Pase).

Question 437: Which of the following is NOT used in gluconeogenesis?

A. Oleate (Correct Answer)
B. Succinate
C. Glutamate
D. Aspartate

Explanation: **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.

Question 438: Which of the following glucose transporters is primarily utilized by adipocytes?

A. GLUT1
B. GLUT2
C. GLUT3
D. GLUT4 (Correct Answer)

Explanation: **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.

Question 439: Which glycolytic enzyme(s) are inhibited by fluoride?

A. Hexokinase
B. Aldolase
C. Enolase (Correct Answer)
D. Pyruvate kinase

Explanation: **Explanation:** **1. Why Enolase is the Correct Answer:** Enolase is the glycolytic enzyme responsible for the dehydration of **2-phosphoglycerate to phosphoenolpyruvate (PEP)**. Fluoride acts as a potent competitive inhibitor of this enzyme. The mechanism involves fluoride ions forming a complex with **magnesium (Mg²⁺)** and inorganic phosphate, which then binds to the active site of enolase. Since enolase requires Mg²⁺ as a cofactor for its activity, the formation of this **magnesium-fluorophosphate complex** effectively traps the enzyme and halts glycolysis. **2. Why Other Options are Incorrect:** * **Hexokinase (A):** This enzyme catalyzes the first step of glycolysis (Glucose to Glucose-6-Phosphate). It is inhibited by its product, Glucose-6-Phosphate, not fluoride. * **Aldolase (B):** This enzyme cleaves Fructose-1,6-bisphosphate into DHAP and Glyceraldehyde-3-phosphate. It is not sensitive to fluoride. * **Pyruvate Kinase (D):** This catalyzes the final step of glycolysis. It is regulated by allosteric effectors (inhibited by ATP/Alanine; activated by Fructose-1,6-bisphosphate) and covalent modification, but not by fluoride. **3. Clinical Pearls for NEET-PG:** * **Blood Glucose Estimation:** In clinical practice, blood samples for glucose estimation are collected in **Grey-topped tubes** containing **Sodium Fluoride (NaF)**. This prevents "in vitro" glycolysis by RBCs and WBCs, ensuring the measured glucose level reflects the patient's actual blood sugar at the time of draw. * **Anticoagulant Pairing:** NaF is usually combined with **Potassium Oxalate** (an anticoagulant) because fluoride alone is a poor anticoagulant. * **Dental Health:** Fluoride's ability to inhibit bacterial enolase in oral flora (like *S. mutans*) is one reason it helps prevent dental caries.

Question 440: What is the term for the six-membered ring structure of monosaccharides?

A. Pyran (Correct Answer)
B. Furan
C. Aldose
D. Ketose

Explanation: **Explanation:** Monosaccharides with five or more carbon atoms usually exist in cyclic forms rather than open chains. This cyclization occurs when the carbonyl group (aldehyde or ketone) reacts with a hydroxyl group on the same molecule. **1. Why Pyran is Correct:** When a monosaccharide forms a **six-membered ring** (consisting of five carbon atoms and one oxygen atom), it is called a **Pyranose** ring. This name is derived from its structural similarity to the heterocyclic compound **Pyran**. For example, the most stable form of glucose in solution is glucopyranose. **2. Analysis of Incorrect Options:** * **B. Furan:** This refers to a **five-membered ring** structure (four carbons and one oxygen), named after the compound **Furan**. While glucose can form a furanose ring, it is less stable than the pyranose form. Fructose commonly exists as a fructofuranose. * **C. Aldose:** This is a classification based on the functional group. An aldose is a sugar containing an **aldehyde group** (e.g., Glucose, Galactose). * **D. Ketose:** This is a sugar containing a **ketone group** (e.g., Fructose, Ribulose). **High-Yield Clinical Pearls for NEET-PG:** * **Anomers:** Cyclization creates a new asymmetric center at Carbon-1 (for aldoses) or Carbon-2 (for ketoses), known as the **anomeric carbon**. This leads to $\alpha$ and $\beta$ configurations. * **Mutarotation:** The process of interconversion between $\alpha$ and $\beta$ anomers in solution until equilibrium is reached. * **Haworth Projections:** These are the standard diagrams used to represent the cyclic pyranose and furanose forms of sugars. * **Glucose Fact:** In equilibrium, D-glucose exists primarily as $\beta$-D-glucopyranose (~63%) because it is more stable than the $\alpha$ form (~36%).

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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