Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 391: Which of the following is a non-reducing sugar?

A. Glucose
B. Maltose
C. Lactose
D. Sucrose (Correct Answer)

Explanation: ### Explanation **Core Concept: Reducing vs. Non-Reducing Sugars** A sugar is classified as "reducing" if it has a free **anomeric carbon** (aldehyde or ketone group) capable of acting as a reducing agent. In chemical tests like Benedict’s or Fehling’s, these sugars reduce cupric ions ($Cu^{2+}$) to cuprous ions ($Cu^+$), resulting in a color change. **Why Sucrose is the Correct Answer:** Sucrose is a disaccharide composed of **Glucose and Fructose**. The glycosidic linkage occurs between the $C_1$ of glucose and the $C_2$ of fructose. Since both anomeric carbons are involved in the bond, there is **no free aldehyde or ketone group** available. Consequently, sucrose cannot reduce alkaline copper solutions, making it a **non-reducing sugar**. **Analysis of Incorrect Options:** * **Glucose (Option A):** A monosaccharide with a free aldehyde group at $C_1$; it is a classic reducing sugar. * **Maltose (Option B):** A disaccharide (Glucose + Glucose) with an $\alpha(1\to4)$ bond. The second glucose molecule retains a free anomeric carbon at $C_1$. * **Lactose (Option C):** A disaccharide (Galactose + Glucose) with a $\beta(1\to4)$ bond. The glucose unit has a free anomeric carbon at $C_1$. **High-Yield Clinical Pearls for NEET-PG:** * **Inversion:** Sucrose is dextrorotatory, but upon hydrolysis, it becomes levorotatory (due to fructose). Hence, hydrolyzed sucrose is called **"Invert Sugar."** * **Benedict’s Test:** Used clinically to detect reducing sugars in urine (e.g., glucose in Diabetes Mellitus or galactose in Galactosemia). * **Trehalose:** Another high-yield non-reducing disaccharide (found in mushrooms) where two glucose units are linked via their anomeric carbons ($1\to1$). * **All monosaccharides** are reducing sugars.

Question 392: What is another name for glucose?

A. Dextrin
B. Dextrose (Correct Answer)
C. Sucrose
D. Saccharin

Explanation: **Explanation:** **Glucose** is a six-carbon monosaccharide (hexose) and is the primary source of energy for the body. It is also known as **Dextrose** because it is **dextrorotatory**; in a solution, it rotates the plane of polarized light to the right (clockwise). In clinical settings, intravenous fluids containing glucose are commonly labeled as "Dextrose" (e.g., D5W). **Analysis of Incorrect Options:** * **A. Dextrin:** These are low-molecular-weight carbohydrates produced by the partial hydrolysis of starch. They are intermediates in the digestion of starch by salivary or pancreatic amylase. * **C. Sucrose:** Known as "Table Sugar," it is a disaccharide composed of one molecule of glucose and one molecule of fructose linked by an $\alpha(1\to2)$ glycosidic bond. It is a non-reducing sugar. * **D. Saccharin:** This is an artificial, non-nutritive sweetener that is chemically unrelated to carbohydrates (it is a sulfonamide derivative). It provides no caloric value. **Clinical Pearls for NEET-PG:** * **Normal Fasting Blood Glucose:** 70–100 mg/dL. * **Renal Threshold for Glucose:** Approximately **180 mg/dL**. Beyond this level, glucose appears in the urine (glucosuria). * **GLUT-4:** The only insulin-dependent glucose transporter, primarily located in skeletal muscle and adipose tissue. * **Brain Metabolism:** The brain is dependent on glucose as its primary fuel; it can only switch to ketone bodies during prolonged starvation. * **Reducing Property:** Glucose is a reducing sugar because it has a free functional group (aldehyde) at the C1 position, which allows it to give a positive Benedict’s test.

Question 393: Which enzyme is involved in fructose metabolism?

A. Glucokinase
B. Glyceraldehyde-3-P Dehydrogenase (Correct Answer)
C. Aldolase A
D. PFK-1

Explanation: **Explanation:** Fructose metabolism (fructolysis) occurs primarily in the liver. The process bypasses the major rate-limiting step of glycolysis (PFK-1), which explains why fructose is metabolized faster than glucose. **Why Glyceraldehyde-3-P Dehydrogenase is correct:** In the liver, Fructose is converted to Fructose-1-Phosphate by **Fructokinase**. This is then cleaved by **Aldolase B** into Dihydroxyacetone phosphate (DHAP) and **Glyceraldehyde**. Glyceraldehyde is phosphorylated to **Glyceraldehyde-3-Phosphate (G3P)** by Triokinase. From this point forward, G3P enters the standard glycolytic pathway, where it is acted upon by **Glyceraldehyde-3-P Dehydrogenase** to form 1,3-bisphosphoglycerate. Thus, this enzyme is a shared and essential component of the downstream metabolism of fructose. **Analysis of Incorrect Options:** * **A. Glucokinase:** This enzyme is specific for glucose phosphorylation in the liver. While Hexokinase can phosphorylate fructose in muscles, Glucokinase cannot. * **C. Aldolase A:** This isoform is found in muscle and RBCs and prefers Fructose-1,6-BP. Fructose metabolism in the liver specifically requires **Aldolase B**, which can utilize Fructose-1-Phosphate as a substrate. * **D. PFK-1:** This is the rate-limiting enzyme of glycolysis. Fructose metabolism enters the pathway *below* this step, allowing it to bypass regulation by insulin and ATP. **High-Yield Clinical Pearls for NEET-PG:** 1. **Essential Fructosuria:** Deficiency of **Fructokinase**. It is a benign condition where fructose appears in the urine (reducing sugar positive). 2. **Hereditary Fructose Intolerance (HFI):** Deficiency of **Aldolase B**. It leads to the accumulation of Fructose-1-Phosphate, causing intracellular phosphate depletion, hypoglycemia, and jaundice. 3. **Speed of Metabolism:** Fructose is metabolized faster than glucose because it bypasses the PFK-1 regulatory step.

Question 394: The differences between the liver and muscle in glucose utilization from blood are due to the different regulatory properties of which enzyme?

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

Explanation: The differential regulation of glucose uptake between the liver and muscle is primarily governed by the tissue-specific isoenzymes of **Hexokinase**. ### Why Hexokinase is the Correct Answer Hexokinase catalyzes the first irreversible step of glycolysis: the phosphorylation of glucose to glucose-6-phosphate. * **Muscle (Hexokinase I & II):** These have a **low Km** (high affinity), allowing muscles to scavenge glucose even at low blood concentrations. They are strongly inhibited by their product (Glucose-6-Phosphate), ensuring the muscle only takes what it needs for immediate energy. * **Liver (Hexokinase IV or Glucokinase):** This isoenzyme has a **high Km** (low affinity) and a high Vmax. It functions only when blood glucose levels are high (post-prandial), allowing the liver to "buffer" blood sugar by converting excess glucose into glycogen. Crucially, it is **not** inhibited by Glucose-6-Phosphate. ### Why Other Options are Incorrect * **Pyruvate Kinase:** While it is a regulatory enzyme of glycolysis, its tissue-specific isoforms (L and M) are regulated by phosphorylation/hormones rather than being the primary gatekeepers of glucose entry from the blood. * **Phosphoglucomutase:** This is a reversible enzyme involved in glycogen metabolism (converting G1P to G6P); it is not a rate-limiting or regulatory step for glucose utilization. * **Aldolase:** This is a reversible enzyme in the glycolytic pathway. While Aldolase B is specific to the liver, it is primarily involved in fructose metabolism rather than the differential regulation of glucose uptake. ### High-Yield Clinical Pearls for NEET-PG * **Glucokinase (Hexokinase IV)** acts as a **glucose sensor** in the Beta-cells of the pancreas. * **MODY Type 2:** Mutations in the Glucokinase gene lead to Maturity-Onset Diabetes of the Young. * **Insulin Influence:** Insulin induces the synthesis of Glucokinase in the liver, but does not significantly affect the synthesis of Hexokinase in muscles.

Question 395: Which of the following is NOT an example of substrate-level phosphorylation?

A. Phosphofructokinase (Correct Answer)
B. Succinyl thiokinase
C. Pyruvate kinase
D. Phosphoglycerate kinase

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, independent of the electron transport chain or molecular oxygen. **Why Phosphofructokinase (PFK) is the correct answer:** PFK-1 is the rate-limiting enzyme of glycolysis that catalyzes the conversion of Fructose-6-phosphate to Fructose-1,6-bisphosphate. This reaction **consumes** one molecule of ATP rather than generating it. Therefore, it is an ATP-utilizing step, not a phosphorylation step that produces energy. **Analysis of Incorrect Options (Examples of SLP):** * **Phosphoglycerate Kinase:** Catalyzes the conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate in glycolysis, yielding 1 ATP. * **Pyruvate Kinase:** Catalyzes the final step of glycolysis (Phosphoenolpyruvate to Pyruvate), yielding 1 ATP. This is a highly exergonic, irreversible step. * **Succinyl Thiokinase (Succinyl-CoA Synthetase):** The only example of SLP in the **TCA Cycle**. It converts Succinyl-CoA to Succinate, yielding 1 GTP (which is energetically equivalent to ATP). **High-Yield NEET-PG Pearls:** 1. **Total SLP yield:** In aerobic glycolysis, SLP produces 4 ATP (net 2); in the TCA cycle, SLP produces 1 GTP per turn. 2. **Location:** SLP occurs in both the cytoplasm (glycolysis) and the mitochondria (TCA cycle). 3. **Arsenate Poisoning:** Arsenate competes with inorganic phosphate in the GAPDH reaction, bypassing the first SLP step (Phosphoglycerate kinase), resulting in zero net ATP gain from glycolysis. 4. **Key Distinction:** Unlike Oxidative Phosphorylation, SLP can occur under **anaerobic** conditions (e.g., in mature RBCs which lack mitochondria).

Question 396: Ribose phosphate is a product in which of the following pathways?

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

Explanation: **Explanation:** The **Pentose Phosphate Pathway (PPP)**, also known as the Hexose Monophosphate (HMP) Shunt, is a unique pathway of glucose oxidation that occurs in the cytosol. Unlike glycolysis, its primary purpose is not the generation of ATP, but the production of two vital intermediates: **NADPH** and **Pentoses (Ribose-5-phosphate)**. 1. **Why Option C is Correct:** In the **oxidative phase** of the PPP, Glucose-6-phosphate is converted into Ribulose-5-phosphate. In the subsequent **non-oxidative phase**, this is isomerized into **Ribose-5-phosphate**. This 5-carbon sugar is the essential precursor for the synthesis of nucleotides (DNA and RNA), ATP, NADH, and Coenzyme A. 2. **Why Other Options are Incorrect:** * **Lactic acid cycle (Cori Cycle):** This involves the conversion of lactate (produced by anaerobic glycolysis in muscles) back to glucose in the liver. It does not involve 5-carbon sugars. * **Citric acid cycle (TCA Cycle):** This is the final common pathway for the oxidation of carbohydrates, fats, and proteins. Its primary products are NADH, $FADH_2$, and $CO_2$; it does not produce ribose phosphate. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Glucose-6-Phosphate Dehydrogenase (G6PD). * **G6PD Deficiency:** Leads to hemolytic anemia due to the inability to produce NADPH, which is required to keep glutathione reduced to neutralize reactive oxygen species (ROS) in RBCs. * **Tissue Distribution:** The PPP is highly active in tissues requiring NADPH for fatty acid or steroid synthesis (e.g., Adrenal cortex, Liver, Mammary glands) and in RBCs to maintain reduced glutathione. * **Transketolase:** An enzyme in the non-oxidative phase that requires **Thiamine (Vitamin B1)** as a cofactor; its activity is used to clinically diagnose Thiamine deficiency.

Question 397: Which of the following is NOT a factor responsible for ketosis in a patient of von Gierke's disease?

A. Hypoglycemia
B. Impaired gluconeogenesis
C. Impaired glycogenolysis
D. Low fat mobilization (Correct Answer)

Explanation: **Explanation:** **Von Gierke’s Disease (GSD Type I)** is caused by a deficiency of **Glucose-6-Phosphatase**. This enzyme is the final common step for both glycogenolysis and gluconeogenesis, meaning the liver cannot release free glucose into the blood. **Why "Low fat mobilization" is the correct answer:** In Von Gierke’s disease, patients actually experience **increased fat mobilization**, not low. Due to the inability to maintain blood glucose, there is a chronic state of hypoglycemia. This leads to a low Insulin:Glucagon ratio, which stimulates **Hormone-Sensitive Lipase (HSL)** in adipose tissue. This results in massive mobilization of free fatty acids (FFAs) to the liver. These FFAs undergo β-oxidation, providing the acetyl-CoA necessary for **ketogenesis**. Therefore, "Low fat mobilization" is factually incorrect regarding the disease's pathophysiology. **Analysis of Incorrect Options:** * **Hypoglycemia:** This is the primary trigger. Low blood glucose stimulates glucagon and epinephrine, which drive the lipolysis required for ketosis. * **Impaired Gluconeogenesis & Glycogenolysis:** Both are hallmark features of GSD Type I. Because the liver cannot convert Glucose-6-Phosphate into free glucose, the body is forced to switch to alternative fuel sources (fats and ketones) to meet energy demands. **Clinical Pearls for NEET-PG:** * **The "Doll-like" Face:** Caused by fat deposition in cheeks due to hyperlipidemia. * **Biochemical Tetrad:** Hyperuricemia (gout), Hyperlactatemia (lactic acidosis), Hyperlipidemia, and Hypoglycemia. * **Key Distinction:** Unlike GSD Type III (Cori’s), Von Gierke’s presents with **severe lactic acidosis** because the lactate cannot be recycled into glucose via gluconeogenesis.

Question 398: Glycolysis is the metabolic pathway that involves 10 enzyme-mediated steps. In which of the following cell organelles does it occur?

A. Cytosol (Correct Answer)
B. Mitochondria
C. Nucleus
D. Lysosome

Explanation: ### Explanation **1. Why Cytosol is Correct:** Glycolysis (the Embden-Meyerhof pathway) is the primary pathway for glucose metabolism. All ten enzymes required for this process are located in the **cytosol** (cytoplasm). This localization is universal across all cell types in the human body, allowing cells to generate energy (ATP) even in the absence of oxygen (anaerobic glycolysis) or specialized organelles like mitochondria (e.g., in Mature RBCs). **2. Why Other Options are Incorrect:** * **Mitochondria:** This is the site for the **TCA cycle (Krebs cycle)**, Electron Transport Chain (ETC), and Beta-oxidation of fatty acids. While the product of glycolysis (Pyruvate) enters the mitochondria for further oxidation, the glycolytic pathway itself does not occur here. * **Nucleus:** This organelle houses the genetic material (DNA) and is the site for replication and transcription. It does not contain the enzymatic machinery for glucose breakdown. * **Lysosome:** Known as the "suicide bag" of the cell, it contains hydrolytic enzymes for the degradation of macromolecules and cellular debris, not for metabolic energy production. **3. NEET-PG High-Yield Clinical Pearls:** * **RBC Dependency:** Mature Red Blood Cells lack mitochondria; therefore, they rely **entirely** on cytosolic glycolysis for their energy needs. * **Rate-Limiting Step:** The conversion of Fructose-6-phosphate to Fructose-1,6-bisphosphate by **Phosphofructokinase-1 (PFK-1)** is the key regulatory and rate-limiting step. * **Rapoport-Luebering Cycle:** A shunt of glycolysis occurring in RBCs that produces **2,3-BPG**, which decreases hemoglobin's affinity for oxygen, facilitating oxygen delivery to tissues. * **Arsenic Poisoning:** Arsenite inhibits the conversion of Glyceraldehyde-3-phosphate to 1,3-Bisphosphoglycerate, resulting in zero net ATP gain from glycolysis.

Question 399: Which of the following enzymes does NOT participate in the conversion of lactate to phosphoenolpyruvate?

A. Lactate dehydrogenase
B. Pyruvate kinase (Correct Answer)
C. Pyruvate carboxylase
D. Phosphoenolpyruvate carboxykinase

Explanation: The conversion of lactate to phosphoenolpyruvate (PEP) is the initial phase of **Gluconeogenesis**. This process is essentially the reversal of glycolysis; however, three irreversible steps of glycolysis must be bypassed. ### Why Pyruvate Kinase is the Correct Answer **Pyruvate kinase** is a glycolytic enzyme that catalyzes the *irreversible* conversion of PEP to pyruvate. In gluconeogenesis, this "bottleneck" must be bypassed using two different enzymes and an intermediate (Oxaloacetate). Pyruvate kinase does not participate in the synthesis of PEP; rather, it destroys PEP to form pyruvate. ### Analysis of Other Options * **Lactate Dehydrogenase (A):** This is the first step. It oxidizes lactate into pyruvate in the cytosol, simultaneously reducing $NAD^+$ to $NADH$. * **Pyruvate Carboxylase (C):** This mitochondrial enzyme converts pyruvate into **Oxaloacetate (OAA)**. It requires **Biotin** as a cofactor and ATP. This is the first bypass step. * **Phosphoenolpyruvate Carboxykinase (PEPCK) (D):** This enzyme converts OAA into **Phosphoenolpyruvate (PEP)**. It requires **GTP** as an energy source. This completes the bypass of the pyruvate kinase reaction. ### High-Yield Clinical Pearls for NEET-PG * **The "Bypass" Enzymes:** Gluconeogenesis has 4 unique enzymes: Pyruvate carboxylase, PEPCK, Fructose-1,6-bisphosphatase, and Glucose-6-phosphatase. * **Obligatory Activator:** Pyruvate carboxylase is allosterically activated by **Acetyl-CoA**. High levels of Acetyl-CoA signal that the TCA cycle is saturated and pyruvate should be diverted to gluconeogenesis. * **Cori Cycle:** Lactate produced by anaerobic glycolysis in muscles/RBCs travels to the liver to be converted back to glucose via these enzymes. * **Location:** Pyruvate carboxylase is found only in the **mitochondria**, while PEPCK can be both cytosolic and mitochondrial.

Question 400: Which of the following statements about NADPH is false?

A. NADPH produces ATP in Red Blood Cells. (Correct Answer)
B. Glucose-6-phosphate dehydrogenase deficiency causes decreased synthesis of NADPH.
C. NADPH is required for reductive biosynthesis.
D. NADPH stabilizes the membrane of Red Blood Cells.

Explanation: **Explanation:** **1. Why Option A is the correct (False) statement:** NADPH (Nicotinamide Adenine Dinucleotide Phosphate) is primarily used for **reductive biosynthesis** and maintaining **antioxidant defenses**, not for energy production. Unlike NADH, which enters the Electron Transport Chain (ETC) to generate ATP via oxidative phosphorylation, NADPH does not serve as a substrate for ATP synthesis. In Red Blood Cells (RBCs), ATP is generated exclusively through **anaerobic glycolysis** (the Embden-Meyerhof pathway). **2. Analysis of other options:** * **Option B:** True. The **Hexose Monophosphate (HMP) Shunt** is the sole source of NADPH in RBCs. Glucose-6-phosphate dehydrogenase (G6PD) is the rate-limiting enzyme of this pathway; its deficiency directly leads to decreased NADPH levels. * **Option C:** True. NADPH provides the reducing equivalents necessary for the synthesis of fatty acids, cholesterol, and steroid hormones. * **Option D:** True. NADPH is essential for regenerating **reduced glutathione** from its oxidized form. Reduced glutathione neutralizes reactive oxygen species (like H₂O₂) that would otherwise cause lipid peroxidation and oxidative damage to the RBC membrane, leading to hemolysis. **High-Yield Clinical Pearls for NEET-PG:** * **G6PD Deficiency:** The most common enzyme deficiency worldwide. It presents as episodic hemolytic anemia triggered by oxidative stress (e.g., Fava beans, Primaquine, or infections). * **Heinz Bodies:** Denatured hemoglobin precipitates seen in G6PD deficiency due to oxidative stress. * **Bite Cells:** Formed when splenic macrophages pluck out Heinz bodies from RBCs. * **Key NADPH Sources:** HMP Shunt (major) and Malic Enzyme (minor, in tissues with mitochondria).

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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