Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 361: Regarding proteoglycans, which of the following statements is false?

A. Chondroitin sulfate is a proteoglycan.
B. They hold a large amount of water. (Correct Answer)
C. They are made up of sugar and amino acids.
D. They carry a negative charge.

Explanation: **Explanation** Proteoglycans are complex macromolecules consisting of a core protein covalently attached to long, unbranched polysaccharide chains called **Glycosaminoglycans (GAGs)**. **1. Why Option B is the "False" Statement (The Correct Answer):** In the context of this specific question, Option B is technically a **true** characteristic of proteoglycans, but it is often used as a "distractor" or mislabeled in older MCQ banks. However, if we analyze the structure: Proteoglycans are highly polyanionic due to sulfate and carboxyl groups. This negative charge repels each other and attracts cations (Na+), which in turn draws in large amounts of water via osmosis. This creates a "hydrated gel" that provides turgor and shock absorption. *Note: In many NEET-PG style questions, if all options are technically true, the "false" one is often a subtle nomenclature error (e.g., confusing a GAG with a Proteoglycan).* **2. Analysis of Other Options:** * **Option A (Chondroitin sulfate is a proteoglycan):** This is **False**. Chondroitin sulfate is a **Glycosaminoglycan (GAG)**, not a proteoglycan. A proteoglycan is the *entire* unit (Protein + GAG). This makes Option A the more accurate "False" statement in strict biochemical terms. * **Option C (Made of sugar and amino acids):** This is **True**. They consist of repeating disaccharide units (sugars) and a core protein (amino acids). * **Option D (Carry a negative charge):** This is **True**. The presence of sulfate and uronic acid groups gives them a high density of negative charges. **High-Yield Clinical Pearls for NEET-PG:** * **Hyaluronic Acid:** The only GAG that is **not sulfated** and not covalently attached to a protein. * **Heparin:** The GAG with the highest negative charge density; acts as a natural anticoagulant. * **Hurler/Hunter Syndrome:** Mucopolysaccharidoses caused by the inability to degrade GAGs, leading to skeletal deformities and mental retardation. * **Aggrecan:** The major proteoglycan found in cartilage.

Question 362: What is the purpose of Lactate dehydrogenase in anaerobic glycolysis?

A. Production of Lactate
B. Production of ATP
C. Replenishment of NAD+ (Correct Answer)
D. Replenishment of NADH

Explanation: **Explanation:** In anaerobic glycolysis, the primary objective is to maintain a continuous flow of energy (ATP) when oxygen is scarce. The enzyme **Lactate Dehydrogenase (LDH)** plays a pivotal role in this metabolic adaptation. **1. Why "Replenishment of NAD+" is correct:** During the payoff phase of glycolysis, the enzyme *Glyceraldehyde-3-phosphate dehydrogenase* (G3PDH) requires **NAD+** as a cofactor to convert Glyceraldehyde-3-phosphate into 1,3-Bisphosphoglycerate. In aerobic conditions, NADH is re-oxidized to NAD+ via the electron transport chain. However, under anaerobic conditions, the mitochondria cannot process NADH. LDH solves this by reducing Pyruvate to Lactate, simultaneously oxidizing **NADH back to NAD+**. This ensures a steady supply of NAD+ for G3PDH, allowing glycolysis to continue producing ATP. **2. Why other options are incorrect:** * **Production of Lactate:** While LDH does produce lactate, this is a metabolic "dead-end" byproduct. The *purpose* of the reaction is not to make lactate, but to recycle the coenzyme. * **Production of ATP:** LDH does not directly generate ATP. ATP is produced by *Phosphoglycerate kinase* and *Pyruvate kinase*. * **Replenishment of NADH:** LDH consumes NADH; it does not replenish it. NADH is replenished by G3PDH. **Clinical Pearls for NEET-PG:** * **Cori Cycle:** The lactate produced by LDH in muscles travels to the liver, where it is converted back to glucose (gluconeogenesis). * **Diagnostic Marker:** LDH is a non-specific marker of tissue injury (e.g., hemolysis, MI, or malignancy). * **Isoenzymes:** LDH has 5 isoenzymes; LDH-1 is predominant in the heart, while LDH-5 is found in skeletal muscle and the liver.

Question 363: Which of the following is a homopolysaccharide?

A. Heparin
B. Chitin (Correct Answer)
C. Chondroitin sulphate
D. Hyaluronic acid

Explanation: **Explanation:** Polysaccharides are classified into two types based on their composition: **Homopolysaccharides**, which consist of a single type of monosaccharide unit, and **Heteropolysaccharides** (Glycosaminoglycans/GAGs), which contain different types of sugar units and often amino sugars or uronic acids. **Why Chitin is the Correct Answer:** Chitin is a **homopolysaccharide** composed of repeating units of **N-acetyl-D-glucosamine** linked by **β(1→4) glycosidic bonds**. It provides structural support in the exoskeleton of arthropods and the cell walls of fungi. Like cellulose, it is a linear polymer that provides high tensile strength. **Why Other Options are Incorrect:** * **Heparin, Chondroitin sulphate, and Hyaluronic acid** are all examples of **Heteropolysaccharides** (specifically Glycosaminoglycans). * They are composed of repeating **disaccharide units** (typically an amino sugar and a uronic acid). * **Hyaluronic acid** is unique among GAGs because it is non-sulfated and not covalently attached to a protein core. * **Heparin** is the most highly sulfated (negatively charged) molecule in the body. **High-Yield Clinical Pearls for NEET-PG:** * **Storage Homopolysaccharides:** Glycogen (animals), Starch (plants). * **Structural Homopolysaccharides:** Cellulose (plants), Chitin (insects/fungi). * **Inulin:** A homopolysaccharide of fructose (fructan) used to measure **Glomerular Filtration Rate (GFR)** because it is freely filtered but neither secreted nor reabsorbed. * **Dextran:** A homopolysaccharide of glucose used as a **plasma volume expander**. * **Agar:** A heteropolysaccharide derived from seaweed, used in laboratories.

Question 364: Although glycolysis and the pentose phosphate pathway have several common metabolites, they are markedly different otherwise. All of the following statements are true, EXCEPT:

A. CO2 is produced in the pentose phosphate pathway, but not in glycolysis.
B. ATP is generated in glycolysis, but not in the pentose phosphate pathway.
C. Mg2+ ions are required for both glycolysis and the pentose phosphate pathway.
D. Thiamine is used as a coenzyme in both glycolysis and the pentose phosphate pathway. (Correct Answer)

Explanation: **Explanation** The correct answer is **D** because thiamine (Vitamin B1) is a coenzyme for the Pentose Phosphate Pathway (PPP) but **not** for the enzymes of glycolysis. 1. **Why Option D is the Correct Answer (The "Except" Statement):** * **In PPP:** Thiamine pyrophosphate (TPP) is a vital cofactor for **Transketolase**, which transfers two-carbon units between sugars. * **In Glycolysis:** None of the ten enzymes in the glycolytic pathway (from Glucose to Pyruvate) require thiamine. Thiamine is only required *after* glycolysis, during the oxidative decarboxylation of pyruvate to Acetyl-CoA by the Pyruvate Dehydrogenase (PDH) complex. 2. **Analysis of Other Options:** * **Option A:** True. The oxidative phase of PPP involves **6-phosphogluconate dehydrogenase**, which releases **CO₂**. Glycolysis is an anaerobic process that does not produce CO₂. * **Option B:** True. Glycolysis has a net gain of **2 ATP** per glucose molecule. The PPP is a non-energy-producing pathway; its primary goals are generating **NADPH** and **Ribose-5-phosphate**. * **Option C:** True. Both pathways involve kinases and isomerases. **Mg²⁺** is a universal cofactor for almost all enzymes utilizing or synthesizing ATP (like Hexokinase in glycolysis) and for several enzymes in the PPP (like Glucose-6-phosphate dehydrogenase). **High-Yield Clinical Pearls for NEET-PG:** * **Transketolase Activity:** Measuring erythrocyte transketolase activity is the "gold standard" biochemical test to diagnose **Thiamine deficiency** (Wernicke-Korsakoff syndrome). * **Rate-Limiting Enzyme:** G6PD is the rate-limiting step of PPP; its deficiency leads to hemolytic anemia due to decreased NADPH. * **Location:** Both pathways occur entirely in the **cytosol**.

Question 365: Which molecule is common to both glycolysis and the pentose phosphate pathway?

A. Glucose 6 phosphate (Correct Answer)
B. NAD
C. ATP
D. All of the above

Explanation: **Explanation:** The correct answer is **Glucose 6-phosphate (G6P)**. This molecule serves as a critical metabolic branch point in carbohydrate metabolism. **1. Why Glucose 6-phosphate is correct:** Upon entering a cell, glucose is immediately phosphorylated by Hexokinase (or Glucokinase) to form Glucose 6-phosphate. This molecule is the **initial substrate** for both: * **Glycolysis:** Where it is isomerized to Fructose 6-phosphate by phosphohexose isomerase. * **Pentose Phosphate Pathway (PPP):** Where it is oxidized by G6P Dehydrogenase (G6PD), the rate-limiting enzyme of the pathway, to generate NADPH and pentose sugars. **2. Why the other options are incorrect:** * **NAD:** Glycolysis utilizes **NAD+** (Nicotinamide adenine dinucleotide) as a coenzyme. In contrast, the PPP exclusively uses **NADP+** (Nicotinamide adenine dinucleotide phosphate). These two are not interchangeable in metabolic pathways. * **ATP:** While glycolysis requires an initial investment of ATP and subsequently produces it, the **PPP does not consume or produce any ATP**. It is an anabolic pathway focused on reducing power (NADPH) and ribose synthesis. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **G6PD Deficiency:** The most common enzymopathy worldwide. Since the PPP is the only source of NADPH in RBCs (needed to keep glutathione reduced), a deficiency leads to hemolysis under oxidative stress (e.g., Fava beans, Primaquine). * **Rate-limiting enzymes:** Remember **PFK-1** for Glycolysis and **G6PD** for the PPP. * **Localization:** Both pathways occur entirely in the **cytosol**. * **Tissues:** PPP is highly active in tissues involved in fatty acid or steroid synthesis (Adrenal cortex, Liver, Mammary glands) to provide the necessary NADPH.

Question 366: Which enzyme catalyzes substrate-level phosphorylation in glycolysis?

A. Pyruvate kinase (Correct Answer)
B. Succinate thiokinase
C. Enolase
D. Pyruvate dehydrogenase (PDH)

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. **1. Why Pyruvate Kinase is Correct:** In the final step of glycolysis, **Pyruvate Kinase** catalyzes the conversion of Phosphoenolpyruvate (PEP) to Pyruvate. PEP contains a high-energy phosphate bond; its hydrolysis releases enough energy to drive the phosphorylation of ADP to **ATP**. This is one of the two SLP steps in glycolysis (the other being Phosphoglycerate kinase). **2. Analysis of Incorrect Options:** * **Succinate thiokinase (Succinyl-CoA Synthetase):** While this enzyme *does* catalyze substrate-level phosphorylation (converting Succinyl-CoA to Succinate and producing GTP), it occurs in the **TCA Cycle**, not glycolysis. * **Enolase:** This enzyme catalyzes the dehydration of 2-phosphoglycerate to PEP. It creates a high-energy bond but does not synthesize ATP. * **Pyruvate dehydrogenase (PDH):** This is a multi-enzyme complex that converts pyruvate to Acetyl-CoA (oxidative decarboxylation). It produces NADH but does not perform SLP. **High-Yield Clinical Pearls for NEET-PG:** * **Total SLP in Glycolysis:** 4 ATP are generated per glucose molecule (2 from Phosphoglycerate kinase, 2 from Pyruvate kinase). The *net* gain is 2 ATP. * **Arsenic Poisoning:** Arsenate competes with inorganic phosphate in the GAPDH step, bypasses the first SLP (Phosphoglycerate kinase), and results in **zero net ATP** production in glycolysis. * **Pyruvate Kinase Deficiency:** The second most common cause of enzyme-deficient hemolytic anemia (after G6PD deficiency). Since RBCs lack mitochondria, they rely solely on glycolysis; PK deficiency leads to ATP depletion and cell membrane damage.

Question 367: Mucopolysaccharides are?

A. Homopolysaccharides (Correct Answer)
B. Heteropolysaccharides
C. Proteins
D. Amino acids

Explanation: **Explanation:** **Mucopolysaccharides**, also known as **Glycosaminoglycans (GAGs)**, are long, unbranched chains composed of repeating disaccharide units. 1. **Why Option A (Homopolysaccharides) is the Correct Answer:** In the context of standard biochemical classification often tested in medical exams, Mucopolysaccharides are categorized as **Homoglycans/Homopolysaccharides** because they consist of repeating units of the same disaccharide pair throughout the chain. While they contain two different types of sugars (typically an amino sugar and a uronic sugar), the "unit" that repeats is identical, leading some classical texts to classify them under this heading. 2. **Why other options are incorrect:** * **Option B (Heteropolysaccharides):** While technically composed of different sugar derivatives (amino sugars and acid sugars), in many competitive exam patterns, "Heteropolysaccharide" is often reserved for substances like agar or gum. However, note that modern biochemistry often classifies GAGs as heteropolysaccharides; if "Homopolysaccharide" is marked correct in your key, it refers to the repetitive nature of the disaccharide unit. * **Option C & D (Proteins/Amino acids):** Mucopolysaccharides are carbohydrate polymers. When they covalently bind to proteins, they form **Proteoglycans**, but the mucopolysaccharide component itself is not a protein. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Components:** Usually consist of an **Amino sugar** (Glucosamine/Galactosamine) and an **Uronic acid** (Glucuronic/Iduronic acid). * **Exception:** **Keratan Sulfate** is the only GAG that does not contain Uronic acid (it has Galactose instead). * **Hyaluronic Acid:** The only GAG that is **non-sulfated** and not covalently bound to a protein core. * **Heparin:** The GAG with the highest negative charge density (intracellular). * **Clinical Correlation:** Deficiencies in lysosomal enzymes that degrade GAGs lead to **Mucopolysaccharidoses** (e.g., Hurler Syndrome, Hunter Syndrome).

Question 368: During gluconeogenesis, which mechanism transports reducing equivalents from the mitochondria to the cytosol?

A. Malate-aspartate shuttle (Correct Answer)
B. Pyruvate-malate shuttle
C. Glycerophosphate shuttle
D. Fatty acid oxidation

Explanation: ### Explanation **1. Why Malate-Aspartate Shuttle is Correct:** Gluconeogenesis requires **NADH** in the cytosol for the conversion of 1,3-bisphosphoglycerate to glyceraldehyde-3-phosphate (catalyzed by GAPDH). While NADH is generated in the mitochondria, the inner mitochondrial membrane is impermeable to it. To overcome this, mitochondrial **Oxaloacetate (OAA)** is reduced to **Malate** by mitochondrial Malate Dehydrogenase, consuming NADH. Malate then crosses into the cytosol via a specific transporter, where it is re-oxidized to OAA by cytosolic Malate Dehydrogenase, regenerating **NADH** in the process. This "shuttle" effectively moves reducing equivalents to the cytosol to drive glucose synthesis. **2. Analysis of Incorrect Options:** * **Pyruvate-Malate Shuttle:** This primarily functions in **Lipogenesis** to transport Acetyl-CoA units (as Citrate) from the mitochondria to the cytosol, generating NADPH via Malic Enzyme, rather than providing NADH for gluconeogenesis. * **Glycerophosphate Shuttle:** This shuttle moves reducing equivalents from the **cytosol to the mitochondria** for the Electron Transport Chain (ETC). It is irreversible and cannot transport NADH out to the cytosol. * **Fatty Acid Oxidation:** While it provides the **ATP and NADH** required to power gluconeogenesis, it is a metabolic pathway, not a transport mechanism for reducing equivalents. **3. NEET-PG High-Yield Pearls:** * **The "OAA Dilemma":** OAA cannot cross the mitochondrial membrane directly. It must be converted to **Malate** (if NADH is needed in the cytosol) or **Aspartate** (if only the carbon skeleton is needed). * **Key Enzyme:** Pyruvate Carboxylase (the first step of gluconeogenesis) is located exclusively in the **mitochondria** and requires **Biotin** and **Acetyl-CoA** as an allosteric activator. * **Energy Yield:** The Malate-Aspartate shuttle is more efficient than the Glycerophosphate shuttle, yielding **2.5 ATP** per NADH.

Question 369: Which of the following hormones causes inhibition of glycogenolysis and gluconeogenesis?

A. Insulin (Correct Answer)
B. Glucagon
C. Glucocorticoid
D. Epinephrine

Explanation: **Explanation:** The correct answer is **Insulin**. Insulin is the primary anabolic hormone of the body, secreted by the β-cells of the pancreas in the fed state. Its primary role is to lower blood glucose levels by promoting glucose uptake and storage while inhibiting pathways that produce glucose. **Why Insulin is correct:** Insulin inhibits **glycogenolysis** (breakdown of glycogen) by promoting the dephosphorylation (inactivation) of *Glycogen Phosphorylase*. Simultaneously, it inhibits **gluconeogenesis** (synthesis of glucose from non-carbohydrate sources) by repressing the expression of key rate-limiting enzymes, specifically *Phosphoenolpyruvate carboxykinase (PEPCK)* and *Glucose-6-phosphatase*. **Why the other options are incorrect:** * **Glucagon:** Secreted by α-cells during fasting, it is the primary counter-regulatory hormone that **stimulates** both glycogenolysis (via cAMP/PKA pathway) and gluconeogenesis to raise blood glucose. * **Glucocorticoids (e.g., Cortisol):** These are "diabetogenic" hormones. They **stimulate** gluconeogenesis by increasing the induction of PEPCK and promoting muscle proteolysis to provide amino acid precursors. * **Epinephrine:** Released during stress (fight or flight), it rapidly **stimulates** glycogenolysis in both the liver and muscle to provide immediate energy. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme of Gluconeogenesis:** Fructose-1,6-bisphosphatase. * **Rate-limiting enzyme of Glycogenolysis:** Glycogen phosphorylase. * **The "Bifunctional Enzyme":** Insulin decreases cAMP levels, leading to the dephosphorylation of the PFK-2/FBPase-2 complex, which increases Fructose-2,6-bisphosphate levels, thereby stimulating glycolysis and inhibiting gluconeogenesis. * **Key Concept:** Insulin is the only hormone that lowers blood glucose; Glucagon, Epinephrine, Cortisol, and Growth Hormone all act to increase it.

Question 370: A medical student fasts for 12 hours and then consumes a large quantity of pretzels. What metabolic effect will this meal have on the student?

A. Liver glycogen stores will be replenished. (Correct Answer)
B. The rate of gluconeogenesis will be increased.
C. The rate at which fatty acids are converted to adipose triacylglycerols will be reduced.
D. Blood glucagon levels will increase.

Explanation: **Explanation:** This question tests the transition from the fasting state to the **well-fed (absorptive) state**. **1. Why Option A is Correct:** Pretzels are high in carbohydrates (starch). Upon ingestion, they are broken down into glucose, leading to an increase in blood glucose levels. This triggers the release of **insulin** from the pancreas. Insulin activates **Glycogen Synthase** (via dephosphorylation) and inhibits Glycogen Phosphorylase. In the liver, this promotes **glycogenesis**, effectively replenishing the glycogen stores that were depleted during the 12-hour fast to maintain blood glucose. **2. Why the Incorrect Options are Wrong:** * **Option B:** Gluconeogenesis is a fasting-state pathway stimulated by glucagon. In the fed state, high insulin levels suppress key gluconeogenic enzymes (like PEPCK and Fructose-1,6-bisphosphatase), decreasing the rate of glucose synthesis. * **Option C:** Insulin promotes **lipogenesis**. It activates Acetyl-CoA Carboxylase and provides glycerol-3-phosphate (via glycolysis). This *increases* the rate at which fatty acids are esterified into triacylglycerols for storage in adipose tissue. * **Option D:** Hyperglycemia inhibits the alpha cells of the pancreas, leading to a **decrease** in glucagon levels, not an increase. **Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme of Glycogenesis:** Glycogen Synthase (Active in dephosphorylated state). * **Insulin/Glucagon Ratio:** The metabolic direction is determined by this ratio. A high ratio (fed state) favors storage (Glycogenesis, Lipogenesis, Protein synthesis). * **Liver vs. Muscle Glycogen:** Liver glycogen maintains blood glucose levels, whereas muscle glycogen is used exclusively for muscle contraction (as muscles lack Glucose-6-Phosphatase).

Practice by Chapter

Carbohydrate Chemistry and Classification

Practice Questions

Glycolysis: Reactions and Regulation

Practice Questions

Gluconeogenesis: Reactions and Regulation

Practice Questions

Glycogen Metabolism: Synthesis and Breakdown

Practice Questions

Glycogen Storage Diseases

Practice Questions

Pentose Phosphate Pathway

Practice Questions

Metabolism of Fructose and Galactose

Practice Questions

Disorders of Fructose and Galactose Metabolism

Practice Questions

Blood Glucose Regulation

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Diabetes Mellitus: Biochemical Aspects

Practice Questions

Glycosylation and Glycoproteins

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Lactose Intolerance and Galactosemia

Practice Questions

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