Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 311: What is the cofactor for Glycogen phosphorylase in Glycogenolysis?

A. Thiamine pyrophosphate
B. Pyridoxal phosphate (Correct Answer)
C. Citrate
D. FAD

Explanation: **Explanation:** **1. Why Pyridoxal Phosphate (PLP) is correct:** Glycogen phosphorylase is the rate-limiting enzyme of glycogenolysis. It catalyzes the phosphorolytic cleavage of glycogen to produce glucose-1-phosphate. Unlike most enzymes that use PLP for transamination or decarboxylation, glycogen phosphorylase requires **Pyridoxal Phosphate (Vitamin B6)** as an essential cofactor for its catalytic activity. Specifically, the **phosphate group** of PLP acts as a general acid-base catalyst, promoting the attack of inorganic phosphate on the glycosidic bond. This is a unique mechanism where the aldehyde group of PLP (usually the active site) is not directly involved in the catalysis. **2. Why other options are incorrect:** * **Thiamine pyrophosphate (TPP):** This is the active form of Vitamin B1. it is a cofactor for oxidative decarboxylation reactions (e.g., Pyruvate Dehydrogenase, $\alpha$-ketoglutarate dehydrogenase) and the HMP shunt (Transketolase). * **Citrate:** This is an intermediate of the TCA cycle and acts as an allosteric **inhibitor** of Phosphofructokinase-1 (PFK-1) in glycolysis, not a cofactor. * **FAD:** Flavin adenine dinucleotide (Vitamin B2 derivative) acts as an electron carrier in redox reactions, such as those catalyzed by Succinate dehydrogenase. **3. High-Yield Clinical Pearls for NEET-PG:** * **McArdle Disease (GSD Type V):** Caused by a deficiency of muscle glycogen phosphorylase. Patients present with exercise intolerance, muscle cramps, and myoglobinuria. * **Hers Disease (GSD Type VI):** Caused by a deficiency of liver glycogen phosphorylase, leading to hepatomegaly and mild fasting hypoglycemia. * **Regulation:** Glycogen phosphorylase is activated by **phosphorylation** (via phosphorylase kinase) and allosterically activated by **AMP** in the muscle. * **B6 Fact:** Over 80% of the body's Vitamin B6 is stored in the muscle, bound to glycogen phosphorylase.

Question 312: Which of the following is NOT an example of a glycosaminoglycan?

A. Hyaluronic acid
B. Chondroitin sulfate
C. Heparin
D. None of the above (Correct Answer)

Explanation: **Explanation:** The correct answer is **None of the above** because all three options listed (Hyaluronic acid, Chondroitin sulfate, and Heparin) are classic examples of **Glycosaminoglycans (GAGs)**. **Understanding Glycosaminoglycans (GAGs):** GAGs, formerly known as mucopolysaccharides, are long, unbranched heteropolysaccharides consisting of repeating disaccharide units. These units typically contain an **amino sugar** (D-glucosamine or D-galactosamine) and an **uronic acid** (glucuronic or iduronic acid). Due to their high negative charge, they attract water, providing lubrication and shock absorption in the extracellular matrix. * **Option A: Hyaluronic Acid** is a GAG. It is unique because it is the only GAG that is **non-sulfated**, not covalently bound to a protein core, and is found in the vitreous humor and synovial fluid. * **Option B: Chondroitin Sulfate** is the **most abundant GAG** in the body, found prominently in cartilage, bone, and heart valves. * **Option C: Heparin** is an intracellular GAG found in mast cells. It acts as a potent anticoagulant by activating antithrombin III. (Note: Heparan sulfate is its extracellular counterpart found in basement membranes). **High-Yield Clinical Pearls for NEET-PG:** 1. **Keratan Sulfate:** The only GAG that **lacks uronic acid** (contains galactose instead). 2. **Mucopolysaccharidoses (MPS):** Genetic deficiencies of lysosomal enzymes that degrade GAGs (e.g., **Hurler Syndrome** - α-L-iduronidase deficiency; **Hunter Syndrome** - Iduronate sulfatase deficiency, which is X-linked recessive). 3. **Specific Locations:** * **Dermatan sulfate:** Skin, blood vessels, heart valves. * **Heparan sulfate:** Basement membranes; determines charge selectivity in the glomerular filtration barrier.

Question 313: Which enzyme, when absent, would impair the rate-limiting step of glycogenolysis?

A. 1,4-Glucanotransferase
B. Glycogen synthetase
C. Glycogen phosphorylase (Correct Answer)
D. Phosphoglucomutase

Explanation: **Explanation:** **Glycogenolysis** is the biochemical breakdown of glycogen into glucose-1-phosphate and glucose. The correct answer is **Glycogen phosphorylase** because it catalyzes the **rate-limiting and regulatory step** of this pathway. 1. **Why Glycogen Phosphorylase is correct:** This enzyme breaks the $\alpha(1\to4)$ glycosidic bonds by adding an inorganic phosphate (phosphorolysis), releasing glucose-1-phosphate. It acts sequentially from the non-reducing ends of glycogen chains until it reaches a point four glucose residues away from a branch point. Its activity is tightly regulated by hormonal signals (glucagon/epinephrine) via covalent modification (phosphorylation). 2. **Why other options are incorrect:** * **1,4-Glucanotransferase:** This is part of the "debranching enzyme" complex. It moves a trisaccharide unit from one branch to another but is not the rate-limiting step. * **Glycogen synthetase:** This is the rate-limiting enzyme for **glycogenesis** (glycogen synthesis), not glycogenolysis. * **Phosphoglucomutase:** This enzyme catalyzes the reversible conversion of glucose-1-phosphate to glucose-6-phosphate. It is an equilibrium enzyme and not a regulatory bottleneck. **High-Yield Clinical Pearls for NEET-PG:** * **Cofactor:** Glycogen phosphorylase requires **Pyridoxal Phosphate (Vitamin B6)** as an essential cofactor. * **Clinical Correlation:** A deficiency of liver glycogen phosphorylase leads to **Von Gierke's Type VI (Hers Disease)**, while a deficiency in muscle leads to **McArdle Disease (Type V)**. * **Limit Dextrin:** Glycogen phosphorylase cannot break $\alpha(1\to6)$ bonds; the remaining branched structure it leaves behind is called "limit dextrin."

Question 314: A genetic disorder renders fructose 1,6-biphosphatase in the liver less sensitive to regulation by fructose 2,6-biphosphate. All of the following metabolic changes are observed in this disorder except?

A. Level of fructose 1,6-biphosphate is higher than normal (Correct Answer)
B. Level of fructose 1,6-biphosphate is lower than normal
C. Less pyruvate is formed
D. Less ATP is generated

Explanation: **Explanation:** The core of this question lies in understanding the reciprocal regulation of Glycolysis and Gluconeogenesis by **Fructose 2,6-bisphosphate (F2,6-BP)**. **1. Why Option A is the correct answer (The "Except" statement):** Fructose 2,6-BP is the most potent **allosteric inhibitor** of Fructose 1,6-bisphosphatase (F1,6-BPase), the rate-limiting enzyme of gluconeogenesis. If F1,6-BPase becomes **less sensitive** to this inhibition, the enzyme remains constitutively active. This leads to an accelerated conversion of Fructose 1,6-bisphosphate into Fructose 6-phosphate. Consequently, the steady-state levels of **Fructose 1,6-bisphosphate will be lower than normal**, making Option A false and thus the correct answer. **2. Analysis of Incorrect Options:** * **Option B:** As explained above, increased F1,6-BPase activity depletes the substrate, leading to lower levels of Fructose 1,6-bisphosphate. * **Option C:** Fructose 1,6-bisphosphate is a potent **feed-forward activator** of Pyruvate Kinase. Since its levels are low (Option B), Pyruvate Kinase activity decreases, resulting in less pyruvate formation. * **Option D:** With decreased glycolytic flux (due to low F1,6-BP levels and inhibited Pyruvate Kinase), the net production of ATP from the glycolytic pathway is significantly reduced. **NEET-PG High-Yield Pearls:** * **F2,6-BP Dual Role:** It is a potent **activator of PFK-1** (Glycolysis) and a potent **inhibitor of F1,6-BPase** (Gluconeogenesis). * **Bifunctional Enzyme:** F2,6-BP levels are controlled by the PFK-2/FBPase-2 complex. Insulin increases F2,6-BP (promoting glycolysis), while Glucagon decreases it (promoting gluconeogenesis). * **Clinical Correlation:** Deficiency of F1,6-BPase leads to fasting hypoglycemia and lactic acidosis, as the liver cannot generate glucose from non-carbohydrate precursors.

Question 315: Which of the following is NOT a true statement about the pentose phosphate pathway?

A. Carbon dioxide is produced during the pathway.
B. The oxidative phase utilizes NADP+.
C. No ATP is directly generated by the pathway.
D. All of the above statements are true. (Correct Answer)

Explanation: The **Pentose Phosphate Pathway (PPP)**, also known as the Hexose Monophosphate (HMP) Shunt, is a unique alternative route for glucose oxidation that occurs in the cytosol. Unlike glycolysis, its primary purpose is not energy production but the generation of **NADPH** and **Pentoses** (Ribose-5-phosphate). ### **Analysis of Statements:** * **Statement A (True):** During the oxidative phase, Glucose-6-phosphate undergoes decarboxylation. Specifically, the enzyme **6-phosphogluconate dehydrogenase** converts 6-phosphogluconate to Ribulose-5-phosphate, releasing **CO₂**. * **Statement B (True):** The oxidative phase is irreversible and utilizes **NADP⁺** as the electron acceptor. The key rate-limiting enzyme, **Glucose-6-phosphate dehydrogenase (G6PD)**, reduces NADP⁺ to NADPH. * **Statement C (True):** The HMP shunt is unique because **no ATP is directly consumed or produced** during the cycle. It is an anabolic-supporting pathway rather than a catabolic energy-yielding one. Since all statements (A, B, and C) are biochemically accurate, **Option D** is the correct choice. ### **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** G6PD (induced by Insulin). * **Tissues involved:** Occurs in tissues requiring NADPH for fatty acid/steroid synthesis (Adrenal cortex, Liver, Lactating mammary glands) or for maintaining reduced glutathione (Erythrocytes). * **Clinical Correlation:** **G6PD Deficiency** leads to hemolytic anemia due to the inability to neutralize free radicals in RBCs, often triggered by fava beans or oxidative drugs (e.g., Primaquine). * **Transketolase:** This enzyme in the non-oxidative phase requires **Thiamine (Vitamin B1)** as a cofactor; measuring its activity is used to diagnose Thiamine deficiency.

Question 316: After oral glucose feeding of 50 gm, what is the expected metabolic response?

A. Decreased ketone body production
B. Increased lactate production upon exercise
C. Decreased gluconeogenesis
D. All of these (Correct Answer)

Explanation: This question tests your understanding of the **Well-Fed State (Absorptive Phase)**, which is primarily mediated by an increased **Insulin-to-Glucagon ratio**. ### **Explanation of the Correct Answer** When 50g of glucose is ingested, blood glucose levels rise, triggering the release of insulin from pancreatic beta cells. This shifts the body into an anabolic state: * **Decreased Ketone Body Production:** Insulin inhibits **Hormone-Sensitive Lipase (HSL)**, reducing the breakdown of adipose tissue into free fatty acids (FFAs). Since FFAs are the precursors for ketogenesis in the liver, ketone production drops significantly. * **Increased Lactate Production upon Exercise:** In the fed state, muscle glycogen stores are replenished. During subsequent exercise, the abundance of glucose and glycogen leads to increased glycolytic flux. This results in higher pyruvate levels, which are converted to lactate via anaerobic glycolysis. * **Decreased Gluconeogenesis:** Insulin suppresses the expression of key gluconeogenic enzymes (e.g., PEPCK, Fructose-1,6-bisphosphatase) and decreases the availability of substrates like glycerol and amino acids. The body prioritizes using exogenous glucose rather than synthesizing it. Since all three physiological changes occur simultaneously following glucose intake, **Option D** is correct. ### **High-Yield Clinical Pearls for NEET-PG** * **The "Insulin Effect":** Insulin is the only hypoglycemic hormone. It promotes glycolysis, glycogenesis, and lipogenesis while inhibiting gluconeogenesis, glycogenolysis, and ketogenesis. * **Key Enzyme Regulation:** Insulin activates **Phosphofructokinase-1 (PFK-1)** via Fructose-2,6-bisphosphate, making it the "pacemaker" of glycolysis in the fed state. * **Metabolic Switch:** The transition from fasting to fed state is characterized by the brain switching from using ketone bodies (during prolonged fasts) back to exclusive glucose utilization.

Question 317: All the following enzymes catalyze physiologically irreversible reactions of glycolysis except?

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

Explanation: ### Explanation In glycolysis, reactions are categorized as either **reversible** or **irreversible**. Irreversible reactions are the key regulatory points of the pathway because they involve a significant decrease in free energy ($\Delta G$), making them "one-way" valves under physiological conditions. **Why Enolase is the Correct Answer:** Enolase catalyzes the conversion of 2-phosphoglycerate to phosphoenolpyruvate (PEP). This is a **reversible** reaction. Unlike the regulatory steps of glycolysis, this step can easily proceed in the opposite direction during **gluconeogenesis** using the same enzyme. **Analysis of Incorrect Options (Irreversible Enzymes):** The three irreversible steps of glycolysis are often referred to as the "bottlenecks" or "regulatory valves": * **Hexokinase (Step 1):** Converts Glucose to Glucose-6-Phosphate. It is irreversible to "trap" glucose inside the cell. * **Phosphofructokinase-1 (PFK-1) (Step 3):** Converts Fructose-6-Phosphate to Fructose-1,6-Bisphosphate. This is the **rate-limiting step** of glycolysis. * **Pyruvate Kinase (Step 10):** Converts PEP to Pyruvate. This is the final irreversible step and produces ATP via substrate-level phosphorylation. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Fluoride Inhibition:** Enolase is inhibited by **Fluoride**. This is why fluoride is added to gray-top vacutainers (sodium fluoride) when collecting blood for glucose estimation—it stops glycolysis to ensure accurate sugar levels. * **Gluconeogenesis Bypass:** Because Hexokinase, PFK-1, and Pyruvate Kinase are irreversible, gluconeogenesis must use four different enzymes (Pyruvate carboxylase, PEP carboxykinase, Fructose-1,6-bisphosphatase, and Glucose-6-phosphatase) to bypass these steps. * **Mnemonic:** Remember the irreversible steps as **"HPP"** (Hexokinase, PFK, Pyruvate Kinase).

Question 318: Lactose on hydrolysis yields which of the following?

A. 2 molecules of fructose
B. 2 molecules of glucose
C. One molecule of glucose and one molecule of fructose
D. One molecule of glucose and one molecule of galactose (Correct Answer)

Explanation: **Explanation:** Lactose, commonly known as "milk sugar," is a disaccharide found exclusively in mammalian milk. It consists of two monosaccharides—**D-glucose and D-galactose**—joined by a **$\beta$(1→4) glycosidic linkage**. During digestion, the enzyme **Lactase** (a brush-border disaccharidase) hydrolyzes this bond, releasing one molecule of glucose and one molecule of galactose into the bloodstream. **Analysis of Options:** * **Option A (2 molecules of fructose):** This does not occur in human carbohydrate metabolism. * **Option B (2 molecules of glucose):** This is the result of the hydrolysis of **Maltose** (by maltase) or **Isomaltose** (by isomaltase). * **Option C (One glucose and one fructose):** This is the result of the hydrolysis of **Sucrose** (table sugar) by the enzyme sucrase. * **Option D (Correct):** Lactose is specifically composed of glucose and galactose. **High-Yield Clinical Pearls for NEET-PG:** 1. **Lactose Intolerance:** Caused by a deficiency of the enzyme lactase. It leads to osmotic diarrhea, bloating, and flatulence due to the bacterial fermentation of undigested lactose in the colon. 2. **Galactosemia:** A deficiency in enzymes like **GALT** (Galactose-1-phosphate uridyltransferase) prevents the metabolism of galactose (derived from lactose), leading to cataracts, liver damage, and intellectual disability in infants. 3. **Reducing Sugar:** Lactose is a reducing sugar because it retains a free anomeric carbon on the glucose residue. 4. **Source:** It is the least sweet of all common sugars and is the only carbohydrate of animal origin in the human diet.

Question 319: Which enzyme plays an important role in regulating blood glucose levels after feeding?

A. Glucokinase (Correct Answer)
B. Glucose-6-phosphatase
C. Phosphofructokinase
D. Pyruvate kinase

Explanation: **Explanation** **Glucokinase (Hexokinase IV)** is the correct answer because it acts as a "glucose sensor" in the liver and pancreatic beta cells. Unlike Hexokinase I-III, Glucokinase has a **high Km** (low affinity) and a **high Vmax** (high capacity). This means it only becomes significantly active when blood glucose levels are high (postprandial/after feeding). It rapidly phosphorylates glucose to glucose-6-phosphate, trapping it in the liver for glycogen synthesis and glycolysis, thereby lowering blood glucose levels. **Analysis of Incorrect Options:** * **B. Glucose-6-phosphatase:** This enzyme is involved in **gluconeogenesis and glycogenolysis** (fasting state). It converts glucose-6-phosphate back to free glucose to be released into the blood. It increases blood glucose, rather than regulating it after feeding. * **C. Phosphofructokinase-1 (PFK-1):** While it is the rate-limiting enzyme of glycolysis, its primary role is regulating the *flux* of carbohydrate breakdown within the cell to meet energy needs, not the systemic regulation of blood glucose levels. * **D. Pyruvate kinase:** This is the final enzyme of glycolysis. While regulated by insulin, it does not serve as the primary sensor or gatekeeper for blood glucose entry into metabolic pathways. **High-Yield NEET-PG Pearls:** * **Inducibility:** Glucokinase is induced by **Insulin**, whereas Hexokinase is not. * **Inhibition:** Glucokinase is **not** inhibited by its product (Glucose-6-Phosphate), allowing it to continue clearing glucose even when levels are high. * **Clinical Correlation:** Mutations in the Glucokinase gene lead to **MODY type 2** (Maturity-Onset Diabetes of the Young), characterized by a higher threshold for insulin release. * **Localization:** Glucokinase is sequestered in the nucleus by Glucokinase Regulatory Protein (GKRP) during fasting and released into the cytosol after a meal.

Question 320: In muscles, how many ATP molecules are produced from the conversion of one glucose residue in the linear chain of glycogen to lactic acid via anaerobic glycolysis?

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

Explanation: ### Explanation The correct answer is **3 ATP molecules**. #### 1. Why Option C is Correct The key to this question lies in the starting material: **glycogen**. In anaerobic glycolysis starting from free glucose, 2 ATPs are consumed and 4 are produced, resulting in a net gain of 2 ATP. However, when starting from a **glucose residue in glycogen**: * **Glycogenolysis:** Glycogen phosphorylase cleaves the glucose residue as **Glucose-1-Phosphate (G-1-P)**. This step uses inorganic phosphate ($P_i$) rather than ATP. * **Isomerization:** G-1-P is converted to Glucose-6-Phosphate (G-6-P) by phosphoglucomutase. * **Bypassing the Hexokinase Step:** Because the glucose enters the glycolytic pathway already phosphorylated as G-6-P, the **Hexokinase/Glucokinase step (which normally consumes 1 ATP) is bypassed.** * **Energy Yield:** Only 1 ATP is consumed (at the Phosphofructokinase-1 step), while 4 ATPs are generated during the payoff phase. * **Net Yield:** $4 \text{ (produced)} - 1 \text{ (consumed)} = \mathbf{3 \text{ ATP}}$. #### 2. Why Other Options are Incorrect * **Option A (1 ATP):** This does not correspond to any standard net yield in glycolysis. * **Option B (2 ATP):** This is the net yield of anaerobic glycolysis starting from **free blood glucose**. * **Option D (4 ATP):** This is the total (gross) ATP produced in the payoff phase, but it does not account for the ATP consumed at the PFK-1 step. #### 3. Clinical Pearls & High-Yield Facts * **Energetic Advantage:** Glycogen is a more "energy-efficient" fuel for muscles during rapid contraction because it yields 50% more net ATP (3 vs 2) than blood glucose under anaerobic conditions. * **Lactic Acidosis:** In intense exercise, pyruvate is converted to lactate by **Lactate Dehydrogenase (LDH)** to regenerate $NAD^+$, allowing glycolysis to continue. * **McArdle Disease (GSD Type V):** A deficiency in muscle glycogen phosphorylase leads to exercise intolerance and cramps because the muscle cannot access these glucose residues for ATP production.

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

Practice Questions

Diabetes Mellitus: Biochemical Aspects

Practice Questions

Glycosylation and Glycoproteins

Practice Questions

Lactose Intolerance and Galactosemia

Practice Questions

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