Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 491: Which enzyme is active in glycogen metabolism?

A. Phosphorylase a (Correct Answer)
B. Phosphorylase b
C. Glycogen Synthase II
D. Glycogen Synthase C

Explanation: ### Explanation In glycogen metabolism, enzymes are regulated through **covalent modification** (phosphorylation and dephosphorylation). The key to answering this question lies in understanding the reciprocal regulation of glycogen phosphorylase and glycogen synthase. **1. Why Option A is Correct:** **Glycogen Phosphorylase** is the rate-limiting enzyme of glycogenolysis. It exists in two forms: * **Phosphorylase *a*:** The **phosphorylated**, active form. * **Phosphorylase *b*:** The dephosphorylated, inactive form. In the fasting state or during exercise, glucagon or epinephrine triggers a cAMP cascade that activates phosphorylase kinase, which phosphorylates Phosphorylase *b* into the active **Phosphorylase *a***. **2. Why the Other Options are Incorrect:** * **Option B (Phosphorylase *b*):** This is the dephosphorylated, **inactive** state of the enzyme (though it can be allosterically activated by high AMP levels in muscle). * **Option C & D (Glycogen Synthase II/C):** These are not standard biochemical nomenclatures for glycogen synthase. Glycogen synthase exists as **Synthase *a*** (dephosphorylated/active) and **Synthase *b*** (phosphorylated/inactive). Note that phosphorylation *inactivates* synthase but *activates* phosphorylase. ### NEET-PG Clinical Pearls & High-Yield Facts: * **Reciprocal Regulation:** Glucagon/Epinephrine lead to phosphorylation, which **activates** Glycogen Phosphorylase (breakdown) and **inactivates** Glycogen Synthase (synthesis). * **Insulin Effect:** Insulin triggers protein phosphatase-1, which dephosphorylates both enzymes, activating Glycogen Synthase and inactivating Phosphorylase. * **Allosteric Activator:** In the muscle, **AMP** is a potent allosteric activator of Phosphorylase *b*, allowing glycogenolysis to occur even without hormonal stimulation during intense contraction. * **Rate-limiting enzymes:** Glycogen Synthase (Glycogenesis) and Glycogen Phosphorylase (Glycogenolysis).

Question 492: Which disaccharides are not broken down in the small intestine?

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

Explanation: ### Explanation **1. Why Lactulose is the Correct Answer:** Lactulose is a synthetic disaccharide composed of **galactose and fructose**. Unlike natural disaccharides, the human small intestine lacks the specific enzyme (disaccharidase) required to hydrolyze the $\beta$-1,4-glycosidic bond of lactulose. Consequently, it passes unabsorbed into the large intestine. There, resident colonic bacteria ferment it into lactic acid and acetic acid. This process underpins its clinical use as an osmotic laxative and in managing hepatic encephalopathy. **2. Why the Other Options are Incorrect:** The human brush border of the small intestine contains specific enzymes for the digestion of dietary disaccharides: * **Maltose:** Broken down by **Maltase** into two glucose molecules. * **Sucrose:** Broken down by **Sucrase** into glucose and fructose. * **Lactose:** Broken down by **Lactase** into glucose and galactose. Since these enzymes are physiologically present, these sugars are normally absorbed in the small intestine. **3. Clinical Pearls for NEET-PG:** * **Hepatic Encephalopathy:** Lactulose is the first-line treatment. It acidifies the gut lumen ($NH_3 \to NH_4^+$), trapping ammonia as non-absorbable ammonium ions (**Ammonia Trapping**), which are then excreted. * **Lactulose Breath Test:** Used to diagnose **Small Intestinal Bacterial Overgrowth (SIBO)** and to measure orocecal transit time. * **Reducing vs. Non-reducing:** Sucrose is a non-reducing sugar (the only one among common dietary sugars), whereas Lactose and Maltose are reducing sugars. * **Inulin:** Another non-digestible carbohydrate (polysaccharide) often confused with lactulose; it is used to measure GFR as it is freely filtered but neither reabsorbed nor secreted.

Question 493: A child with hypoglycemia is unable to utilize glucose from glycogenolysis or gluconeogenesis. Which of the following enzymes is deficient in the child?

A. Fructokinase
B. Glucokinase
C. Glucose 6-phosphatase (Correct Answer)
D. Transketolase

Explanation: **Explanation:** The child is suffering from **Von Gierke Disease (Glycogen Storage Disease Type I)**. The enzyme **Glucose 6-phosphatase** is the correct answer because it is the "final common pathway" for both glycogenolysis and gluconeogenesis to release free glucose into the bloodstream. 1. **Why it is correct:** In the liver, glycogenolysis breaks down glycogen into Glucose 1-phosphate, which is converted to Glucose 6-phosphate (G6P). Similarly, gluconeogenesis produces G6P from non-carbohydrate precursors. The enzyme **Glucose 6-phosphatase** is required to remove the phosphate group from G6P to form free glucose. Without it, glucose remains trapped inside the hepatocytes, leading to severe fasting hypoglycemia. 2. **Why other options are incorrect:** * **Fructokinase:** Deficiency causes Essential Fructosuria, a benign condition that does not cause hypoglycemia. * **Glucokinase:** This enzyme catalyzes the initial step of glycolysis (Glucose → G6P). Deficiency would impair glucose utilization/sensing but would not prevent the liver from *producing* glucose. * **Transketolase:** An enzyme of the Pentose Phosphate Pathway (HMP Shunt) that requires Thiamine (B1). It is not involved in glucose release. **High-Yield Clinical Pearls for NEET-PG:** * **Von Gierke Disease Presentation:** Hepatomegaly (due to glycogen/fat accumulation), "doll-like" facies, and severe fasting hypoglycemia. * **Biochemical Hallmarks:** Hyperlactatemia (G6P shunts to glycolysis), Hyperuricemia (G6P shunts to HMP shunt → Purine synthesis), and Hyperlipidemia. * **Location:** Glucose 6-phosphatase is located in the **Endoplasmic Reticulum (ER)** lumen. Deficiency of the G6P transporter (T1) results in Type Ib GSD, which also presents with neutropenia.

Question 494: The immediate degradation of glycogen under normal conditions gives rise to which one of the following?

A. More glucose than glucose-1-phosphate
B. More glucose-1-phosphate than glucose (Correct Answer)
C. Equal amounts of glucose and glucose-1-phosphate
D. Neither glucose nor glucose-1-phosphate

Explanation: ### Explanation The degradation of glycogen (glycogenolysis) occurs through the coordinated action of two primary enzymes: **Glycogen Phosphorylase** and the **Debranching Enzyme**. **1. Why Option B is Correct:** * **Glycogen Phosphorylase:** This enzyme cleaves the $\alpha(1\to4)$ glycosidic bonds by adding inorganic phosphate (phosphorolysis). This process releases **Glucose-1-Phosphate (G1P)**. Since the majority of glucose residues in glycogen are linked by $\alpha(1\to4)$ bonds, G1P is the predominant product (approximately 90%). * **Debranching Enzyme:** When a branch point is reached, the $\alpha(1\to6)$ glucosidase activity of the debranching enzyme hydrolytically cleaves the $\alpha(1\to6)$ bond. This releases **free Glucose**. Because branches occur only every 8–12 residues, free glucose accounts for only about 10% of the yield. * **Conclusion:** Therefore, glycogenolysis yields significantly **more Glucose-1-Phosphate than Glucose**. **2. Why Other Options are Wrong:** * **Option A & C:** These are incorrect because the ratio of $\alpha(1\to4)$ bonds to $\alpha(1\to6)$ bonds is roughly 10:1. Phosphorolysis (yielding G1P) is the major pathway, while hydrolysis (yielding glucose) is minor. * **Option D:** This is factually incorrect as these are the two primary products of glycogen breakdown. **3. NEET-PG Clinical Pearls & High-Yield Facts:** * **Energetic Advantage:** Phosphorolysis is energetically favorable because the released glucose is already phosphorylated (G1P), saving one ATP molecule that would otherwise be required by Hexokinase. * **Key Enzyme:** Glycogen Phosphorylase requires **Pyridoxal Phosphate (Vitamin B6)** as a mandatory cofactor. * **Von Gierke Disease (GSD Type I):** Deficiency of Glucose-6-Phosphatase. While G1P is converted to G6P, it cannot be converted to free glucose in the liver, leading to severe fasting hypoglycemia. * **Cori Disease (GSD Type III):** Deficiency of Debranching Enzyme. This results in the accumulation of "Limit Dextrins" (abnormal glycogen with short outer branches).

Question 495: All of the following are true regarding proteoglycans, EXCEPT?

A. Chondroitin sulfate is a glycosaminoglycan.
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 chains of **Glycosaminoglycans (GAGs)**. **Why Option B is the "Except" (Correct Answer):** In the context of this specific question format, Option B is often marked as the "incorrect statement" if the examiner defines proteoglycans strictly by their chemical composition rather than their physical properties. However, physiologically, proteoglycans **do** hold large amounts of water. If this is the designated answer, it implies a technicality: Proteoglycans *attract* water due to their osmotic pressure, but the water is not a structural component of the molecule itself. *Note: In many standard textbooks, B is actually a true statement, making this a controversial "recall" question. If B is the keyed answer, it is likely because the examiner considers "holding water" a function of the GAG component, not the proteoglycan as a whole.* **Analysis of Other Options:** * **Option A:** **True.** Chondroitin sulfate is the most abundant GAG in the body, found in cartilage and bone. * **Option C:** **True.** They consist of a protein core (amino acids) and GAGs (long sugar chains). * **Option D:** **True.** GAGs contain sulfate and carboxyl groups, giving them a high **negative charge**. This causes the chains to repel each other, creating the "bottle-brush" appearance and allowing them to act as biological lubricants. **High-Yield NEET-PG Pearls:** * **Hyaluronic Acid:** The only GAG that is **not sulfated** and not covalently bound to a protein core. * **Heparin:** The GAG with the highest negative charge density; acts as an intracellular anticoagulant. * **Mucopolysaccharidoses (MPS):** Genetic disorders (e.g., Hurler, Hunter syndromes) caused by the deficiency of lysosomal enzymes that degrade GAGs. * **Aggrecan:** The major proteoglycan in cartilage.

Question 496: Which of the following statements regarding the oxidation of one molecule of acetyl CoA by the citric acid cycle is FALSE?

A. NADH is produced, and each molecule yields 3 ATP.
B. FADH2 is produced, yielding 2 ATP.
C. One molecule of ATP is generated from succinyl CoA to succinate.
D. A total of 14 ATP are generated. (Correct Answer)

Explanation: ### Explanation The Citric Acid Cycle (TCA cycle) is the final common pathway for the oxidation of carbohydrates, lipids, and proteins. To identify the false statement, we must calculate the energy yield per turn of the cycle starting from **one molecule of Acetyl CoA**. **1. Why Option D is the Correct (False) Statement:** The total ATP yield from one molecule of Acetyl CoA is **12 ATP** (based on traditional calculations) or **10 ATP** (based on modern P:O ratios). It is **never 14 ATP**. * 3 NADH × 3 ATP = 9 ATP * 1 FADH₂ × 2 ATP = 2 ATP * 1 GTP (Substrate-level phosphorylation) = 1 ATP * **Total = 12 ATP.** *(Note: Using modern ratios of 2.5 per NADH and 1.5 per FADH₂, the total is 10 ATP).* **2. Analysis of Incorrect (True) Options:** * **Option A:** During the cycle, 3 molecules of NADH are produced (at Isocitrate dehydrogenase, α-Ketoglutarate dehydrogenase, and Malate dehydrogenase steps). In the electron transport chain (ETC), each NADH traditionally yields 3 ATP. * **Option B:** One molecule of FADH₂ is produced at the Succinate dehydrogenase step, yielding 2 ATP via the ETC. * **Option C:** This refers to **substrate-level phosphorylation**. The conversion of Succinyl CoA to Succinate by *Succinate thiokinase* generates one molecule of GTP (energetically equivalent to ATP). **3. NEET-PG High-Yield Pearls:** * **Rate-limiting enzyme:** Isocitrate dehydrogenase. * **Only membrane-bound enzyme:** Succinate dehydrogenase (also part of Complex II of ETC). * **Inhibitors:** Fluoroacetate inhibits Aconitase; Arsenite inhibits α-Ketoglutarate dehydrogenase. * **Amphibolic nature:** The TCA cycle serves both catabolic (energy production) and anabolic (providing precursors for gluconeogenesis and amino acid synthesis) functions.

Question 497: Which of the following is a homopolysaccharide of fructose?

A. Chitin
B. Dextran
C. Inulin (Correct Answer)
D. All of the above

Explanation: ### Explanation **Correct Answer: C. Inulin** **Why Inulin is correct:** Inulin is a **homopolysaccharide** (a polymer consisting of only one type of monosaccharide unit) composed of **D-fructose** units. These fructose units are linked by **β(2→1) glycosidic bonds**. It typically ends with a terminal glucose residue. Inulin is found in plants like chicory, dahlia bulbs, and garlic. Because it is not digested by human enzymes, it serves as a soluble fiber. **Why other options are incorrect:** * **A. Chitin:** This is a homopolysaccharide of **N-acetyl-D-glucosamine** (NAG) linked by β(1→4) bonds. It is a structural component found in the exoskeletons of arthropods and fungal cell walls. * **B. Dextran:** This is a branched homopolysaccharide of **D-glucose**. It is produced by bacteria and yeast, characterized by α(1→6) main chains and α(1→3) branches. (Note: Do not confuse Dextran with Dextrin, which is an intermediate product of starch hydrolysis). **High-Yield Clinical Pearls for NEET-PG:** 1. **Renal Function:** Inulin is the "Gold Standard" for measuring **Glomerular Filtration Rate (GFR)** because it is freely filtered by the glomeruli but is neither reabsorbed nor secreted by the renal tubules. 2. **Fructans:** Inulin belongs to a class of carbohydrates called fructans. 3. **Other Homopolysaccharides:** Remember that Starch, Glycogen, and Cellulose are all homopolysaccharides of **Glucose**, differing only in their linkages and branching patterns. 4. **Diagnostic Use:** While Inulin is the gold standard for GFR, **Creatinine clearance** is more commonly used in clinical practice as it is endogenous and does not require a continuous intravenous infusion.

Question 498: What is the number of hydroxyl (-OH) groups in ribose?

A. 4 (Correct Answer)
B. 5
C. 6
D. 2

Explanation: **Explanation:** Ribose is a **pentose sugar** (a 5-carbon monosaccharide) with the chemical formula **$C_5H_{10}O_5$**. In its open-chain structure, ribose is an aldopentose, meaning it contains one aldehyde group (-CHO) at Carbon-1 and hydroxyl (-OH) groups on the remaining four carbons. **Why Option A is correct:** The structural formula of ribose in its open-chain form is $CH_2OH-(CHOH)_3-CHO$. There are hydroxyl groups attached to carbons C-2, C-3, C-4, and C-5. Even in its cyclic (furanose) form—which is how it exists in RNA—there are **4 hydroxyl groups** (at C-1, C-2, C-3, and C-5). Note that the C-4 oxygen becomes part of the ring structure (hemiacetal linkage). **Analysis of Incorrect Options:** * **Option B (5):** While ribose has 5 carbons and 5 oxygens, one oxygen is part of the aldehyde group (in open chain) or the ring linkage (in cyclic form), leaving only 4 as hydroxyl groups. * **Option C (6):** This would correspond to hexoses like glucose or galactose, which have more carbon atoms. * **Option D (2):** This is incorrect for a pentose. However, it is important to distinguish ribose from **2-deoxyribose** (found in DNA), which has **3 hydroxyl groups** because the -OH at C-2 is replaced by a hydrogen atom. **High-Yield Clinical Pearls for NEET-PG:** * **RNA vs. DNA:** Ribose is the sugar component of RNA, while 2-deoxyribose is found in DNA. The presence of the extra -OH group at C-2 makes RNA more chemically reactive and less stable than DNA. * **HMP Shunt:** Ribose-5-phosphate is a key product of the Pentose Phosphate Pathway (HMP Shunt), essential for nucleotide synthesis. * **Energy Molecules:** Ribose is a structural component of ATP, NADH, FAD, and Coenzyme A.

Question 499: When glucose concentration in blood increases, there is a linear increase in which of the following hormones?

A. Insulin (Correct Answer)
B. Glucagon
C. Growth Hormone
D. Cortisol

Explanation: **Explanation:** The correct answer is **Insulin**. This question tests the fundamental understanding of glycemic control and the kinetics of hormone secretion. **1. Why Insulin is Correct:** Insulin is the primary anabolic hormone secreted by the **β-cells of the Islets of Langerhans** in response to hyperglycemia. The relationship between blood glucose and insulin secretion is **linear and sigmoidal**. As glucose levels rise, glucose enters β-cells via **GLUT-2** (a high-capacity, low-affinity transporter). It is then phosphorylated by **Glucokinase**, which acts as the "glucose sensor." This metabolic flux leads to an increase in ATP, closure of K+ channels, and subsequent insulin release. Within physiological ranges, the higher the glucose concentration, the greater the insulin output to facilitate glucose uptake into peripheral tissues (muscle and adipose). **2. Why the Other Options are Incorrect:** * **Glucagon:** Secreted by α-cells, it is a counter-regulatory hormone. Its secretion is **inhibited** by high blood glucose levels; it increases only during hypoglycemia. * **Growth Hormone (GH):** GH is a diabetogenic hormone that increases blood glucose by antagonizing insulin action. Its secretion is **suppressed** by hyperglycemia and stimulated by hypoglycemia. * **Cortisol:** Known as the "stress hormone," it increases glucose via gluconeogenesis. Like GH, its secretion is not triggered by high glucose; rather, it contributes to hyperglycemia. **High-Yield Clinical Pearls for NEET-PG:** * **Glucokinase vs. Hexokinase:** Glucokinase (found in liver/β-cells) has a high Km, allowing it to respond linearly to high glucose levels, unlike Hexokinase which is easily saturated. * **Biphasic Release:** Insulin release is biphasic—an immediate "burst" (stored insulin) followed by a sustained "plateau" (newly synthesized insulin). * **Incretin Effect:** Oral glucose causes a much higher insulin response than intravenous glucose due to the release of GLP-1 and GIP from the gut.

Question 500: Gluconeogenesis is inhibited by:

A. Insulin (Correct Answer)
B. Glucagon
C. Glucocorticoids
D. Gonadotropin-releasing hormone

Explanation: **Explanation:** **Gluconeogenesis** is the metabolic pathway that results in the generation of glucose from non-carbohydrate precursors (like lactate, glycerol, and glucogenic amino acids). It occurs primarily in the liver and kidneys during periods of fasting to maintain blood glucose levels. **1. Why Insulin is the correct answer:** Insulin is an **anabolic hormone** secreted by the beta cells of the pancreas in the fed state. Its primary goal is to lower blood glucose. It inhibits gluconeogenesis by: * **Transcriptional Regulation:** Decreasing the expression of key rate-limiting enzymes, specifically **Phosphoenolpyruvate carboxykinase (PEPCK)** and **Glucose-6-phosphatase**. * **Allosteric Regulation:** Increasing the levels of Fructose-2,6-bisphosphate, which stimulates glycolysis and inhibits gluconeogenesis. **2. Why the other options are incorrect:** * **Glucagon:** This is the primary stimulator of gluconeogenesis. It increases cAMP levels, activating Protein Kinase A, which induces the transcription of the PEPCK gene. * **Glucocorticoids (e.g., Cortisol):** These are "diabetogenic" hormones. They stimulate gluconeogenesis by increasing the breakdown of muscle proteins into amino acids (substrate supply) and inducing the synthesis of gluconeogenic enzymes. * **Gonadotropin-releasing hormone (GnRH):** This hormone regulates the reproductive axis (FSH/LH secretion) and has no direct regulatory role in carbohydrate metabolism. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step of Gluconeogenesis:** Conversion of Fructose-1,6-bisphosphate to Fructose-6-phosphate by **Fructose-1,6-bisphosphatase**. * **Key Stimulators:** Glucagon, Epinephrine, Cortisol, and Acetyl-CoA (obligatory activator of Pyruvate Carboxylase). * **Key Inhibitor:** Insulin and Alcohol (NADH/NAD+ ratio imbalance inhibits gluconeogenesis, leading to fasting hypoglycemia).

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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