Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 41: Which of the following enzymes does not catalyze a reversible step in glycolysis?

A. Phosphofructokinase-1 (PFK) (Correct Answer)
B. Enolase
C. Phosphoglyceromutase
D. Glyceraldehyde-3-phosphate dehydrogenase

Explanation: ### Explanation In glycolysis, most reactions are reversible and occur near equilibrium. However, there are **three irreversible steps** that serve as the primary regulatory points of the pathway. These reactions have a large negative Gibbs free energy ($\Delta G$), making them "one-way" valves in the cell. **1. Why Phosphofructokinase-1 (PFK-1) is the correct answer:** PFK-1 catalyzes the conversion of Fructose-6-phosphate to Fructose-1,6-bisphosphate. This is the **second irreversible step** and the **rate-limiting step** of glycolysis. Because it is highly exergonic, it cannot be reversed by the same enzyme; instead, gluconeogenesis must use a different enzyme (Fructose-1,6-bisphosphatase) to bypass this step. **2. Analysis of Incorrect Options:** * **Enolase:** Catalyzes the reversible dehydration of 2-phosphoglycerate to phosphoenolpyruvate (PEP). It is inhibited by **fluoride** (used in gray-top blood collection tubes to prevent glycolysis). * **Phosphoglyceromutase:** Catalyzes the reversible shift of the phosphate group from the 3rd to the 2nd carbon of glycerate. * **Glyceraldehyde-3-phosphate dehydrogenase (GAPDH):** Catalyzes the reversible oxidative phosphorylation of GAP to 1,3-bisphosphoglycerate. This step is notable for producing NADH and being inhibited by **arsenite**. **3. High-Yield Clinical Pearls for NEET-PG:** * **The Three Irreversible Enzymes:** Remember the mnemonic **"H-P-K"**: **H**exokinase (Step 1), **P**hosphofructokinase-1 (Step 3), and **P**yruvate **K**inase (Step 10). * **PFK-1 Regulation:** It is allosterically activated by **AMP** and **Fructose-2,6-bisphosphate**, and inhibited by **ATP** and **Citrate**. * **Arsenite Poisoning:** Arsenite competes with inorganic phosphate in the GAPDH reaction, leading to the bypass of ATP synthesis at the substrate level.

Question 42: Oxidative decarboxylation of pyruvate requires which of the following?

A. NADP+
B. Cytochromes
C. Pyridoxal phosphate
D. COASH (Correct Answer)

Explanation: **Explanation:** The oxidative decarboxylation of pyruvate to Acetyl-CoA is a critical link between glycolysis and the TCA cycle. This reaction is catalyzed by the **Pyruvate Dehydrogenase (PDH) Complex**, a multi-enzyme system located in the mitochondrial matrix. **Why COASH is correct:** The PDH complex requires five specific cofactors to function: **T**hiamine pyrophosphate (TPP/B1), **L**ipoic acid, **C**oenzyme A (CoA-SH/B5), **F**AD (B2), and **N**AD+ (B3). **CoA-SH** acts as the final acceptor of the acetyl group, forming **Acetyl-CoA**, which then enters the Krebs cycle. Without CoA-SH, the two-carbon unit cannot be activated for further metabolism. **Why other options are incorrect:** * **NADP+:** This is primarily used in reductive biosynthesis (e.g., fatty acid synthesis) and the HMP shunt. The PDH complex specifically uses **NAD+** as an electron acceptor. * **Cytochromes:** These are components of the Electron Transport Chain (ETC) involved in redox reactions for ATP production, not in the decarboxylation of keto-acids. * **Pyridoxal phosphate (PLP):** This is the active form of Vitamin B6, essential for **transamination** and decarboxylation of amino acids, but it plays no role in the PDH complex. **Clinical Pearls for NEET-PG:** * **Mnemonic:** "The Lovely Co-enzymes For Nerds" (TPP, Lipoate, CoA, FAD, NAD). * **Arsenic Poisoning:** Arsenite inhibits the PDH complex by binding to the -SH groups of **Lipoic acid**, leading to lactic acidosis and neurological symptoms. * **Deficiency:** Thiamine (B1) deficiency (Wernicke-Korsakoff) impairs PDH activity, significantly affecting ATP-dependent organs like the brain.

Question 43: Which of the following is a component of dietary fibre?

A. Collagen
B. Pectin (Correct Answer)
C. Proteoglycans
D. Starch

Explanation: **Explanation:** Dietary fiber consists of edible parts of plants or analogous carbohydrates that are resistant to digestion and absorption in the human small intestine, with complete or partial fermentation in the large intestine. **Why Pectin is Correct:** **Pectin** is a structural heteropolysaccharide found in the primary cell walls of terrestrial plants (especially fruits like apples and citrus). It is a **soluble fiber** that forms a gel-like substance in the gut. Chemically, it is primarily composed of methylated galacturonic acid units. Because humans lack the enzymes to break its specific glycosidic bonds, it remains undigested, fulfilling the definition of dietary fiber. **Why Other Options are Incorrect:** * **Collagen:** This is an animal-derived structural protein. While it provides "fiber-like" strength to connective tissues, it is not a carbohydrate nor a component of plant-based dietary fiber. * **Proteoglycans:** These are molecules consisting of a core protein covalently attached to glycosaminoglycans (GAGs). They are found in the human extracellular matrix (e.g., cartilage) and are not dietary fibers. * **Starch:** This is a homopolymer of glucose (amylose and amylopectin) and is the primary **digestible** storage polysaccharide in plants. Unlike fiber, starch is easily hydrolyzed by salivary and pancreatic amylase. **High-Yield Clinical Pearls for NEET-PG:** * **Classification:** Dietary fibers are divided into **Soluble** (Pectin, Gums, Mucilages) and **Insoluble** (Cellulose, Hemicellulose, Lignin). Note: **Lignin** is the only non-carbohydrate component of dietary fiber. * **Metabolic Benefit:** Soluble fibers like pectin delay gastric emptying (increasing satiety) and slow glucose absorption, making them beneficial for diabetics. * **Cardiovascular Effect:** They bind to bile acids, increasing their excretion and effectively lowering LDL cholesterol levels. * **Caloric Value:** Fiber provides approximately **2 kcal/g** due to short-chain fatty acids (SCFAs) produced during colonic fermentation.

Question 44: Which of the following statements about glycolysis is true?

A. Glycolysis occurs in the mitochondria.
B. Glycolysis is the complete breakdown of glucose.
C. Glycolysis involves the conversion of glucose to 3-carbon units. (Correct Answer)
D. 3 ATP molecules are used in the anaerobic pathway of glycolysis.

Explanation: **Explanation:** **1. Why Option C is Correct:** Glycolysis (the Embden-Meyerhof pathway) is the process of breaking down one molecule of glucose (a **6-carbon hexose**) into two molecules of pyruvate (a **3-carbon alpha-keto acid**). This conversion occurs through a series of ten enzymatic steps. Even in anaerobic conditions, the end product is lactate, which also maintains a 3-carbon structure. **2. Why Other Options are Incorrect:** * **Option A:** Glycolysis occurs entirely in the **cytosol** of the cell. It is the only metabolic pathway that can function in cells lacking mitochondria, such as mature erythrocytes (RBCs). * **Option B:** Glycolysis is an **incomplete** breakdown of glucose. Complete oxidation occurs only when pyruvate enters the mitochondria to undergo oxidative decarboxylation (via PDH complex) and the TCA cycle, eventually yielding $CO_2$ and $H_2O$. * **Option C:** In the "Investment Phase" of glycolysis, exactly **2 ATP** molecules are consumed (catalyzed by Hexokinase/Glucokinase and Phosphofructokinase-1). There is no stage where 3 ATP molecules are used. **3. NEET-PG High-Yield Clinical Pearls:** * **Rate-Limiting Enzyme:** Phosphofructokinase-1 (PFK-1) is the key regulatory and rate-limiting enzyme. * **Net ATP Yield:** Under anaerobic conditions, the net gain is **2 ATP**. Under aerobic conditions (via Malate-Aspartate shuttle), it is **7 or 5 ATP** (depending on the shuttle used). * **Rapoport-Luebering Cycle:** In RBCs, a bypass of glycolysis produces **2,3-BPG**, which decreases hemoglobin's affinity for oxygen, facilitating oxygen delivery to tissues. * **Arsenic Poisoning:** Arsenate competes with inorganic phosphate in the GAPDH reaction, resulting in zero net ATP production during glycolysis.

Question 45: In glycolysis, insulin affects all of the following enzymes except?

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

Explanation: ### Explanation The correct answer is **Hexokinase**. In carbohydrate metabolism, insulin acts as an anabolic hormone that promotes glycolysis to lower blood glucose levels. It achieves this by inducing the synthesis of three key regulatory enzymes. However, **Hexokinase (Type I, II, and III)** is an exception because it is a constitutive enzyme. **1. Why Hexokinase is the correct answer:** Hexokinase is found in most extrahepatic tissues. It is **not induced by insulin**. Instead, it is primarily regulated by **allosteric inhibition** by its product, glucose-6-phosphate. This ensures that cells do not over-consume glucose if they already have sufficient energy, regardless of insulin levels. **2. Why the other options are incorrect:** Insulin regulates the three "irreversible" steps of glycolysis via gene induction (increasing enzyme synthesis): * **Glucokinase (Hexokinase IV):** Unlike other hexokinases, the liver-specific Glucokinase is **induced by insulin**. This allows the liver to trap glucose effectively after a meal. * **Phosphofructokinase-1 (PFK-1):** Insulin increases the synthesis of PFK-1 and also stimulates the production of Fructose-2,6-bisphosphate, its most potent allosteric activator. * **Pyruvate Kinase:** Insulin induces the synthesis of Pyruvate Kinase and keeps it in its active (dephosphorylated) state. **High-Yield Clinical Pearls for NEET-PG:** * **Glucokinase vs. Hexokinase:** Glucokinase has a **high Km** (low affinity) and **high Vmax**, making it ideal for glucose sensing in the liver and pancreas. Hexokinase has a **low Km** (high affinity), allowing tissues to take up glucose even at low blood concentrations. * **Mnemonic:** Insulin **"Induces"** the **"Key"** enzymes: **G**lucokinase, **P**FK, and **P**yruvate kinase (**G**o **P**lay **P**iano). * **MODY Type 2:** Mutations in the *GCK* gene (Glucokinase) lead to Maturity-Onset Diabetes of the Young.

Question 46: In glycolysis, the conversion of 1 mol of fructose 1,6-bisphosphate to 2 mol of pyruvate results in the formation of which of the following molecules?

A. 1 mol NAD+ and 2 mol of ATP
B. 2 mol NAD+ and 4 mol of ATP
C. 2 mol NADH and 2 mol of ATP
D. 2 mol NADH and 4 mol of ATP (Correct Answer)

Explanation: **Explanation:** In the glycolytic pathway, the conversion of **Fructose 1,6-bisphosphate (F1,6BP)** to Pyruvate represents the "Pay-off Phase." 1. **Cleavage:** F1,6BP (6 carbons) is cleaved by Aldolase A into two 3-carbon units: Glyceraldehyde 3-phosphate (G3P) and Dihydroxyacetone phosphate (DHAP). DHAP is isomerized to G3P, resulting in **2 moles of G3P**. 2. **Oxidation (NADH generation):** Each G3P is oxidized by *G3P Dehydrogenase*. This step reduces NAD+ to NADH. Since there are 2 moles of G3P, **2 mol of NADH** are produced. 3. **ATP Generation (Substrate-Level Phosphorylation):** * Conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate (by Phosphoglycerate kinase) yields 1 ATP per G3P. * Conversion of Phosphoenolpyruvate (PEP) to Pyruvate (by Pyruvate kinase) yields 1 ATP per G3P. * Total yield: 2 ATP × 2 moles of G3P = **4 mol of ATP**. **Why incorrect options are wrong:** * **A & B:** These suggest the formation of NAD+. In glycolysis, NAD+ is *consumed* (reduced), not formed. NAD+ is only regenerated during anaerobic glycolysis (lactate formation). * **C:** This option accounts for only 2 ATP. While the *net* gain of glycolysis starting from Glucose is 2 ATP, the question starts from **Fructose 1,6-bisphosphate**, bypassing the preparatory phase where 2 ATP are normally consumed. **Clinical Pearls for NEET-PG:** * **Net ATP Yield:** From Glucose to Pyruvate is 2 ATP; from F1,6BP to Pyruvate is 4 ATP. * **Arsenate Poisoning:** Arsenate competes with inorganic phosphate in the G3P Dehydrogenase reaction, resulting in 0 net ATP production despite glycolysis continuing. * **Rapoport-Luebering Cycle:** In RBCs, 1,3-BPG can be converted to 2,3-BPG, bypassing the first ATP-generating step, which shifts the O2 dissociation curve to the right.

Question 47: The uptake of glucose by the liver increases following a carbohydrate meal because:

A. There is an increase in the phosphorylation of glucose by glucokinase. (Correct Answer)
B. GLUT-2 is stimulated by insulin.
C. Glucokinase has a low Km for glucose.
D. Hexokinase in the liver has a high affinity for glucose.

Explanation: ### Explanation **1. Why Option A is Correct:** Following a carbohydrate meal, blood glucose levels rise. The liver expresses **Glucokinase (Hexokinase IV)**, which plays a pivotal role in glucose sensing. Unlike hexokinase in other tissues, glucokinase has a **high $K_m$** (low affinity) and a **high $V_{max}$**. This means it is not easily saturated and its activity increases proportionally as the blood glucose concentration rises. By rapidly phosphorylating glucose into glucose-6-phosphate, glucokinase maintains a steep concentration gradient, allowing more glucose to enter the hepatocytes for glycogen synthesis and glycolysis. **2. Why the Other Options are Incorrect:** * **Option B:** While GLUT-2 is the primary transporter in the liver, it is **insulin-independent**. It has a high capacity and high $K_m$, allowing glucose to enter the liver freely according to the concentration gradient. Insulin primarily stimulates **GLUT-4** (found in muscle and adipose tissue), not GLUT-2. * **Option C:** Glucokinase has a **high $K_m$** (approx. 10 mmol/L), not a low one. This high $K_m$ ensures that the liver only clears significant amounts of glucose when blood levels are high (postprandial), sparing glucose for the brain during fasting. * **Option D:** Hexokinase (Types I-III) has a **low $K_m$** (high affinity), meaning it works at maximum velocity even at very low glucose levels. Hexokinase is inhibited by its product (G6P), whereas glucokinase is not, allowing the liver to continue "trapping" glucose even when energy stores are full. **3. NEET-PG High-Yield Pearls:** * **Glucokinase Location:** Liver and Beta-cells of the pancreas. * **Molecular Switch:** Glucokinase acts as a "glucose sensor" for insulin release. * **Clinical Correlation:** Mutations in the glucokinase gene lead to **MODY type 2** (Maturity-Onset Diabetes of the Young). * **Regulation:** Glucokinase is regulated by the **Glucokinase Regulatory Protein (GKRP)**, which sequesters it in the nucleus during fasting.

Question 48: Polysaccharides are classified as which of the following types of organic molecules?

A. Polymers (Correct Answer)
B. Acids
C. Proteins
D. Oils

Explanation: **Explanation:** **Why Polymers is the Correct Answer:** Polysaccharides are complex carbohydrates composed of long chains of monosaccharide units (monomers) linked together by **glycosidic bonds**. In biochemistry, a polymer is defined as a large molecule made of repeating structural subunits. Polysaccharides like **glycogen** (the storage form of glucose in humans), **starch**, and **cellulose** fit this definition perfectly. They can be linear or branched and may consist of hundreds to thousands of sugar units. **Why the Other Options are Incorrect:** * **Acids:** While some polysaccharides can be acidic (e.g., Glycosaminoglycans like Hyaluronic acid due to glucuronic acid residues), the category "Polysaccharide" itself defines a structural carbohydrate, not a functional acid. * **Proteins:** These are polymers of **amino acids** linked by peptide bonds. While proteins can link with carbohydrates to form glycoproteins, they are chemically distinct. * **Oils:** These belong to the **Lipid** family, specifically triacylglycerols that are liquid at room temperature. They are composed of glycerol and fatty acids, not sugar units. **NEET-PG High-Yield Clinical Pearls:** * **Glycogen:** A highly branched homopolymer of glucose. It contains $\alpha(1\to4)$ linkages in the linear chain and $\alpha(1\to6)$ linkages at branch points. * **Inulin:** A polymer of fructose (fructosan) used to determine **Glomerular Filtration Rate (GFR)** because it is freely filtered but neither reabsorbed nor secreted by renal tubules. * **Glycosaminoglycans (GAGs):** These are heteropolysaccharides (e.g., Heparin, Chondroitin sulfate) that form the ground substance of the extracellular matrix. * **Cellulose:** A glucose polymer with **$\beta(1\to4)$ linkages**. Humans cannot digest it because we lack the enzyme cellulase, making it a key component of dietary fiber.

Question 49: What is glyconeogenesis?

A. Synthesis of glucose from non-carbohydrate sources
B. Synthesis of glycogen from glucose
C. Synthesis of glucose from glycerol
D. Synthesis of glycogen from non-carbohydrate sources (Correct Answer)

Explanation: **Explanation:** The term **Glyconeogenesis** is often confused with *Gluconeogenesis*, but they represent distinct metabolic pathways. 1. **Why Option D is Correct:** **Glyconeogenesis** refers specifically to the synthesis of **glycogen** from **non-carbohydrate precursors** (such as lactate, amino acids, or glycerol). This process occurs when these precursors are first converted into glucose-6-phosphate (via the gluconeogenesis pathway) and then immediately channeled into glycogen synthesis (glycogenesis) without the glucose ever entering the systemic circulation. This is a vital process in the liver during recovery from exercise or during fasting. 2. **Analysis of Incorrect Options:** * **Option A (Gluconeogenesis):** This is the synthesis of *glucose* from non-carbohydrate sources. While related, the end product is free glucose, not glycogen. * **Option B (Glycogenesis):** This is the synthesis of glycogen specifically from *glucose* molecules. * **Option C:** This is a specific subset of gluconeogenesis, not the definition of glyconeogenesis. **High-Yield Clinical Pearls for NEET-PG:** * **Site:** Primarily occurs in the **liver**. * **Key Difference:** * *Gluconeogenesis:* Non-carbohydrate $\rightarrow$ Glucose. * *Glycogenesis:* Glucose $\rightarrow$ Glycogen. * *Glyconeogenesis:* Non-carbohydrate $\rightarrow$ Glycogen. * **Cori Cycle Connection:** A classic example of glyconeogenesis is the conversion of muscle-derived **lactate** back into liver glycogen during the recovery phase after strenuous muscular activity. * **Enzymes:** It requires the coordinated action of both gluconeogenic enzymes (like PEPCK and Fructose-1,6-bisphosphatase) and glycogenic enzymes (like Glycogen Synthase).

Question 50: What is the source of maltose?

A. Beet sugar
B. Milk
C. Germinating cereals (Correct Answer)
D. Yeast

Explanation: **Explanation:** **Maltose**, also known as "malt sugar," is a disaccharide composed of two glucose units linked by an **α(1→4) glycosidic bond**. **Why Germinating Cereals is Correct:** The primary source of maltose is the enzymatic breakdown of starch. During the germination of cereals (like barley), the enzyme **α-amylase** is activated. This enzyme hydrolyzes the starch stored in the grain into maltose. This process is fundamental in the brewing industry (malting), where germinated grains provide the fermentable sugars necessary for alcohol production. In the human body, maltose is similarly produced as an intermediate during the digestion of dietary starch by salivary and pancreatic amylase. **Analysis of Incorrect Options:** * **A. Beet sugar:** This is a primary source of **Sucrose** (cane sugar), which is a disaccharide of glucose and fructose. * **B. Milk:** This is the source of **Lactose** (milk sugar), a disaccharide composed of glucose and galactose. * **D. Yeast:** While yeast ferments maltose into ethanol and CO₂, it is not a "source" of the sugar itself. Yeast contains the enzyme maltase to break down maltose for energy. **NEET-PG High-Yield Pearls:** * **Reducing Sugar:** Maltose is a reducing sugar because it retains a free anomeric carbon (unlike sucrose). * **Isomaltose:** A structural isomer of maltose where glucose units are linked by an **α(1→6)** bond; it is a limit dextrin produced during starch digestion. * **Maltotriose:** A trisaccharide consisting of three glucose units, also produced during starch hydrolysis. * **Enzymatic Defect:** Deficiency of the **sucrase-isomaltase complex** leads to osmotic diarrhea and abdominal distention after ingesting starch or sucrose.

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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