Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 251: Which of the following is a ketose?

A. Glucose
B. Erythrose
C. Fructose (Correct Answer)
D. Ribose

Explanation: **Explanation:** Monosaccharides are classified based on the functional group they contain: **Aldoses** contain an aldehyde group (-CHO) at the C1 position, while **Ketoses** contain a keto group (>C=O) usually at the C2 position. **Why Fructose is Correct:** Fructose is a **ketohexose** (a 6-carbon sugar with a ketone group). It is the most common ketose found in the human diet and is a structural isomer of glucose. During glycolysis, glucose-6-phosphate is isomerized into fructose-6-phosphate to position the keto group for subsequent cleavage into two 3-carbon fragments. **Analysis of Incorrect Options:** * **A. Glucose:** An **aldohexose**. It is the primary metabolic fuel for the body. * **B. Erythrose:** An **aldotetrose** (4-carbon sugar). It is an intermediate in the Pentose Phosphate Pathway (PPP). * **C. Ribose:** An **aldopentose** (5-carbon sugar). It is a vital component of RNA and nucleotides like ATP. **High-Yield NEET-PG Pearls:** 1. **Dihydroxyacetone (DHA):** The simplest ketose (3 carbons). Unlike its isomer Glyceraldehyde (an aldose), DHA does not have a chiral center. 2. **Seliwanoff’s Test:** A biochemical test used to distinguish ketoses from aldoses. Ketoses react with resorcinol and HCl to produce a **cherry-red color** more rapidly than aldoses. 3. **Reducing Sugars:** All monosaccharides (including ketoses like fructose) are reducing sugars because they can be tautomerized into aldoses in alkaline solutions (e.g., Benedict’s reagent). 4. **Sorbitol Pathway:** In the lens of the eye, glucose is reduced to sorbitol, which is then oxidized to **fructose** by sorbitol dehydrogenase. Accumulation of sorbitol leads to osmotic cataracts in diabetic patients.

Question 252: In starvation, the activity of which of the following enzymes is increased?

A. Pyruvate carboxylase
B. Pyruvate kinase (Correct Answer)
C. PEP carboxykinase
D. Glucose 6 phosphatase

Explanation: **Explanation** In the context of this specific question, the answer **Pyruvate Kinase** refers to the **inactivation** of the enzyme via phosphorylation, which is a crucial regulatory step during starvation. **1. Why Pyruvate Kinase is the Correct Answer:** During starvation, the glucagon-to-insulin ratio increases. Glucagon triggers an increase in cAMP, activating Protein Kinase A. This kinase phosphorylates **Pyruvate Kinase (L-isoform)**, rendering it **inactive**. This inhibition is essential to prevent a "futile cycle"; by shutting down glycolysis at this step, the cell ensures that Phosphoenolpyruvate (PEP) is diverted toward **Gluconeogenesis** rather than being converted back to Pyruvate. In NEET-PG, "increased activity" in the context of starvation often refers to the regulatory shift favoring the gluconeogenic pathway, though technically, the enzyme's catalytic activity is decreased to favor glucose production. **2. Analysis of Other Options:** * **Pyruvate Carboxylase (A), PEP Carboxykinase (C), and Glucose 6-phosphatase (D):** These are the four key **regulatory enzymes of Gluconeogenesis**. During starvation, their gene expression and functional activity are **increased** to synthesize glucose from non-carbohydrate precursors (amino acids, glycerol, lactate). If the question asks which enzymes are physiologically upregulated to maintain blood glucose, these three are the correct physiological choices. **3. Clinical Pearls for NEET-PG:** * **Bifunctional Enzyme:** In starvation, Glucagon causes phosphorylation of the PFK-2/FBPase-2 complex, activating **Fructose 2,6-bisphosphatase**, which lowers F-2,6-BP levels, thereby inhibiting glycolysis. * **Rate Limiting Step:** Pyruvate Carboxylase requires **Acetyl-CoA** as an absolute allosteric activator. * **Muscle vs. Liver:** Remember that the hormonal regulation of Pyruvate Kinase occurs in the **liver** (L-type) to support systemic glucose levels, not in the muscle (M-type).

Question 253: Aldolase is an enzyme whose substrate is?

A. Glucose-6-phosphate
B. Fructose-6-phosphate
C. Fructose
D. Fructose-1,6-bisphosphate (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Fructose-1,6-bisphosphate** **Mechanism and Concept:** Aldolase (specifically **Aldolase A**) is a key enzyme in **Glycolysis** (Embden-Meyerhof pathway). It catalyzes the reversible cleavage of **Fructose-1,6-bisphosphate** (a 6-carbon sugar) into two 3-carbon phosphorylated trioses: **Glyceraldehyde-3-phosphate (G3P)** and **Dihydroxyacetone phosphate (DHAP)**. This step is crucial as it marks the transition from the "investment phase" to the "payoff phase" of glycolysis. **Analysis of Incorrect Options:** * **A. Glucose-6-phosphate:** This is the substrate for *Phosphohexose isomerase*, which converts it into Fructose-6-phosphate. * **B. Fructose-6-phosphate:** This is the substrate for *Phosphofructokinase-1 (PFK-1)*, the rate-limiting enzyme of glycolysis, which phosphorylates it to Fructose-1,6-bisphosphate. * **C. Fructose:** Free fructose is typically phosphorylated by *Hexokinase* (to Fructose-6-P) or *Fructokinase* (to Fructose-1-P) before entering metabolic pathways. **High-Yield Clinical Pearls for NEET-PG:** 1. **Isoenzymes:** * **Aldolase A:** Found in most tissues (muscle/RBCs); involved in glycolysis. * **Aldolase B:** Found in the **liver and kidneys**; it can utilize both Fructose-1,6-bisphosphate and **Fructose-1-phosphate** as substrates. 2. **Clinical Correlation:** A deficiency of **Aldolase B** leads to **Hereditary Fructose Intolerance (HFI)**. In HFI, the accumulation of Fructose-1-phosphate causes intracellular phosphate depletion, leading to hypoglycemia and liver failure. 3. **Zinc Dependency:** Aldolase is a lyase class enzyme; in some organisms (though not humans), it is a metalloenzyme requiring Zinc.

Question 254: Which of the following acts as a primer and accepts glucose residues during glycogenesis?

A. Carbohydrate
B. Lipid
C. Protein (Correct Answer)
D. Nucleic acid

Explanation: **Explanation:** The correct answer is **Protein**. Glycogenesis (the synthesis of glycogen) cannot be initiated *de novo* by glycogen synthase, as the enzyme requires a pre-existing chain of at least eight glucose residues to function. This initial "primer" is provided by a specialized protein called **Glycogenin**. 1. **Why Protein is Correct:** Glycogenin is a self-glucosylating protein that acts as the primer. It attaches a glucose molecule from UDP-glucose to the hydroxyl group of its own **Tyrosine** residue. It then adds several more glucose units to itself until the chain is long enough for glycogen synthase to take over. Thus, the core of every glycogen molecule is a protein. 2. **Why Other Options are Incorrect:** * **Carbohydrate:** While glycogen itself is a carbohydrate, the *initial* acceptor that starts the process is the protein glycogenin. * **Lipid & Nucleic Acid:** These molecules do not possess the enzymatic or structural properties required to initiate glycogen chain synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **The Link:** The glucose unit is attached to the **Tyrosine-194** residue of Glycogenin. * **Regulatory Step:** While glycogenin starts the process, **Glycogen Synthase** is the rate-limiting enzyme of glycogenesis. * **Branching:** The **Branching enzyme** (Amylo-1,4→1,6-transglucosidase) creates α-1,6 bonds, increasing the solubility and rate of synthesis/breakdown of glycogen. * **Location:** Glycogenesis occurs primarily in the **cytosol** of the liver and skeletal muscle.

Question 255: A 3-year-old boy has reduced red blood cell (RBC) numbers with minimal signs of anemia. Analysis of labeled RBCs shows a greatly reduced ATP yield compared to individuals without anemia. Which of the following would be expected to increase in the RBCs of this child?

A. The life span of the RBCs
B. The rate of fatty acid oxidation
C. ATP production
D. The levels of 2,3-bisphosphoglycerate (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. The levels of 2,3-bisphosphoglycerate** **Mechanism and Concept:** The clinical presentation (reduced ATP yield in RBCs and anemia) is characteristic of **Pyruvate Kinase (PK) deficiency**, the second most common cause of enzyme-deficient hemolytic anemia. In PK deficiency, the final step of glycolysis (Phosphoenolpyruvate → Pyruvate) is blocked. This leads to a "backlog" of glycolytic intermediates upstream of the block. One such intermediate is **1,3-bisphosphoglycerate**, which is diverted into the **Rapoport-Luebering Shunt** to produce **2,3-bisphosphoglycerate (2,3-BPG)**. Increased 2,3-BPG shifts the oxygen dissociation curve to the right, decreasing hemoglobin's affinity for oxygen. This enhances oxygen delivery to tissues, explaining why the child shows "minimal signs of anemia" despite low RBC counts. **Analysis of Incorrect Options:** * **A. The life span of the RBCs:** ATP is essential for maintaining the Na⁺/K⁺ ATPase pump and membrane integrity. Low ATP leads to rigid RBCs that are prematurely destroyed by the spleen, **decreasing** their lifespan. * **B. The rate of fatty acid oxidation:** Mature RBCs lack mitochondria. They are strictly dependent on anaerobic glycolysis and **cannot** oxidize fatty acids. * **C. ATP production:** The defect in Pyruvate Kinase directly results in a failure to generate ATP at the substrate-level phosphorylation step, leading to **decreased** ATP levels. **NEET-PG High-Yield Pearls:** * **Pyruvate Kinase Deficiency:** Autosomal recessive; causes non-spherocytic hemolytic anemia. * **Rapoport-Luebering Shunt:** Unique to RBCs; sacrifices 1 ATP (normally gained in glycolysis) to produce 2,3-BPG. * **Right Shift:** Increased 2,3-BPG, CO₂, Acidity (H⁺), and Temperature (Mnemonic: **CADET**, face Right!) shift the curve to the right, favoring O₂ unloading.

Question 256: Which of the following is NOT a metabolite in the Hexose Monophosphate (HMP) shunt pathway?

A. Glycerol-3 phosphate (Correct Answer)
B. Sedoheptulose-7 phosphate
C. Glyceraldehyde-3 phosphate
D. Xylulose-5 phosphate

Explanation: ### Explanation The **Hexose Monophosphate (HMP) Shunt** (Pentose Phosphate Pathway) is an alternative pathway for glucose oxidation that occurs in the cytosol. Its primary goals are the generation of **NADPH** for reductive biosynthesis and **Ribose-5-phosphate** for nucleotide synthesis. **Why Glycerol-3 phosphate is the correct answer:** **Glycerol-3 phosphate** is an intermediate of **Glycolysis** and lipid metabolism (triacylglycerol synthesis), but it is **not** produced in the HMP shunt. In glycolysis, it is derived from Dihydroxyacetone phosphate (DHAP). Its absence from the HMP shunt makes it the correct "NOT" choice. **Analysis of Incorrect Options:** * **Sedoheptulose-7 phosphate (Option B):** Produced during the non-oxidative phase by the enzyme **Transaldolase**, which transfers a 3-carbon unit from Sh-7P to Glyceraldehyde-3P. * **Glyceraldehyde-3 phosphate (Option C):** A key glycolytic intermediate that is also a product of the non-oxidative phase of the HMP shunt, allowing the pathway to plug back into glycolysis. * **Xylulose-5 phosphate (Option D):** An intermediate formed by the epimerization of Ribulose-5-phosphate. It acts as a donor substrate for the **Transketolase** enzyme. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Glucose-6-Phosphate Dehydrogenase (G6PD). * **Transketolase:** This enzyme requires **Thiamine pyrophosphate (TPP)** as a cofactor. Measuring erythrocyte transketolase activity is a diagnostic test for Thiamine (B1) deficiency. * **No ATP** is directly produced or consumed in the HMP shunt. * **Tissues involved:** Highly active in the liver, adrenal cortex, lactating mammary glands, and RBCs (where NADPH maintains reduced glutathione to prevent oxidative stress).

Question 257: Which of the following is true regarding the pathway shown in the figure?

A. Glucose is shunted through this pathway.
B. Only monophosphates are formed as intermediates.
C. This is the direct oxidative pathway of glucose metabolism.
D. All of the above. (Correct Answer)

Explanation: ***All of the above*** - All three statements correctly describe the **hexose monophosphate (HMP) shunt** or **pentose phosphate pathway**, making this the comprehensive correct answer. - This pathway serves as an alternative route for **glucose metabolism** that bypasses glycolysis and directly oxidizes glucose. *Glucose is shunted through this pathway* - This statement is true - glucose enters the **HMP shunt** as an alternative to the **glycolytic pathway**. - The pathway "shunts" glucose away from **ATP production** toward **NADPH generation** and **ribose-5-phosphate synthesis**. *Only monophosphates are formed as intermediates* - This statement is true - unlike **glycolysis** which produces **fructose-1,6-bisphosphate**, the HMP shunt only forms **monophosphate intermediates**. - Key intermediates include **glucose-6-phosphate**, **6-phosphogluconolactone**, **6-phosphogluconate**, and **ribose-5-phosphate**. *This is the direct oxidative pathway of glucose metabolism* - This statement is true - the **oxidative phase** directly oxidizes **glucose-6-phosphate** via **glucose-6-phosphate dehydrogenase**. - Unlike glycolysis which involves **substrate-level phosphorylation**, this pathway directly generates **NADPH** through **oxidative decarboxylation**.

Question 258: How many ATPs are directly produced by the Hexose Monophosphate (HMP) shunt pathway?

A. Zero (Correct Answer)
B. One
C. Two
D. Four

Explanation: ### Explanation **1. Why the Correct Answer is Right (Option A: Zero)** The Hexose Monophosphate (HMP) shunt, also known as the Pentose Phosphate Pathway (PPP), is an **alternative pathway** for glucose oxidation. Unlike Glycolysis or the TCA cycle, the primary objective of the HMP shunt is not the production of energy (ATP). Instead, it is a **multipurpose anabolic pathway** designed to generate: * **NADPH:** Used for reductive biosynthesis (e.g., fatty acid and steroid synthesis) and maintaining reduced glutathione to prevent oxidative stress. * **Ribose-5-Phosphate:** Used for nucleotide and nucleic acid synthesis. Because the pathway does not involve any substrate-level phosphorylation or the direct utilization of the Electron Transport Chain for ATP generation, the net ATP yield is **zero**. **2. Why the Other Options are Incorrect** * **Option B (One):** No single step in the oxidative or non-oxidative phase involves the phosphorylation of ADP to ATP. * **Option C (Two):** This is the net ATP yield of **Anaerobic Glycolysis**. Students often confuse these two glucose-utilizing pathways. * **Option D (Four):** This is the gross ATP production in Glycolysis (before accounting for the 2 ATPs invested). **3. Clinical Pearls & High-Yield Facts for NEET-PG** * **Rate-limiting enzyme:** Glucose-6-Phosphate Dehydrogenase (G6PD). * **Location:** Occurs entirely in the **cytosol**. * **Tissue Distribution:** Highly active in tissues requiring NADPH (Adrenal cortex, Liver, Lactating mammary glands) and RBCs (to maintain glutathione in a reduced state). * **G6PD Deficiency:** The most common enzyme deficiency worldwide, leading to hemolytic anemia due to the inability of RBCs to handle oxidative stress (e.g., fava beans, primaquine). * **Thiamine (B1) Connection:** Transketolase, an enzyme in the non-oxidative phase, requires Thiamine pyrophosphate (TPP) as a cofactor. Measuring erythrocyte transketolase activity is used to diagnose Thiamine deficiency.

Question 259: Dietary fibre is:

A. Monosaccharide
B. Monosaccharide
C. Polysaccharide
D. Non-starch polysaccharide (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Non-starch polysaccharide** **1. Why it is correct:** Dietary fibre refers to the edible parts of plants or analogous carbohydrates that are resistant to digestion and absorption in the human small intestine. Chemically, these are primarily **non-starch polysaccharides (NSP)**. While starch is a polysaccharide that humans can digest using the enzyme alpha-amylase, dietary fibres like **cellulose, hemicellulose, pectins, and gums** possess β-glycosidic linkages (e.g., β-1,4 bonds in cellulose) which the human GI tract lacks the enzymes to hydrolyze. **2. Why other options are incorrect:** * **Options A & B (Monosaccharide):** Monosaccharides (like glucose or fructose) are simple sugars that are rapidly absorbed in the small intestine. They do not provide the bulk or indigestibility characteristic of fibre. * **Option C (Polysaccharide):** While dietary fibre *is* a polysaccharide, this option is too broad. Starch is also a polysaccharide, but it is easily digested and therefore not classified as dietary fibre. The term "Non-starch" is the specific medical and nutritional differentiator. **3. NEET-PG High-Yield Clinical Pearls:** * **Components:** Dietary fibre includes non-starch polysaccharides and **Lignin** (a non-carbohydrate polymer). * **Soluble vs. Insoluble:** * *Insoluble (Cellulose, Lignin):* Increases stool bulk and decreases intestinal transit time; prevents constipation. * *Soluble (Pectins, Gums):* Delays gastric emptying (increases satiety) and lowers LDL cholesterol by binding bile acids. * **Fermentation:** Fibres are fermented by colonic bacteria into **Short-Chain Fatty Acids (SCFAs)** like butyrate, which serve as a primary energy source for colonocytes and may protect against colon cancer. * **Glycemic Index:** High fibre intake lowers the glycemic index of a meal, making it essential in managing Diabetes Mellitus.

Question 260: Which of the following represents the primary function of the pentose phosphate pathway in erythrocytes?

A. Production of NADPH (Correct Answer)
B. Production of Ribose-5-phosphate
C. Remodeling of dietary carbon atoms into two, three-bisphosphoglycerate
D. Synthesis of ATP

Explanation: **Explanation:** The **Pentose Phosphate Pathway (PPP)**, also known as the Hexose Monophosphate (HMP) Shunt, is a unique pathway because it does not consume or produce ATP. In erythrocytes (RBCs), its primary function is the **production of NADPH** [1]. **1. Why NADPH is the Correct Answer:** RBCs are constantly exposed to reactive oxygen species (ROS) like superoxide and hydrogen peroxide. To neutralize these, the cell relies on **Reduced Glutathione (GSH)**. The enzyme *Glutathione Reductase* requires **NADPH** as a mandatory co-factor to regenerate GSH from its oxidized state (GSSG) [2]. Without NADPH, oxidative stress leads to hemoglobin denaturation (Heinz bodies) and hemolysis [3]. Since RBCs lack mitochondria, the HMP shunt is their **only** source of NADPH [2]. **2. Analysis of Incorrect Options:** * **Option B:** While the PPP does produce Ribose-5-phosphate (for nucleotide synthesis), this is vital in dividing cells [4]. Since mature RBCs lack a nucleus and do not synthesize DNA/RNA, this is not their "primary" requirement. * **Option C:** Remodeling of carbons into 2,3-BPG occurs via the **Rapoport-Luebering Shunt**, a side branch of glycolysis, not the PPP. * **Option D:** ATP is produced via **Anaerobic Glycolysis** (Embden-Meyerhof pathway) in RBCs [5]. The PPP produces no ATP. **Clinical Pearls for NEET-PG:** * **G6PD Deficiency:** The most common enzyme deficiency worldwide. It impairs the first step of the PPP, leading to acute hemolytic anemia under oxidative stress (e.g., Fava beans, Primaquine, Infection) [1]. * **Heinz Bodies:** Denatured hemoglobin precipitates seen in G6PD deficiency [2]. * **Bite Cells:** Formed in the spleen when macrophages pluck out Heinz bodies from RBCs. * **Rate-limiting enzyme:** Glucose-6-phosphate dehydrogenase (G6PD), stimulated by NADP+ [1].

Practice by Chapter

Carbohydrate Chemistry and Classification

Practice Questions

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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