Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 291: The conversion of glucose 6-phosphate to fructose 6-phosphate is an example of which type of reaction?

A. Phosphate transfer
B. Isomerization (Correct Answer)
C. Dehydration
D. Aldol cleavage

Explanation: **Explanation:** The conversion of Glucose 6-phosphate (G6P) to Fructose 6-phosphate (F6P) is the second step of **Glycolysis**, catalyzed by the enzyme **Phosphohexose Isomerase** (also known as Phosphoglucose Isomerase). **1. Why Isomerization is Correct:** Isomerization is a chemical process where a molecule is transformed into another molecule which has exactly the same atoms, but a different arrangement. In this reaction, an **aldose** sugar (Glucose 6-P) is converted into its **ketose** isomer (Fructose 6-P). This step is crucial because it shifts the carbonyl group from C1 to C2, preparing the molecule for subsequent phosphorylation and symmetrical cleavage into two 3-carbon units. **2. Why other options are incorrect:** * **Phosphate transfer:** This involves moving a phosphate group from one molecule to another (e.g., ATP to Glucose, catalyzed by Hexokinase). In this step, the phosphate remains at the C6 position. * **Dehydration:** This involves the removal of a water molecule (e.g., 2-phosphoglycerate to Phosphoenolpyruvate by Enolase). No water is lost here. * **Aldol cleavage:** This refers to the splitting of a 6-carbon sugar into two 3-carbon fragments (e.g., Fructose 1,6-bisphosphate into DHAP and Glyceraldehyde 3-P by Aldolase). **High-Yield NEET-PG Pearls:** * **Enzyme Requirement:** Phosphohexose isomerase requires **$Mg^{2+}$** as a cofactor. * **Reversibility:** Unlike the first step (Hexokinase), this reaction is **freely reversible** under physiological conditions. * **Clinical Link:** Inherited deficiency of Phosphoglucose Isomerase is the second most common cause of **enzymopathic hemolytic anemia** (after G6PD deficiency), as RBCs depend entirely on glycolysis for energy.

Question 292: Which metabolic pathway can produce energy even in the absence of oxygen?

A. TCA cycle
B. Glycolysis (Correct Answer)
C. Fatty acid oxidation
D. Respiratory chain

Explanation: **Explanation:** **Glycolysis** is the correct answer because it is the only metabolic pathway among the options that can function under both aerobic and anaerobic conditions. In the absence of oxygen (anaerobic glycolysis), pyruvate is converted into **lactate** by the enzyme *Lactate Dehydrogenase*. This process allows for the regeneration of **NAD+**, which is essential to keep the pathway running, yielding a net of **2 ATP** per molecule of glucose. This is vital for tissues with few or no mitochondria, such as mature erythrocytes. **Why the other options are incorrect:** * **TCA Cycle (Krebs Cycle):** While the cycle itself doesn't use $O_2$ directly, it requires the regeneration of $NAD^+$ and $FAD$ from the electron transport chain (ETC). Since the ETC requires oxygen as the final electron acceptor, the TCA cycle ceases in anaerobic conditions. * **Fatty Acid Oxidation ($\beta$-oxidation):** This process occurs exclusively in the mitochondria and is strictly aerobic. It generates $NADH$ and $FADH_2$, which must be oxidized via the respiratory chain to produce ATP. * **Respiratory Chain (Oxidative Phosphorylation):** This is the final stage of cellular respiration where oxygen acts as the terminal electron acceptor to form water. Without oxygen, this chain stalls completely. **High-Yield Clinical Pearls for NEET-PG:** * **Mature RBCs** derive 100% of their energy from glycolysis because they lack mitochondria. * **Rapoport-Luebering Shunt:** A side pathway of glycolysis in RBCs that produces **2,3-BPG**, which shifts the oxygen dissociation curve to the right (facilitating $O_2$ release to tissues). * **Lactic Acidosis:** Occurs during severe hypoxia (e.g., septic shock) because the body relies solely on anaerobic glycolysis, leading to an accumulation of lactate.

Question 293: What is the first substrate of the Krebs cycle?

A. Pyruvate (Correct Answer)
B. Glycine
C. Alanine
D. Lipoprotein

Explanation: ### Explanation **Correct Option: A (Pyruvate)** The Krebs cycle (TCA cycle) occurs in the mitochondrial matrix. While **Acetyl-CoA** is the direct molecule that enters the cycle by condensing with Oxaloacetate, **Pyruvate** is considered the primary substrate that initiates the transition into the cycle. Pyruvate, derived from glycolysis in the cytosol, is transported into the mitochondria and undergoes **oxidative decarboxylation** by the **Pyruvate Dehydrogenase (PDH) complex** to form Acetyl-CoA. In the context of metabolic entry points, Pyruvate serves as the essential precursor that links glycolysis to the Krebs cycle. **Why Incorrect Options are Wrong:** * **B (Glycine):** This is the simplest non-essential amino acid. While it can be glucogenic, it primarily enters metabolism through the creation of heme, glutathione, or conversion to serine/pyruvate, but it is not the primary substrate for the TCA cycle. * **C (Alanine):** Alanine is a key glucogenic amino acid. Through the **Cahill cycle**, it is transaminated to Pyruvate in the liver. While it can *become* a substrate, it is a secondary source compared to the direct glycolytic product, Pyruvate. * **D (Lipoprotein):** These are complex particles (like LDL, HDL) that transport lipids in the blood. They are not metabolic intermediates of the Krebs cycle. **NEET-PG High-Yield Pearls:** 1. **The Bridge Reaction:** The conversion of Pyruvate to Acetyl-CoA is irreversible and is catalyzed by the PDH complex, which requires five cofactors: **T**hiamine (B1), **R**iboflavin (B2), **N**iacin (B3), **P**antothenic acid (B5), and **L**ipoic acid (Mnemonic: **T**ender **R**oving **N**ights **P**lease **L**ove). 2. **Rate-Limiting Step:** The conversion of Isocitrate to alpha-ketoglutarate by **Isocitrate Dehydrogenase** is the rate-limiting step of the Krebs cycle. 3. **ATP Yield:** One turn of the TCA cycle produces **10 ATP** equivalents (3 NADH, 1 FADH2, 1 GTP).

Question 294: All steps of N-glycosylation occur in the ER except?

A. Dolichol synthesis
B. Glycosyl transferase activity
C. Protein-oligosaccharide transferase activity
D. Final trimming of the oligosaccharide (Correct Answer)

Explanation: **Explanation:** N-glycosylation is a complex process where a pre-formed oligosaccharide is attached to the nitrogen atom of an **Asparagine (Asn)** residue. This process is spatially divided between the **Endoplasmic Reticulum (ER)** and the **Golgi apparatus**. **Why "Final trimming" is the correct answer:** The initial assembly of the core oligosaccharide and the initial trimming (removal of glucose and some mannose residues) occur in the **ER**. However, the **final trimming** and subsequent complex modifications (addition of galactose, sialic acid, or fucose) occur exclusively in the **Golgi apparatus**. Therefore, final trimming is not an ER-resident step. **Analysis of Incorrect Options:** * **A. Dolichol synthesis:** Dolichol phosphate is the essential lipid carrier located in the **ER membrane** upon which the oligosaccharide chain is built. * **B. Glycosyl transferase activity:** These enzymes are responsible for the sequential addition of sugars (N-acetylglucosamine and mannose) to the dolichol carrier within the **ER**. * **C. Protein-oligosaccharide transferase:** Also known as Oligosaccharyltransferase (OST), this enzyme complex resides in the **ER lumen** and catalyzes the transfer of the 14-sugar precursor from dolichol to the nascent protein. **High-Yield Clinical Pearls for NEET-PG:** * **Site of N-glycosylation:** Starts in ER, finishes in Golgi. (Contrast: **O-glycosylation** occurs exclusively in the Golgi). * **Sequence Motif:** N-glycosylation occurs at the **Asn-X-Ser/Thr** motif (where X is any amino acid except proline). * **Tunicamycin:** An antibiotic that inhibits the first step of N-glycosylation (blocks the formation of Dolichol-P-P-GlcNAC). * **I-Cell Disease:** Caused by a deficiency in phosphotransferase in the Golgi, leading to failure of mannose-6-phosphate tagging, causing lysosomal enzymes to be secreted extracellularly rather than reaching lysosomes.

Question 295: Caffeine, a methyl xanthine, has been added to a variety of cell types. Which one of the following would be expected in various cell types treated with caffeine and epinephrine?

A. Decreased activity of liver PKA
B. Decreased activity of muscle PKA
C. Increased activity of liver pyruvate kinase
D. Decreased activity of liver glycogen synthase (Correct Answer)

Explanation: ### Explanation The correct answer is **D. Decreased activity of liver glycogen synthase.** **Mechanism of Action:** The combination of **Epinephrine** and **Caffeine** acts synergistically to elevate intracellular **cyclic AMP (cAMP)** levels through two distinct mechanisms: 1. **Epinephrine:** Stimulates Adenylyl Cyclase via Gs-protein coupled receptors, increasing cAMP production. 2. **Caffeine (Methylxanthine):** Inhibits **Phosphodiesterase (PDE)**, the enzyme responsible for degrading cAMP into 5'-AMP. Elevated cAMP activates **Protein Kinase A (PKA)**. PKA then phosphorylates key enzymes in carbohydrate metabolism. In the liver, PKA phosphorylates **Glycogen Synthase**, converting it from its active ('a') form to its **inactive ('b') phosphorylated form**. Thus, glycogen synthesis is inhibited (decreased activity). --- ### Why other options are incorrect: * **A & B (Decreased activity of PKA):** Incorrect. Both Epinephrine and Caffeine increase cAMP, which directly **increases** the activity of PKA in both liver and muscle cells. * **C (Increased activity of liver pyruvate kinase):** Incorrect. In the liver, PKA phosphorylates **Pyruvate Kinase**, which **inactivates** it. This inhibition is a crucial step to prevent glycolysis during gluconeogenesis (sparing glucose for the brain). --- ### NEET-PG High-Yield Pearls: * **Phosphorylation Rule:** In the post-absorptive/fasting state (Glucagon/Epinephrine), most regulatory enzymes are **phosphorylated**. For most pathways (Glycogenolysis, Gluconeogenesis), phosphorylation **activates** enzymes, but for **Glycogen Synthase** and **Pyruvate Kinase**, phosphorylation **inhibits** them. * **The "C" in Caffeine:** Remember **C**affeine inhibits **C**AMP phosphodiesterase, leading to prolonged sympathetic-like effects. * **Muscle vs. Liver:** While Epinephrine inhibits Pyruvate Kinase in the **liver**, it does **not** do so in the **muscle** (muscle PK is a different isoenzyme not regulated by PKA), allowing muscles to use glucose for energy during "fight or flight."

Question 296: Which of the following reactions produces CO2?

A. Amino acid synthesis
B. HMP shunt pathway (Correct Answer)
C. Glycolysis
D. None of the above

Explanation: **Explanation:** The correct answer is **B. HMP shunt pathway**. **1. Why HMP Shunt is correct:** The Hexose Monophosphate (HMP) Shunt, also known as the Pentose Phosphate Pathway (PPP), consists of an oxidative and a non-oxidative phase. In the **oxidative phase**, the enzyme **6-phosphogluconate dehydrogenase** catalyzes the conversion of 6-phosphogluconate to Ribulose-5-phosphate. This reaction involves **oxidative decarboxylation**, where a carbon atom is released as **CO₂** while simultaneously generating NADPH. This is the primary source of CO₂ in this pathway. **2. Why other options are incorrect:** * **A. Amino acid synthesis:** While some specific amino acid degradative pathways (like the Glucose-Alanine cycle or Urea cycle) involve CO₂ fixation or release, general synthesis pathways are typically anabolic and do not characteristically produce CO₂ as a primary byproduct in the same way respiratory pathways do. * **C. Glycolysis:** This is a high-yield distinction. Glycolysis is the anaerobic breakdown of glucose to pyruvate (in the cytoplasm). It involves 10 enzymatic steps, **none of which release CO₂**. CO₂ is only released once pyruvate enters the mitochondria and undergoes the Link Reaction (Pyruvate Dehydrogenase complex) or the TCA cycle. **Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme of HMP Shunt:** Glucose-6-Phosphate Dehydrogenase (G6PD). * **Key Products:** NADPH (for fatty acid synthesis and keeping glutathione reduced) and Ribose-5-phosphate (for nucleotide synthesis). * **Site:** Occurs entirely in the **cytosol**. It is highly active in tissues requiring NADPH, such as the adrenal cortex, liver, and RBCs. * **G6PD Deficiency:** Leads to hemolytic anemia due to the inability to neutralize free radicals in RBCs (low NADPH).

Question 297: Which of the following is NOT a product of the uronic acid pathway in humans?

A. Vitamin C (Correct Answer)
B. Pentoses
C. NADH
D. Glucuronic acid

Explanation: The **Uronic Acid Pathway** is an alternative pathway for glucose oxidation that occurs primarily in the liver. It does not generate ATP but is essential for the synthesis of specialized sugars and detoxification. ### Why Vitamin C is the Correct Answer In most mammals, the uronic acid pathway leads to the synthesis of **Ascorbic acid (Vitamin C)**. However, **humans, primates, and guinea pigs cannot synthesize Vitamin C**. This is due to the evolutionary absence of the enzyme **L-gulonolactone oxidase**. Therefore, Vitamin C is an essential dietary requirement for humans and is not a product of this pathway. ### Explanation of Incorrect Options * **Glucuronic Acid:** This is the primary product of the pathway. It is crucial for the **conjugation** of bilirubin, steroid hormones, and drugs (making them water-soluble for excretion). It is also a precursor for Glycosaminoglycans (GAGs). * **Pentoses:** The pathway produces **L-xylulose**, which is subsequently converted to D-xylulose 5-phosphate. This allows the uronic acid pathway to interface with the Pentose Phosphate Pathway (HMP Shunt). * **NADH:** While the pathway primarily involves **NADPH** and **NAD+**, the oxidation of L-gulonate to xylulose involves the reduction of NAD+ to **NADH**. (Note: Some texts focus on NADPH, but NADH is indeed a byproduct of the L-gulonate dehydrogenase step). ### High-Yield Clinical Pearls for NEET-PG * **Essential Pentosuria:** A rare autosomal recessive condition caused by a deficiency of **L-xylulose reductase**. Patients excrete large amounts of L-xylulose in the urine. It is a benign condition but can give a false-positive result for reducing sugars (Benedict’s test). * **Drug Interaction:** Drugs like **Phenobarbital** and **Aminopyrine** can induce the enzymes of the uronic acid pathway, increasing the rate of glucuronate formation. * **Key Enzyme:** Remember **L-gulonolactone oxidase** as the "missing enzyme" in humans.

Question 298: Which of the following is NOT an enzyme that catalyzes substrate-level phosphorylation?

A. Phosphoglycerate kinase
B. Pyruvate kinase
C. Succinate thiokinase
D. Phosphofructokinase (Correct Answer)

Explanation: **Explanation:** **Substrate-level phosphorylation (SLP)** is a metabolic process where a phosphate group is directly transferred from a high-energy intermediate to ADP (or GDP) to form ATP (or GTP), independent of the electron transport chain and oxygen. **Why Phosphofructokinase (PFK) is the correct answer:** PFK is the rate-limiting enzyme of glycolysis. It catalyzes the conversion of Fructose-6-phosphate to Fructose-1,6-bisphosphate. Crucially, this reaction **consumes** one molecule of ATP rather than generating it. Therefore, it is an ATP-utilizing step, not an SLP step. **Analysis of Incorrect Options (SLP Enzymes):** * **Phosphoglycerate Kinase (Glycolysis):** Converts 1,3-bisphosphoglycerate to 3-phosphoglycerate, generating the first ATP of glycolysis via SLP. * **Pyruvate Kinase (Glycolysis):** Converts Phosphoenolpyruvate (PEP) to Pyruvate, generating the second ATP of glycolysis via SLP. * **Succinate Thiokinase (TCA Cycle):** Also known as Succinyl-CoA synthetase, it converts Succinyl-CoA to Succinate. This is the **only** step in the TCA cycle that generates a high-energy phosphate (GTP in liver/ATP in muscle) via SLP. **High-Yield NEET-PG Pearls:** 1. **Total SLP in Glycolysis:** 4 ATP are produced per glucose molecule (2 per triose), but the **net** gain is 2 ATP (due to 2 consumed in the preparatory phase). 2. **Arsenate Poisoning:** Arsenate competes with inorganic phosphate in the GAPDH reaction, bypassing the SLP step of Phosphoglycerate kinase, resulting in **zero net ATP** production in glycolysis. 3. **Mature RBCs:** Since RBCs lack mitochondria, they depend entirely on SLP for their energy requirements.

Question 299: What is an example of an oligosaccharide?

A. Glucose
B. Fructose
C. Maltose (Correct Answer)
D. Dextrin

Explanation: ### Explanation **Correct Answer: C. Maltose** **Understanding the Concept:** Carbohydrates are classified based on the number of sugar units they contain. **Oligosaccharides** typically consist of 2 to 10 monosaccharide units linked by glycosidic bonds. **Maltose** (malt sugar) is a disaccharide—the simplest form of an oligosaccharide—composed of two glucose units joined by an **α(1→4) glycosidic bond**. It is a major product of the enzymatic hydrolysis of starch by amylase. **Analysis of Options:** * **A & B (Glucose & Fructose):** These are **monosaccharides** (hexoses). They are the simplest forms of carbohydrates and cannot be hydrolyzed further into smaller sugar units. * **D (Dextrin):** Dextrins are **polysaccharides**. They are intermediate-length polymers of glucose produced during the partial hydrolysis of starch. They contain many more than 10 sugar units, placing them outside the oligosaccharide category. **NEET-PG High-Yield Clinical Pearls:** 1. **Reducing Sugars:** All monosaccharides and most disaccharides (Maltose, Lactose) are reducing sugars because they have a free anomeric carbon. **Sucrose** is a notable non-reducing disaccharide. 2. **Maltose Digestion:** In the intestinal brush border, the enzyme **maltase** cleaves maltose into two glucose molecules. 3. **Isomaltose:** This is an isomer of maltose where glucose units are linked by an **α(1→6) bond**, representing the branch points in glycogen and amylopectin. 4. **Blood Group Antigens:** Many cell membrane oligosaccharides serve as biological markers, including the ABO blood group determinants.

Question 300: Which of the following pathways does not directly generate ATP?

A. Citric Acid cycle
B. Kreb's cycle
C. Glycolysis
D. HMP shunt (Correct Answer)

Explanation: ### Explanation The **Hexose Monophosphate (HMP) Shunt**, also known as the Pentose Phosphate Pathway (PPP), is a unique alternative pathway for glucose oxidation. Unlike glycolysis or the TCA cycle, its primary purpose is **not the production of energy (ATP)**. Instead, it serves two major biosynthetic functions: 1. **Generation of NADPH:** Used for reductive biosynthesis (fatty acids, steroids) and maintaining reduced glutathione to prevent oxidative stress. 2. **Production of Ribose-5-phosphate:** A precursor for nucleotide and nucleic acid synthesis. Because the HMP shunt does not involve any substrate-level phosphorylation or the production of NADH/FADH₂ for the electron transport chain, it generates **zero ATP**. #### Analysis of Incorrect Options: * **Glycolysis:** Produces a net of **2 ATP** per glucose molecule via substrate-level phosphorylation (at the Phosphoglycerate kinase and Pyruvate kinase steps). * **Citric Acid Cycle / Kreb's Cycle (Options A & B):** These are synonymous. The cycle generates **1 GTP (equivalent to 1 ATP)** per turn via substrate-level phosphorylation (Succinate thiokinase step). It also produces NADH and FADH₂, which yield significant ATP via oxidative phosphorylation. #### NEET-PG High-Yield Pearls: * **Rate-limiting enzyme:** Glucose-6-Phosphate Dehydrogenase (G6PD). * **Location:** Occurs entirely in the **cytosol**. * **Clinical Correlation:** G6PD deficiency leads to hemolytic anemia because RBCs cannot generate NADPH to combat oxidative stress (e.g., from fava beans or primaquine), leading to **Heinz bodies** and **Bite cells**. * **Tissues involved:** Highly active in the liver, lactating mammary glands, adrenal cortex, and RBCs.

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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