Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 421: Where are the maximum number of enzymes of the Krebs cycle found?

A. Mitochondrial matrix (Correct Answer)
B. Intermembrane space
C. Cytosol
D. Ribosome

Explanation: ### Explanation The **Krebs cycle** (also known as the Citric Acid Cycle or TCA cycle) is the final common pathway for the oxidation of carbohydrates, lipids, and proteins. **1. Why the Mitochondrial Matrix is Correct:** The Krebs cycle occurs entirely within the **mitochondria** to ensure that the generated reducing equivalents (NADH and FADH₂) are in close proximity to the Electron Transport Chain (ETC). **Seven out of the eight enzymes** of the cycle—including Citrate Synthase, Isocitrate Dehydrogenase, and α-Ketoglutarate Dehydrogenase—are located in the **mitochondrial matrix** in a soluble form. **2. Analysis of Incorrect Options:** * **Intermembrane Space:** This area primarily serves as a reservoir for protons ($H^+$) pumped during the ETC to create the electrochemical gradient; it does not house metabolic cycle enzymes. * **Cytosol:** While glycolysis occurs here, the pyruvate produced must be transported into the mitochondria (via pyruvate dehydrogenase complex) to enter the Krebs cycle. * **Ribosome:** These are sites of protein synthesis (translation) and have no role in oxidative metabolism. **3. High-Yield Clinical Pearls for NEET-PG:** * **The Exception:** **Succinate Dehydrogenase** is the only enzyme of the Krebs cycle **not** found in the matrix; it is embedded in the **Inner Mitochondrial Membrane** (acting as Complex II of the ETC). * **Rate-Limiting Step:** **Isocitrate Dehydrogenase** is the key rate-limiting enzyme of the cycle. * **Energy Yield:** One turn of the Krebs cycle produces **10 ATPs** (3 NADH = 7.5, 1 FADH₂ = 1.5, 1 GTP = 1). * **Amphibolic Nature:** The cycle is both catabolic (breaking down acetyl-CoA) and anabolic (providing intermediates like Succinyl-CoA for heme synthesis).

Question 422: Glycogenolysis is activated in muscles due to which of the following hormones?

A. Glucagon
B. Insulin
C. Epinephrine (Correct Answer)
D. Growth hormone

Explanation: **Explanation:** **1. Why Epinephrine is Correct:** In skeletal muscle, glycogenolysis is primarily triggered by **Epinephrine** (Adrenaline) during "fight or flight" situations or vigorous exercise. Epinephrine binds to **$\beta_2$-adrenergic receptors**, activating Adenylate Cyclase to increase cAMP. This triggers a phosphorylation cascade: Protein Kinase A activates **Phosphorylase Kinase**, which in turn converts inactive Glycogen Phosphorylase *b* to active **Glycogen Phosphorylase *a***, the rate-limiting enzyme of glycogenolysis. Additionally, muscle glycogenolysis is uniquely stimulated by **Calcium ions** (via calmodulin) during contraction and **AMP** (allosteric activator) during energy depletion. **2. Why Other Options are Incorrect:** * **Glucagon:** While Glucagon is a potent stimulator of glycogenolysis in the **liver**, it has **no effect on skeletal muscle** because muscle cells lack glucagon receptors. * **Insulin:** Insulin is an anabolic hormone that promotes **glycogenesis** (glycogen synthesis) and inhibits glycogenolysis by activating phosphodiesterase (lowering cAMP) and protein phosphatase-1. * **Growth Hormone:** GH generally promotes gluconeogenesis and lipolysis but does not acutely activate the glycogenolytic pathway in muscles. **3. High-Yield Clinical Pearls for NEET-PG:** * **Organ Specificity:** Glucagon = Liver only; Epinephrine = Liver and Muscle. * **The "Missing" Enzyme:** Muscle glycogen cannot contribute to blood glucose because muscles lack **Glucose-6-Phosphatase**. Muscle glycogen is used strictly for local ATP production. * **Allosteric Regulation:** In muscles, **AMP** can activate Glycogen Phosphorylase *b* even without phosphorylation—a vital mechanism during anoxia. * **Rate-limiting enzyme:** Glycogen Phosphorylase.

Question 423: The Urinary D-Xylose test is used for the study of which of the following?

A. Proximal small bowel mucosal function (Correct Answer)
B. Distal small bowel mucosal function
C. Proximal small bowel serosal function
D. Distal small bowel serosal function

Explanation: ### Explanation **1. Why Option A is Correct:** D-Xylose is a five-carbon monosaccharide (pentose) that is absorbed primarily in the **proximal small intestine (duodenum and jejunum)**. Unlike most carbohydrates, its absorption is passive and does not require pancreatic enzymes or bile salts. Once absorbed, it is not metabolized by the liver and is excreted unchanged in the urine. Therefore, a low level of D-Xylose in the urine after an oral dose indicates **mucosal damage** (malabsorption) in the proximal small bowel, such as in Celiac disease or Tropical sprue. **2. Why Other Options are Incorrect:** * **Distal small bowel (Options B & D):** The distal small bowel (ileum) is primarily responsible for the absorption of Vitamin B12 and bile salts. D-Xylose is absorbed early in the GI tract; hence, it is not a marker for distal function. * **Serosal function (Options C & D):** The serosa is the outermost connective tissue layer of the intestines. Absorption is a function of the **mucosa** (the innermost lining with villi). Serosal integrity is irrelevant to nutrient transport. **3. NEET-PG High-Yield Pearls:** * **Differential Diagnosis:** The D-Xylose test helps differentiate **mucosal causes** of malabsorption (e.g., Celiac disease—test is abnormal) from **pancreatic insufficiency** (e.g., Chronic pancreatitis—test is normal, as enzymes aren't needed for D-Xylose). * **False Positives:** Low urinary D-Xylose (suggesting malabsorption) can occur even with a healthy mucosa in cases of **Small Intestinal Bacterial Overgrowth (SIBO)**, as bacteria metabolize the sugar before it is absorbed, or in **renal failure**, where excretion is impaired. * **Gold Standard:** While historically important, this test is now largely replaced by distal duodenal biopsy for diagnosing mucosal diseases.

Question 424: Which glucose transporter is primarily found in neurons?

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

Explanation: ### Explanation **Correct Option: C (GLUT-3)** Glucose transporters (GLUTs) are transmembrane proteins that facilitate the transport of glucose across cell membranes via facilitated diffusion. **GLUT-3** is known as the "neuronal glucose transporter." It is primarily expressed in the **neurons** of the brain. It possesses a **very low Km** (high affinity) for glucose, ensuring that neurons—which have a high metabolic demand and limited internal energy stores—can prioritize glucose uptake even when blood glucose levels are significantly low. **Analysis of Incorrect Options:** * **GLUT-1:** Found primarily in **Red Blood Cells (RBCs)** and the **Blood-Brain Barrier (BBB)**. While it brings glucose into the brain tissue, GLUT-3 is the specific transporter that moves it into the neurons themselves. * **GLUT-2:** A high-capacity, high-Km (low affinity) transporter found in the **Liver, Pancreatic beta cells, and Kidney**. It acts as a glucose sensor in the pancreas. * **GLUT-4:** The only **insulin-dependent** glucose transporter. It is primarily located in **Skeletal muscle and Adipose tissue**. **High-Yield Clinical Pearls for NEET-PG:** * **Brain Glucose Supply:** Glucose enters the brain via GLUT-1 (BBB) and enters neurons via GLUT-3. * **Insulin Independence:** The brain (GLUT-1/3), RBCs (GLUT-1), and Liver (GLUT-2) do not require insulin for glucose uptake. * **SGLT vs. GLUT:** Remember that SGLT (Sodium-Glucose Linked Transporters) perform **active transport** (secondary), whereas GLUTs perform **passive transport** (facilitated diffusion). * **GLUT-5:** Specifically functions as a **fructose** transporter, primarily in the small intestine and spermatozoa.

Question 425: Chitin contains which type of glycosidic bond?

A. Alpha 1-4 bond
B. Alpha 1-6 bond
C. Beta 1-6 bond
D. Beta 1-4 bond (Correct Answer)

Explanation: **Explanation:** **Chitin** is a structural homopolysaccharide found in the exoskeleton of arthropods (like insects and crustaceans) and the cell walls of fungi. It is composed of repeating units of **N-acetyl-D-glucosamine (NAG)**. These monomers are linked together by **$\beta$(1$\rightarrow$4) glycosidic bonds**. The $\beta$(1$\rightarrow$4) linkage is crucial because it allows the polysaccharide chains to form long, straight, and rigid microfibrils. These chains are further stabilized by hydrogen bonding, providing the high tensile strength necessary for structural support, much like cellulose in plants. **Analysis of Incorrect Options:** * **Option A ($\alpha$ 1-4 bond):** This linkage is characteristic of **Starch (Amylose)** and **Glycogen**. It creates a helical structure suitable for energy storage rather than structural rigidity. * **Option B ($\alpha$ 1-6 bond):** This linkage is found at the **branch points** of Glycogen and Amylopectin. * **Option C ($\beta$ 1-6 bond):** This is a less common linkage found in some fungal glucans and specific branching patterns, but it is not the backbone of chitin. **High-Yield NEET-PG Pearls:** * **Chitin vs. Cellulose:** Both have $\beta$(1$\rightarrow$4) bonds. The difference lies in the monomer: Cellulose uses Glucose, while Chitin uses N-acetylglucosamine (an amino sugar derivative). * **Lysozyme:** This enzyme, found in human tears and saliva, can hydrolyze $\beta$(1$\rightarrow$4) glycosidic bonds, acting as an antibacterial agent by attacking bacterial peptidoglycan. * **Clinical Relevance:** Chitin is a potent inducer of cytokine release and is involved in the pathogenesis of certain occupational asthma and allergic reactions.

Question 426: Decreased glucose concentration in hepatic cells triggers all of the following EXCEPT?

A. Increased glucagon levels in blood
B. Activation of fructose-2,6-biphosphatase
C. Inhibition of phosphofructokinase-2
D. Increased fructose-2,6-biphosphate levels (Correct Answer)

Explanation: **Explanation:** The regulation of glycolysis and gluconeogenesis in the liver is primarily controlled by the bifunctional enzyme **PFK-2/FBPase-2**. This enzyme dictates the levels of **Fructose-2,6-bisphosphate (F-2,6-BP)**, the most potent allosteric activator of glycolysis. **Why Option D is the Correct Answer:** When glucose levels decrease, the **Glucagon/Insulin ratio increases**. Glucagon triggers a cAMP-mediated signaling cascade that activates Protein Kinase A (PKA). PKA phosphorylates the bifunctional enzyme, leading to the **inhibition of PFK-2** and **activation of FBPase-2**. This results in the degradation of F-2,6-BP. Therefore, decreased glucose leads to **decreased** F-2,6-BP levels, making Option D the "Except" statement. **Analysis of Other Options:** * **Option A:** Low blood glucose directly stimulates alpha cells of the pancreas to secrete glucagon. * **Option B & C:** As described above, phosphorylation by PKA during hypoglycemia activates the FBPase-2 domain and inhibits the PFK-2 domain of the bifunctional enzyme to favor gluconeogenesis. **High-Yield Facts for NEET-PG:** * **F-2,6-BP** is the most potent stimulator of **Phosphofructokinase-1 (PFK-1)** and a potent inhibitor of **Fructose-1,6-bisphosphatase**. * **Fed State (High Insulin):** Dephosphorylated state → PFK-2 active → High F-2,6-BP → Glycolysis stimulated. * **Fasting State (High Glucagon):** Phosphorylated state → FBPase-2 active → Low F-2,6-BP → Gluconeogenesis stimulated. * **Mnemonic:** **P**hosphorylation by **P**KA makes the enzyme act as a **P**hosphatase (FBPase-2 active).

Question 427: All of the following are true about fructose-1,6-bisphosphatase, EXCEPT?

A. Key gluconeogenic enzyme
B. Fructose-2,6-bisphosphate is an allosteric activator of this enzyme (Correct Answer)
C. Catalyses a hydrolysis reaction
D. Requires magnesium for catalysis

Explanation: **Explanation:** The correct answer is **B**, as Fructose-2,6-bisphosphate (F-2,6-BP) is actually the most potent **allosteric inhibitor** of Fructose-1,6-bisphosphatase (FBPase-1), not an activator. 1. **Why Option B is the exception:** F-2,6-BP acts as a metabolic "switch." High levels of F-2,6-BP (stimulated by insulin) activate Glycolysis (via PFK-1) and simultaneously **inhibit Gluconeogenesis** by inhibiting FBPase-1. This prevents a "futile cycle" where both pathways run at once. Therefore, F-2,6-BP and AMP are the primary inhibitors of this enzyme. 2. **Analysis of other options:** * **Option A:** FBPase-1 is one of the four **irreversible, rate-limiting enzymes** of gluconeogenesis. It bypasses the PFK-1 step of glycolysis. * **Option C:** The enzyme catalyzes the conversion of Fructose-1,6-bisphosphate to Fructose-6-phosphate by removing a phosphate group using water. This is a **hydrolysis** reaction (not a simple reversal of the kinase reaction). * **Option D:** Like most enzymes involving phosphate transfers or removals in carbohydrate metabolism, FBPase-1 requires **divalent cations ($Mg^{2+}$ or $Mn^{2+}$)** for optimal catalytic activity. **High-Yield Clinical Pearls for NEET-PG:** * **Deficiency:** FBPase-1 deficiency leads to impaired gluconeogenesis, presenting as fasting hypoglycemia and lactic acidosis (due to inability to utilize lactate/glycerol for glucose). * **Reciprocal Regulation:** Glucagon decreases F-2,6-BP levels, thereby relieving the inhibition on FBPase-1 and promoting gluconeogenesis. * **Location:** This enzyme is primarily found in the liver and kidneys.

Question 428: Which of the following is a primary fructose transporter?

A. GLUT-1
B. GLUT-3
C. GLUT-5 (Correct Answer)
D. GLUT-4

Explanation: **Explanation:** **GLUT-5** is the correct answer because it is a specialized facilitative transporter with a high affinity for **fructose**. Unlike other GLUT transporters that primarily transport glucose, GLUT-5 is unique in its specificity for fructose. It is predominantly expressed in the apical membrane of the **small intestine** (enterocytes), where it facilitates the absorption of dietary fructose, as well as in the **spermatozoa**, which utilize fructose as their primary energy source. **Analysis of Incorrect Options:** * **GLUT-1:** Found in RBCs, the blood-brain barrier, and the kidneys. It provides basal glucose uptake and has a very low affinity for fructose. * **GLUT-3:** Primarily located in neurons and the placenta. It is a high-affinity glucose transporter ensuring constant glucose supply to the brain. * **GLUT-4:** The only **insulin-dependent** glucose transporter. It is found in skeletal muscle and adipose tissue. It is sequestered in intracellular vesicles and translocates to the cell membrane only in the presence of insulin. **High-Yield Clinical Pearls for NEET-PG:** * **SGLT-1 vs. GLUT-5:** Glucose and Galactose are absorbed via SGLT-1 (active transport), whereas Fructose is absorbed via GLUT-5 (facilitated diffusion). * **GLUT-2:** A bidirectional, high-capacity transporter found in the liver, pancreas, and the basolateral membrane of the intestine. It can transport glucose, galactose, and fructose. * **Fructose Metabolism:** Fructose bypasses the rate-limiting step of glycolysis (Phosphofructokinase-1), leading to more rapid lipogenesis compared to glucose.

Question 429: A hypotonic baby shows an increased ratio of Pyruvate to Acetyl CoA. The baby also exhibits features of lactic acidosis. The underlying defect prevents pyruvate from forming Acetyl CoA in fibroblasts. Which of the following can revert this situation?

A. Biotin
B. Pyridoxine
C. Free fatty acid
D. Thiamin (Correct Answer)

Explanation: ### Explanation **Correct Option: D (Thiamin)** The clinical presentation of hypotonia, lactic acidosis, and a high Pyruvate/Acetyl CoA ratio indicates a defect in the **Pyruvate Dehydrogenase (PDH) Complex**. This multienzyme complex is responsible for the oxidative decarboxylation of pyruvate into Acetyl CoA, bridging glycolysis and the TCA cycle. The PDH complex requires five essential cofactors: **T**hiamine pyrophosphate (TPP/B1), **R**iboflavin (FAD/B2), **N**iacin (NAD/B3), **P**antothenic acid (CoA/B5), and **L**ipoic acid (Mnemonic: **T**ender **R**oving **N**ights **P**lease **L**uck). In many cases of PDH deficiency, the E1 subunit (pyruvate decarboxylase) has a reduced affinity for its cofactor, **Thiamin (TPP)**. Administering high doses of Thiamin can stabilize the enzyme, increase its catalytic activity, and effectively lower lactate levels by diverting pyruvate toward Acetyl CoA formation. **Analysis of Incorrect Options:** * **A. Biotin:** A cofactor for carboxylases (e.g., Pyruvate Carboxylase). Deficiency would impair gluconeogenesis, not the conversion of pyruvate to Acetyl CoA. * **B. Pyridoxine (B6):** Primarily involved in transamination (ALT/AST) and heme synthesis. It does not play a role in the PDH complex. * **C. Free fatty acids:** These are metabolized into Acetyl CoA via beta-oxidation. While they provide an alternative fuel source, they do not fix the enzymatic defect or reduce the accumulation of pyruvate/lactate. **Clinical Pearls for NEET-PG:** * **PDH Deficiency:** The most common cause of congenital lactic acidosis. It is often an **X-linked dominant** inheritance (E1-alpha subunit mutation). * **Dietary Management:** Patients are often placed on a **Ketogenic Diet** (high fat, low carb) to provide energy via ketones/Acetyl CoA, bypassing the PDH block. * **Leucine and Lysine:** These are purely ketogenic amino acids and are preferred in PDH deficiency as they do not contribute to pyruvate formation.

Question 430: What is the major contributor to gluconeogenesis?

A. Ketones
B. Alanine (Correct Answer)
C. Lactate
D. Glycine

Explanation: **Explanation:** Gluconeogenesis is the metabolic pathway that generates glucose from non-carbohydrate precursors during fasting or intense exercise. While several molecules contribute, **Alanine** is considered the major contributor among amino acids and a primary substrate for gluconeogenesis. **Why Alanine is the correct answer:** Alanine serves as the principal vehicle for transporting amino groups from the muscle to the liver via the **Cahill Cycle (Glucose-Alanine Cycle)**. In the liver, alanine undergoes transamination to form **pyruvate**, which directly enters the gluconeogenic pathway. During prolonged fasting, muscle protein breakdown releases amino acids, and alanine accounts for a significant portion of the glucose produced to maintain blood sugar levels. **Analysis of Incorrect Options:** * **Lactate (Option C):** While lactate is a significant precursor (via the Cori Cycle), especially during anaerobic exercise, alanine is often cited as the primary substrate in the context of systemic protein turnover and fasting metabolism. * **Glycine (Option D):** Glycine is a glucogenic amino acid, but its quantitative contribution to the total glucose pool is much smaller compared to alanine. * **Ketones (Option A):** Ketones (e.g., acetoacetate, β-hydroxybutyrate) **cannot** be converted into glucose in humans. They are products of fatty acid oxidation used as an alternative fuel source, but they lack a pathway to contribute to gluconeogenesis. **High-Yield Clinical Pearls for NEET-PG:** * **Key Enzyme:** Pyruvate carboxylase (requires Biotin) is the first regulatory step of gluconeogenesis. * **The "Rule of Two":** Leucine and Lysine are the only **purely ketogenic** amino acids; they cannot contribute to gluconeogenesis. * **Odd-chain Fatty Acids:** Unlike even-chain fats, odd-chain fatty acids can be gluconeogenic because they yield **Propionyl-CoA**, which enters the TCA cycle as Succinyl-CoA.

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

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Lactose Intolerance and Galactosemia

Practice Questions

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