Carbohydrate Metabolism Practice Questions

Q: Which of the following is NOT a component enzyme of the Pyruvate Dehydrogenase complex?

Pyruvate Decarboxylase. **Explanation:** The **Pyruvate Dehydrogenase Complex (PDH)** is a multi-enzyme mitochondrial complex that catalyzes the oxidative decarboxylation of pyruvate into Acetyl-CoA, linking glycolysis to the TCA cycle. **Why Option D is correct:** **Pyruvate Decarboxylase** is an enzyme involved in **ethanol fermentation** (found in yeast and some bacteria), which converts pyruvate into acetaldehyde. It is **not** part of the human PDH complex. While the first subunit of PDH (E1) does perform decarboxylation, its formal name is Pyruvate Dehydrogenase or Pyruvate Decarboxylase *component*, but "Pyruvate Decarboxylase" as a standalone term refers to the fermentation enzyme. **Why the other options are incorrect:** The PDH complex consists of three distinct catalytic subunits: * **Option A (E1): Pyruvate Dehydrogenase.** It uses Thiamine Pyrophosphate (TPP) as a cofactor to decarboxylate pyruvate. * **Option B (E2): Dihydrolipoyl Transacetylase.** It transfers the acetyl group to Coenzyme A and utilizes Lipoic acid. * **Option C (E3): Dihydrolipoyl Dehydrogenase.** It regenerates the oxidized form of lipoic acid using FAD and NAD+. **High-Yield Clinical Pearls for NEET-PG:** * **Cofactors:** PDH requires five cofactors: **T**hiamine (B1), **R**iboflavin (B2/FAD), **N**iacin (B3/NAD), **P**antothenic acid (B5/CoA), and **L**ipoic acid (Mnemonic: **T**ender **R**eversed **N**eck **P**ads **L**oose). * **Arsenic Poisoning:** Arsenite inhibits the PDH complex by binding to the -SH groups of **Lipoic acid**, leading to lactic acidosis and garlic breath. * **Deficiency:** PDH deficiency is a common cause of congenital lactic acidosis; management involves a **ketogenic diet** (high fat, low carb).

Q: Which disaccharide is not digested in the intestine?

Sucralose. **Explanation:** The correct answer is **Sucralose**. **1. Why Sucralose is the correct answer:** Sucralose is an artificial, non-caloric sweetener. Chemically, it is a **trichlorinated derivative of sucrose** (1,6-dichloro-1,6-dideoxy-β-D-fructofuranosyl-4-chloro-4-deoxy-α-D-galactopyranoside). The substitution of three hydroxyl groups with chlorine atoms creates a molecule that the human body cannot recognize as a carbohydrate. Consequently, there are no specific enzymes (disaccharidases) in the human small intestine capable of breaking the bonds in sucralose. It passes through the digestive tract largely unchanged and is excreted, providing zero calories. **2. Why the other options are incorrect:** * **Sucrose (A):** A natural disaccharide (Glucose + Fructose) digested by the enzyme **Sucrase** located in the brush border of the small intestine. * **Isomaltose (B):** An isomer of maltose with an α(1→6) glycosidic bond. It is a product of starch digestion and is hydrolyzed by the enzyme **Isomaltase** (part of the Sucrase-Isomaltase complex). * **Trehalose (C):** A disaccharide found in mushrooms and yeast (Glucose + Glucose with an α1→1 linkage). It is digested by the brush border enzyme **Trehalase**. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Sucrase-Isomaltase Deficiency:** A congenital condition leading to osmotic diarrhea and abdominal distension upon consuming sugar or starch. * **Trehalase Deficiency:** Rare, but presents with symptoms similar to lactose intolerance after eating mushrooms. * **Lactose:** The only major dietary disaccharide with a **β(1→4) linkage**; digested by Lactase. * **Sucralose** is approximately 600 times sweeter than sucrose and is heat-stable, making it popular for baking.

Q: Strenuous exercise is not indicated in which of the following glycogen storage diseases?

McArdle disease. **Explanation:** **McArdle Disease (GSD Type V)** is caused by a deficiency of **muscle glycogen phosphorylase (myophosphorylase)**. This enzyme is essential for glycogenolysis in skeletal muscle, converting glycogen into glucose-1-phosphate to fuel muscle contraction. During strenuous exercise, muscles rely heavily on glycogen for rapid ATP production. In McArdle disease, the inability to mobilize glycogen leads to an energy crisis, resulting in severe muscle cramps, exercise intolerance, and **myoglobinuria** (due to rhabdomyolysis). A classic clinical feature is the **"second wind phenomenon,"** where patients improve after a brief period of exercise as the body switches to using free fatty acids and blood glucose. **Why other options are incorrect:** * **Anderson Disease (Type IV):** Caused by a branching enzyme deficiency. It primarily affects the liver, leading to cirrhosis and hepatosplenomegaly in infancy; it is not primarily characterized by exercise-induced muscle crises. * **Pompe Disease (Type II):** Caused by lysosomal acid alpha-glucosidase deficiency. It leads to generalized glycogen accumulation (especially in the heart), causing hypertrophic cardiomyopathy and respiratory failure, rather than acute exercise-induced cramps. * **Von Gierke Disease (Type I):** Caused by glucose-6-phosphatase deficiency. It affects the liver and kidneys, presenting with severe fasting hypoglycemia, lactic acidosis, and hepatomegaly, but does not directly impair muscle glycogenolysis. **High-Yield Facts for NEET-PG:** * **Ischemic Forearm Exercise Test:** In McArdle disease, there is a failure of blood lactate to rise, but a significant rise in ammonia levels. * **Biopsy finding:** Subsarcolemmal deposits of glycogen in muscle fibers. * **Management:** Patients are advised to consume simple sugars before exercise and avoid sudden, strenuous bursts of activity.

Q: Which enzyme is inhibited by insulin?

Glycogen phosphorylase. **Explanation:** The regulation of carbohydrate metabolism by insulin is a high-yield topic for NEET-PG. Insulin is an **anabolic hormone** secreted by the pancreatic beta cells in response to high blood glucose levels. Its primary goal is to lower blood glucose by promoting glucose utilization (glycolysis), storage (glycogenesis), and inhibiting glucose production. **1. Why Glycogen Phosphorylase is the Correct Answer:** Glycogen phosphorylase is the rate-limiting enzyme of **glycogenolysis** (breakdown of glycogen into glucose). Insulin triggers a signaling cascade that activates **Protein Phosphatase-1 (PP1)**. This phosphatase dephosphorylates glycogen phosphorylase, converting it from its active form (Phosphorylase *a*) to its inactive form (Phosphorylase *b*). By inhibiting this enzyme, insulin prevents the mobilization of glucose stores. **2. Analysis of Incorrect Options:** * **Glucokinase (Option A):** This enzyme facilitates glucose uptake in the liver. Insulin **induces** the synthesis of Glucokinase to promote glucose utilization. * **PFK-1 (Option B):** Phosphofructokinase-1 is the rate-limiting enzyme of glycolysis. Insulin stimulates PFK-1 indirectly by increasing levels of Fructose-2,6-bisphosphate, its most potent allosteric activator. * **Glycogen Synthase (Option D):** This is the rate-limiting enzyme for glycogen synthesis. Insulin **activates** this enzyme via dephosphorylation (by PP1), promoting the storage of glucose as glycogen. **Clinical Pearls for NEET-PG:** * **Rule of Thumb:** Insulin generally **dephosphorylates** enzymes. In most cases, dephosphorylation **activates** anabolic enzymes (e.g., Glycogen synthase) and **inhibits** catabolic enzymes (e.g., Glycogen phosphorylase, Hormone-sensitive lipase). * **Glucagon/Epinephrine** act as antagonists to insulin by increasing cAMP, which activates Protein Kinase A (PKA), leading to the phosphorylation (activation) of glycogen phosphorylase.

Q: All of the following substrates are glucogenic except:

Acetyl CoA. **Explanation:** The core concept tested here is the **irreversibility of the Pyruvate Dehydrogenase (PDH) complex**. In humans, the conversion of Pyruvate to Acetyl CoA is a one-way reaction. **1. Why Acetyl CoA is the Correct Answer:** Acetyl CoA cannot be used for the net synthesis of glucose (gluconeogenesis). When Acetyl CoA enters the TCA cycle, it condenses with Oxaloacetate (OAA) to form Citrate. During the cycle, two carbons are lost as $CO_2$ before OAA is regenerated. Therefore, there is **no net gain of carbon atoms** to be diverted toward glucose synthesis. Furthermore, the PDH reaction is irreversible, meaning Acetyl CoA cannot be converted back into Pyruvate. **2. Why the other options are Glucogenic:** * **Pyruvate:** It is the primary substrate for gluconeogenesis. It is converted to OAA by *Pyruvate Carboxylase*, bypassing the irreversible step of glycolysis. * **Glycerol:** Derived from triacylglycerol breakdown in adipose tissue, it is phosphorylated to Glycerol-3-Phosphate and then oxidized to Dihydroxyacetone phosphate (DHAP), an intermediate of gluconeogenesis. * **Lactate:** Produced via anaerobic glycolysis (e.g., in RBCs or exercising muscle), it is converted back to Pyruvate by *Lactate Dehydrogenase* in the liver (**Cori Cycle**) to form glucose. **High-Yield NEET-PG Pearls:** * **Odd-chain fatty acids** are glucogenic because their terminal product, **Propionyl CoA**, enters the TCA cycle as Succinyl CoA. * **Even-chain fatty acids** are strictly non-glucogenic (they produce only Acetyl CoA). * **Leucine and Lysine** are the only two amino acids that are purely ketogenic (non-glucogenic). * **Key Regulatory Enzyme:** Pyruvate Carboxylase (requires Biotin and is activated by Acetyl CoA).

Q: Which of the following metabolic pathways occurs in two cellular compartments?

Gluconeogenesis. **Explanation:** The correct answer is **Gluconeogenesis**. In biochemistry, metabolic pathways are often compartmentalized to ensure efficient regulation. While many pathways occur entirely in the cytoplasm, gluconeogenesis is a classic example of a "dual-compartment" pathway. **1. Why Gluconeogenesis is correct:** Gluconeogenesis begins in the **mitochondria** and ends in the **cytosol**. * **Mitochondria:** Pyruvate is converted to Oxaloacetate (OAA) by *Pyruvate Carboxylase*. Since OAA cannot cross the mitochondrial membrane, it is reduced to Malate, exported to the cytosol, and then re-oxidized back to OAA. * **Cytosol:** The remaining steps, starting from the conversion of OAA to Phosphoenolpyruvate (PEP) by *PEPCK*, occur in the cytoplasm. * *Note:* The final step (Glucose-6-Phosphate to Glucose) occurs in the **Endoplasmic Reticulum** of liver and kidney cells. **2. Why other options are incorrect:** * **Glycolysis:** Occurs entirely in the **cytosol** of all cells. * **Glycogenesis (Glycogen synthesis):** Occurs entirely in the **cytosol**, primarily in the liver and skeletal muscle. * **Glycogenolysis (Glycogen breakdown):** Occurs in the **cytosol**. (Small amounts of glycogen are degraded in lysosomes by acid maltase, but the primary metabolic pathway is cytosolic). **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Dual-Compartment Pathways:** Remember **"HUG"** — **H**eme synthesis, **U**rea cycle, and **G**luconeogenesis. * **Rate-limiting enzyme of Gluconeogenesis:** Fructose-1,6-bisphosphatase. * **Key Organelle:** The enzyme **Glucose-6-Phosphatase** is located on the luminal surface of the **Smooth ER**. Its deficiency leads to Von Gierke’s Disease (GSD Type I).

Q: Reducing sugar in urine is seen in which of the following conditions?

Galactosemia. **Explanation:** **Why Galactosemia is correct:** Galactosemia is a disorder of galactose metabolism, most commonly due to a deficiency of **Galactose-1-phosphate uridyltransferase (GALT)**. This leads to an accumulation of galactose in the blood (galactosemia) and its subsequent excretion in the urine (**galactosuria**). Since galactose is a **reducing sugar** (containing a free aldehyde group), it reacts positively with Benedict’s test or Fehling’s solution, resulting in a positive test for reducing substances in the urine. **Analysis of Incorrect Options:** * **Fanconi Anemia:** This is a DNA repair defect characterized by bone marrow failure and physical anomalies. It should not be confused with *Fanconi Syndrome* (a renal proximal tubule defect), which can cause glucosuria. * **Lactose Intolerance:** This is a deficiency of the enzyme lactase in the brush border of the small intestine. It leads to the malabsorption of lactose in the gut, causing osmotic diarrhea. Lactose does not enter the bloodstream in significant amounts and is therefore not typically found in the urine. * **Phenylketonuria (PKU):** This is a disorder of amino acid metabolism (deficiency of Phenylalanine hydroxylase). It results in the excretion of phenylketones (like phenylpyruvate), which are not reducing sugars. **High-Yield Clinical Pearls for NEET-PG:** * **Benedict’s Test vs. Dipstick:** Benedict’s test detects all reducing sugars (glucose, galactose, fructose, lactose). The standard urine dipstick uses the **glucose oxidase** method, which is specific for glucose only. A "positive Benedict’s and negative Dipstick" is a classic clue for Galactosemia or Essential Fructosuria. * **Common Reducing Sugars in Urine:** Glucose (Diabetes), Galactose (Galactosemia), Fructose (Essential Fructosuria), and Pentose (Essential Pentosuria). * **Galactosemia Triad:** Cataracts (due to galactitol accumulation), Hepatomegaly, and Intellectual disability.

Q: Which glucose transporter in myocytes is stimulated by insulin?

GLUT 4. ### Explanation **Correct Option: D (GLUT 4)** GLUT 4 is the only **insulin-dependent** glucose transporter. It is primarily expressed in **skeletal muscle (myocytes)** and **adipose tissue**. In the resting state, GLUT 4 is sequestered in intracellular vesicles. When insulin binds to its receptor, it triggers a signaling cascade (via PI3-kinase) that causes these vesicles to fuse with the plasma membrane, increasing glucose uptake. In myocytes, GLUT 4 translocation is also stimulated by **exercise/muscle contraction** (AMPK pathway), which is why exercise helps manage blood glucose in diabetics. **Incorrect Options:** * **GLUT 1:** This is a basal glucose transporter found in almost all tissues, especially **RBCs** and the **Blood-Brain Barrier**. It is insulin-independent and ensures a constant baseline glucose supply. * **GLUT 2:** A high-capacity, low-affinity transporter found in the **Liver, Pancreatic beta-cells, and Kidney**. It acts as a "glucose sensor" in the pancreas and allows bidirectional transport in the liver. It is insulin-independent. * **GLUT 3:** A high-affinity transporter found primarily in **Neurons**. It ensures glucose uptake even during hypoglycemia, independent of insulin. **High-Yield Clinical Pearls for NEET-PG:** * **GLUT 5:** Unique because it is a **Fructose** transporter (found in the small intestine and spermatozoa). * **SGLT 1/2:** These are active transporters (Sodium-Glucose Co-transporters). SGLT-2 inhibitors (e.g., Dapagliflozin) are modern drugs used in Diabetes to promote glucose excretion in urine. * **Mnemonics:** Remember "**4** is for the **Door**" (Insulin opens the door for glucose in muscle/fat).

Question 1

Which is the rate-limiting step in the Hexose Monophosphate (HMP) shunt?

Accepted Answer

Formation of 6-phosphogluconolactone from glucose-6-phosphate

Answer

Formation of 6-phosphogluconate from 6-phosphogluconolactone

Answer

Formation of ribulose-5-phosphate from 6-phosphogluconate

Answer

Formation of xylulose-5-phosphate from ribulose-5-phosphate

Question 2

Which of the following is NOT a component enzyme of the Pyruvate Dehydrogenase complex?

Accepted Answer

Pyruvate Decarboxylase

Answer

Pyruvate Dehydrogenase

Answer

Dihydrolipoyl Transacetylase

Answer

Dihydrolipoyl Dehydrogenase

Question 3

Which disaccharide is not digested in the intestine?

Accepted Answer

Sucralose

Answer

Sucrose

Answer

Isomaltose

Answer

Trehalose

Question 4

Strenuous exercise is not indicated in which of the following glycogen storage diseases?

Accepted Answer

McArdle disease

Answer

Anderson disease

Answer

Pompe disease

Answer

Von Gierke disease

Question 5

Which enzyme is inhibited by insulin?

Accepted Answer

Glycogen phosphorylase

Answer

Glucokinase

Answer

PFK-1

Answer

Glycogen synthase

Question 6

All of the following substrates are glucogenic except:

Accepted Answer

Acetyl CoA

Answer

Pyruvate

Answer

Glycerol

Answer

Lactate

Question 7

Which of the following metabolic pathways occurs in two cellular compartments?

Accepted Answer

Gluconeogenesis

Answer

Glycolysis

Answer

Glycogenesis

Answer

Glycogenolysis

Question 8

Glycogen storage diseases include all of the following except?

Accepted Answer

None of the above

Answer

Krabbe's disease

Answer

Fabry's disease

Answer

Glycogen storage disease

Question 9

Reducing sugar in urine is seen in which of the following conditions?

Accepted Answer

Galactosemia

Answer

Fanconi anemia

Answer

Lactose intolerance

Answer

Phenylketonuria

Question 10

Which glucose transporter in myocytes is stimulated by insulin?

Accepted Answer

GLUT 4

Answer

GLUT 1

Answer

GLUT 2

Answer

GLUT 3

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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