Carbohydrate Metabolism Practice Questions

Q: Which of the following enzymes is present inside the given organelle?

Glucose-6-phosphatase. ***Glucose-6-phosphatase*** - This enzyme is specifically localized to the **endoplasmic reticulum (ER)** membrane and plays a crucial role in **gluconeogenesis** and **glycogenolysis**. - It catalyzes the final step in glucose production by converting **glucose-6-phosphate** to free glucose, which can then be released into the bloodstream. *Glucokinase* - This enzyme is primarily found in the **cytosol** of hepatocytes and pancreatic beta cells, not in the ER. - It functions as a **glucose sensor** and catalyzes the phosphorylation of glucose to glucose-6-phosphate during **glycolysis**. *Isocitrate dehydrogenase* - This enzyme is located in the **mitochondrial matrix** as part of the **citric acid cycle (Krebs cycle)**. - It catalyzes the oxidative decarboxylation of **isocitrate** to **α-ketoglutarate**, generating **NADH** and **CO₂**. *Fructose 2, 6 bisphosphatase* - This enzyme is found in the **cytosol** and is involved in regulating **glycolysis** and **gluconeogenesis**. - It hydrolyzes **fructose-2,6-bisphosphate** to **fructose-6-phosphate**, acting as a key regulatory enzyme in glucose metabolism.

Q: Which of the following is NOT a product of the Hexose Monophosphate (HMP) shunt?

Glycerol-3-phosphate. ### Explanation The **Hexose Monophosphate (HMP) Shunt** (Pentose Phosphate Pathway) occurs in the cytosol and is divided into an irreversible oxidative phase and a reversible non-oxidative phase. **Why Glycerol-3-phosphate is the correct answer:** Glycerol-3-phosphate is **not** a product of the HMP shunt. It is an intermediate of **Glycolysis** (derived from Dihydroxyacetone phosphate via Glycerol-3-phosphate dehydrogenase) or lipid metabolism. While the HMP shunt provides precursors for various pathways, it does not directly generate Glycerol-3-phosphate. **Analysis of Incorrect Options:** * **Glyceraldehyde 3-phosphate (A):** This is a key product of the **non-oxidative phase**. Transketolase and transaldolase enzymes recycle pentose sugars back into glycolytic intermediates like Glyceraldehyde 3-phosphate and Fructose 6-phosphate. * **3CO₂ (C):** In the **oxidative phase**, for every 3 molecules of Glucose 6-phosphate entering the shunt, 3 molecules of CO₂ are released during the decarboxylation of 6-phosphogluconate to Ribulose 5-phosphate. * **6NADPH (D):** The primary purpose of the oxidative phase is the generation of reducing equivalents. For 3 molecules of Glucose 6-phosphate, **6 molecules of NADPH** are produced (2 per glucose molecule). **High-Yield NEET-PG Pearls:** * **Rate-limiting enzyme:** Glucose-6-phosphate dehydrogenase (G6PD). * **Tissues involved:** Highly active in the liver, lactating mammary glands, adrenal cortex, and RBCs (where NADPH maintains reduced glutathione to prevent oxidative stress). * **Clinical Correlation:** G6PD deficiency leads to hemolytic anemia due to the inability to neutralize free radicals, characterized by **Heinz bodies** and **Bite cells** on blood smears. * **Thiamine (B1) Connection:** Transketolase requires Thiamine pyrophosphate (TPP) as a cofactor; measuring erythrocyte transketolase activity is a diagnostic test for Thiamine deficiency.

Q: Which of the following is NOT used in gluconeogenesis?

Oleate. **Explanation:** The core concept in gluconeogenesis is that a substrate must be capable of a net conversion into **Oxaloacetate (OAA)** or other intermediates of the TCA cycle to enter the gluconeogenic pathway. **Why Oleate is the correct answer:** Oleate is a long-chain **fatty acid** (C18:1). In humans, the oxidation of even-chain fatty acids produces **Acetyl-CoA**. Acetyl-CoA cannot be used for the net synthesis of glucose because: 1. The **Pyruvate Dehydrogenase (PDH) reaction** is irreversible; Acetyl-CoA cannot be converted back to Pyruvate. 2. In the TCA cycle, the two carbons of Acetyl-CoA are lost as $CO_2$ before reaching Oxaloacetate, resulting in **no net gain** of carbon atoms for gluconeogenesis. **Analysis of Incorrect Options:** * **Succinate:** This is a TCA cycle intermediate. It is oxidized to Malate and then to Oxaloacetate, which enters gluconeogenesis via PEP carboxykinase. * **Glutamate:** A glucogenic amino acid. It is converted to **$\alpha$-ketoglutarate** (via transamination or glutamate dehydrogenase), which enters the TCA cycle to form Oxaloacetate. * **Aspartate:** A glucogenic amino acid. It undergoes transamination to directly form **Oxaloacetate**. **High-Yield Clinical Pearls for NEET-PG:** * **Odd-chain fatty acids** ARE gluconeogenic because their terminal metabolism yields **Propionyl-CoA**, which enters the TCA cycle as Succinyl-CoA. * **Leucine and Lysine** are the only purely ketogenic amino acids (cannot form glucose). * **Glycerol** (from triacylglycerol breakdown) is gluconeogenic as it enters the pathway at the level of Dihydroxyacetone phosphate (DHAP). * **Acetyl-CoA** is an allosteric **activator of Pyruvate Carboxylase**, thus promoting gluconeogenesis while inhibiting the PDH complex.

Q: Which of the following glucose transporters is primarily utilized by adipocytes?

GLUT4. **Explanation:** The correct answer is **GLUT4**. Glucose transporters (GLUTs) are transmembrane proteins that facilitate the movement of glucose across cell membranes via facilitated diffusion. **Why GLUT4 is correct:** GLUT4 is the only **insulin-dependent** glucose transporter. It is primarily expressed in **adipocytes (fat cells)** and **skeletal muscle**. In the fasting state, GLUT4 remains sequestered in intracellular vesicles. Upon insulin secretion (post-prandial state), these vesicles translocate and fuse with the plasma membrane, allowing for rapid glucose uptake. This mechanism is crucial for lowering post-prandial blood glucose levels. **Why the other options are incorrect:** * **GLUT1:** This is a basal glucose transporter found in almost all tissues. It is highly expressed in **Red Blood Cells (RBCs)** and the **Blood-Brain Barrier**. It ensures a steady, insulin-independent supply of glucose. * **GLUT2:** A high-capacity, low-affinity transporter found in the **Liver, Pancreas (beta cells), Kidney, and Small Intestine**. It acts as a "glucose sensor" in the pancreas and allows for bidirectional glucose flux in the liver. * **GLUT3:** A high-affinity transporter primarily found in **Neurons**. It ensures that the brain receives adequate glucose even when blood sugar levels are low. **High-Yield Clinical Pearls for NEET-PG:** * **GLUT4 & Exercise:** Muscle contraction can trigger GLUT4 translocation to the cell membrane independently of insulin, which is why exercise helps manage blood sugar in Type 2 Diabetes. * **GLUT5:** This is unique because it is primarily a **Fructose** transporter, located in the small intestine and spermatozoa. * **SGLT1/2:** Unlike GLUTs, Sodium-Glucose Linked Transporters (SGLTs) utilize **active transport** (secondary) and are found in the intestinal mucosa and renal tubules. SGLT2 inhibitors (e.g., Dapagliflozin) are now major drugs for Diabetes and Heart Failure.

Q: What are the enzymes required for the formation of Phosphoenolpyruvate from Pyruvate?

Pyruvate carboxylase and Phosphoenolpyruvate Carboxykinase. ### Explanation The conversion of Pyruvate to Phosphoenolpyruvate (PEP) is the first major hurdle in **Gluconeogenesis**. In Glycolysis, the conversion of PEP to Pyruvate by *Pyruvate Kinase* is irreversible. To bypass this, the body employs a two-step "bypass" mechanism: 1. **Pyruvate Carboxylase (PC):** Located in the mitochondria, this enzyme converts Pyruvate into Oxaloacetate (OAA). This reaction requires **Biotin** as a cofactor and ATP. 2. **Phosphoenolpyruvate Carboxykinase (PEPCK):** This enzyme converts OAA into PEP. This step occurs in the cytosol (or mitochondria) and requires **GTP**. **Analysis of Incorrect Options:** * **Pyruvate Dehydrogenase (PDH):** This enzyme converts Pyruvate to Acetyl-CoA. This is an irreversible step that leads into the TCA cycle, not gluconeogenesis. "Acetyl-CoA cannot be converted back to glucose." * **Lactate Dehydrogenase (LDH):** This catalyzes the reversible conversion of Pyruvate to Lactate (anaerobic glycolysis). It does not participate in the bypass of Pyruvate Kinase. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** Pyruvate Carboxylase is a **mitochondrial** enzyme, while PEPCK is found in both the mitochondria and cytosol. * **Obligatory Activator:** Pyruvate Carboxylase is allosterically activated by **Acetyl-CoA**. High levels of Acetyl-CoA signal that the cell has enough energy, shunting pyruvate toward gluconeogenesis. * **Cofactor:** Remember the mnemonic **ABC** for carboxylases: **A**TP, **B**iotin, and **C**O₂ are required. * **Rate-limiting step:** PEPCK is often considered a key regulatory point in gluconeogenesis.

Q: Which of the following enzymes becomes deficient following an episode of severe infectious gastroenteritis?

Lactase. **Explanation:** The correct answer is **Lactase**. This question tests the concept of **Secondary Lactose Intolerance** following mucosal injury. **Why Lactase is the correct answer:** Lactase is a brush-border disaccharidase located at the **tips of the intestinal villi**. In cases of severe infectious gastroenteritis (viral, bacterial, or protozoal like Giardia), the inflammatory process leads to villous atrophy and sloughing of the enterocytes. Because lactase is located most superficially on the villi compared to other disaccharidases (like sucrase or maltase), it is the first enzyme to be lost and the last to recover during mucosal healing. This leads to a temporary inability to digest lactose, resulting in osmotic diarrhea, bloating, and flatulence after milk ingestion. **Why the other options are incorrect:** * **Amylase, Trypsin, and Lipase:** These are **pancreatic enzymes** secreted by the acinar cells of the pancreas into the duodenal lumen. Since infectious gastroenteritis primarily affects the intestinal mucosa and not the pancreatic parenchyma, the production and secretion of these enzymes remain largely unaffected. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** Lactase is found on the apical surface of differentiated enterocytes (brush border). * **Diagnosis:** Secondary lactose intolerance is often diagnosed via a **Hydrogen Breath Test** (increased $H_2$ due to bacterial fermentation of undigested lactose) or finding a **low stool pH** (acidic) with the presence of **reducing substances**. * **Recovery:** It may take 2–4 weeks for the brush border to regenerate and lactase levels to return to normal post-infection. * **Congenital vs. Acquired:** While congenital lactase deficiency is rare, secondary (acquired) deficiency is a common post-sequela of Rotavirus infection in children.

Q: Dehydrogenases of the HMP shunt are specific for which coenzyme?

Nicotinamide adenine dinucleotide phosphate (NADP+). **Explanation:** The Hexose Monophosphate (HMP) Shunt, also known as the Pentose Phosphate Pathway, occurs in the cytosol and is primarily an anabolic pathway. The rate-limiting step and the subsequent oxidative step are catalyzed by **Glucose-6-Phosphate Dehydrogenase (G6PD)** and **6-Phosphogluconate Dehydrogenase**, respectively. Both of these enzymes are strictly specific for **NADP+** as their coenzyme, reducing it to **NADPH**. Unlike NADH, which is used in the electron transport chain for ATP production, NADPH is essential for: 1. **Reductive Biosynthesis:** Synthesis of fatty acids and steroids. 2. **Antioxidant Defense:** Maintaining glutathione in its reduced state to protect cells against reactive oxygen species (ROS), particularly in RBCs. **Analysis of Incorrect Options:** * **A. Thiamine pyrophosphate (TPP):** This is a derivative of Vitamin B1. While it is a coenzyme in the HMP shunt, it is used by **Transketolase** (non-oxidative phase), not by dehydrogenases. * **C & D. FMN and FAD:** These are flavin-derived coenzymes (Vitamin B2) typically involved in redox reactions within the mitochondria (e.g., TCA cycle or Complex I/II of ETC). They are not utilized by the oxidative enzymes of the HMP shunt. **High-Yield Clinical Pearls for NEET-PG:** * **G6PD Deficiency:** The most common enzyme deficiency worldwide. It leads to hemolytic anemia because RBCs cannot generate NADPH, leaving them vulnerable to oxidative stress (e.g., fava beans, primaquine, infections) and resulting in **Heinz bodies** and **Bite cells**. * **Tissue Distribution:** The HMP shunt is most active in tissues requiring high NADPH, such as the adrenal cortex (steroidogenesis), liver (fatty acid synthesis), and lactating mammary glands. * **Transketolase Activity:** Measuring erythrocyte transketolase activity is the gold standard for diagnosing **Thiamine deficiency**.

Q: Glucagon receptors are NOT found in which organ?

Cornea. **Explanation:** Glucagon is a peptide hormone primarily known for its role in glucose homeostasis. It acts via **G-protein coupled receptors (GPCRs)** that activate the Adenylate Cyclase-cAMP pathway. **Why Cornea is the Correct Answer:** Glucagon receptors are primarily localized in tissues involved in fuel mobilization and metabolic regulation. The **cornea** is an avascular, specialized ocular tissue that relies on atmospheric oxygen and glucose from the aqueous humor. It does not possess glucagon receptors because it does not participate in systemic glucose regulation or glycogenolysis. **Analysis of Other Options:** * **Kidney:** Glucagon receptors are present in the renal cortex and distal tubules. Glucagon plays a role in stimulating gluconeogenesis in the kidney and influencing electrolyte excretion (e.g., increasing glomerular filtration rate). * **Stomach:** Receptors are found in the gastric mucosa. Glucagon inhibits gastric acid secretion and reduces gastrointestinal motility. * **Adrenal Gland:** Glucagon receptors are present in the adrenal medulla, where glucagon can stimulate the release of catecholamines (epinephrine and norepinephrine). **High-Yield NEET-PG Pearls:** 1. **Primary Site:** The **Liver** has the highest density of glucagon receptors (stimulating glycogenolysis and gluconeogenesis). 2. **Muscle Paradox:** Glucagon receptors are **NOT** found in **Skeletal Muscle**. This is a frequent exam trap; muscle glycogen is used only for local contraction and cannot be released into the blood because muscle lacks Glucose-6-Phosphatase. 3. **Adipose Tissue:** Glucagon receptors are present here to stimulate lipolysis via Hormone-Sensitive Lipase (HSL). 4. **Clinical Correlation:** Glucagon is used as an antidote for **Beta-blocker overdose** because it bypasses the beta-receptor to increase cAMP in cardiac myocytes, improving heart rate and contractility.

Q: NADPH+ and H+ are generated in the reaction catalyzed by which enzyme?

Glucose-6-phosphate dehydrogenase (G-6-PD). **Explanation:** The correct answer is **Glucose-6-phosphate dehydrogenase (G-6-PD)**. This enzyme catalyzes the first and rate-limiting step of the **Hexose Monophosphate (HMP) Shunt** (Pentose Phosphate Pathway). In this reaction, Glucose-6-phosphate is oxidized to 6-phosphogluconolactone, and **NADP+ is reduced to NADPH + H+**. This is the primary source of NADPH in the body, which is essential for reductive biosynthesis (e.g., fatty acid and steroid synthesis) and maintaining the pool of reduced glutathione to protect cells against oxidative stress. **Analysis of Incorrect Options:** * **Lactate dehydrogenase (LDH):** Involved in anaerobic glycolysis, it interconverts pyruvate and lactate using **NADH/NAD+** as the coenzyme, not NADPH. * **Glyceraldehyde-3-phosphate dehydrogenase (G-3-PD):** A key enzyme in glycolysis that converts G-3-P to 1,3-bisphosphoglycerate, generating **NADH**. * **Alcohol dehydrogenase:** Responsible for ethanol metabolism in the cytosol, it converts ethanol to acetaldehyde using **NAD+** as an electron acceptor to produce **NADH**. **Clinical Pearls for NEET-PG:** * **G6PD Deficiency:** The most common enzymopathy worldwide. It leads to **hemolytic anemia** under oxidative stress (e.g., fava beans, primaquine, infections) because the RBCs cannot generate enough NADPH to maintain reduced glutathione. * **Bite Cells & Heinz Bodies:** Classic peripheral smear findings in G6PD deficiency. * **Tissue Distribution:** The HMP shunt is highly active in the adrenal cortex, liver, mammary glands, and RBCs. * **Rule of Thumb:** Enzymes using **NAD+** are generally involved in **catabolic** pathways (energy production), while those using **NADP+** are involved in **anabolic** pathways (biosynthesis) or detoxification.

Question 1

Which of the following enzymes is present inside the given organelle?

Accepted Answer

Glucose-6-phosphatase

Answer

Glucokinase

Answer

Isocitrate dehydrogenase

Answer

Fructose 2, 6 bisphosphatase

Question 2

Which of the following is NOT a product of the Hexose Monophosphate (HMP) shunt?

Accepted Answer

Glycerol-3-phosphate

Answer

Glyceraldehyde 3-phosphate

Answer

3CO2

Answer

6NADPH

Question 3

Which of the following is NOT used in gluconeogenesis?

Accepted Answer

Oleate

Answer

Succinate

Answer

Glutamate

Answer

Aspartate

Question 4

Which of the following glucose transporters is primarily utilized by adipocytes?

Accepted Answer

GLUT4

Answer

GLUT1

Answer

GLUT2

Answer

GLUT3

Question 5

What are the enzymes required for the formation of Phosphoenolpyruvate from Pyruvate?

Accepted Answer

Pyruvate carboxylase and Phosphoenolpyruvate Carboxykinase

Answer

Pyruvate dehydrogenase and Pyruvate carboxylase

Answer

Lactate Dehydrogenase and Pyruvate Dehydrogenase

Answer

Phosphoenolpyruvate carboxykinase and Pyruvate Dehydrogenase

Question 6

All of the following are true about changes in brain metabolism after traumatic brain injury, except:

Accepted Answer

Increased CSF lactate is associated with good prognosis

Answer

Shut down of pyruvate dehydrogenase activity

Answer

Accumulation of lactate in brain

Answer

Increased lactate uptake from circulation

Question 7

Which of the following enzymes becomes deficient following an episode of severe infectious gastroenteritis?

Accepted Answer

Lactase

Answer

Amylase

Answer

Trypsin

Answer

Lipase

Question 8

Dehydrogenases of the HMP shunt are specific for which coenzyme?

Accepted Answer

Nicotinamide adenine dinucleotide phosphate (NADP+)

Answer

Thiamine pyrophosphate (TPP)

Answer

Flavin mononucleotide (FMN)

Answer

Flavin adenine dinucleotide (FAD)

Question 9

Glucagon receptors are NOT found in which organ?

Accepted Answer

Cornea

Answer

Kidney

Answer

Stomach

Answer

Adrenal gland

Question 10

NADPH+ and H+ are generated in the reaction catalyzed by which enzyme?

Accepted Answer

Glucose-6-phosphate dehydrogenase (G-6-PD)

Answer

Lactate dehydrogenase (LDH)

Answer

Glyceraldehyde-3-phosphate dehydrogenase (G-3-PD)

Answer

Alcohol dehydrogenase

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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