Carbohydrate Metabolism Practice Questions

Q: Which enzyme is common to both glycogenolysis and glycogenesis?

Phosphoglucomutase. **Explanation:** The correct answer is **Phosphoglucomutase**. This enzyme plays a pivotal role in carbohydrate metabolism by catalyzing the reversible interconversion of **Glucose-1-Phosphate (G-1-P)** and **Glucose-6-Phosphate (G-6-P)**. 1. **Why Phosphoglucomutase is correct:** * **In Glycogenesis:** After glucose is phosphorylated to G-6-P by hexokinase/glucokinase, Phosphoglucomutase converts G-6-P into G-1-P. G-1-P is then activated to UDP-glucose to be added to the glycogen chain. * **In Glycogenolysis:** Glycogen phosphorylase releases G-1-P from glycogen. Phosphoglucomutase then converts this G-1-P back into G-6-P, which can enter glycolysis (in muscle) or be dephosphorylated to free glucose (in the liver). 2. **Why other options are incorrect:** * **Glycogen phosphorylase:** The rate-limiting enzyme for **glycogenolysis** only. It breaks $\alpha$-1,4-glycosidic bonds. * **Glycogen synthase:** The rate-limiting enzyme for **glycogenesis** only. It creates $\alpha$-1,4-glycosidic bonds. * **Glucotransferase (Debranching enzyme):** Involved specifically in **glycogenolysis** to handle branch points ($\alpha$-1,6 bonds). **NEET-PG High-Yield Pearls:** * **Rate-limiting steps:** Glycogen Synthase (Synthesis) vs. Glycogen Phosphorylase (Breakdown). * **Cofactor:** Glycogen phosphorylase requires **Pyridoxal Phosphate (Vitamin B6)**. * **Von Gierke’s Disease:** Deficiency of Glucose-6-Phosphatase; it affects the final step of glucose release in the liver but is not part of the glycogen polymer synthesis/breakdown itself. * **Key Intermediate:** UDP-Glucose is the "active" form of glucose used in glycogenesis.

Q: What is the main site of fluoride inhibition in the Embden-Meyerhof pathway?

Enolase. **Explanation:** **Correct Option: B. Enolase** The Embden-Meyerhof pathway (Glycolysis) is inhibited by fluoride at the step where **2-phosphoglycerate** is converted to **phosphoenolpyruvate (PEP)**. This reaction is catalyzed by the enzyme **Enolase**. Fluoride acts as a competitive inhibitor by forming a complex with magnesium ($Mg^{2+}$) and inorganic phosphate ($P_i$), which then binds to the active site of the enzyme. Since Enolase requires $Mg^{2+}$ as a cofactor, the formation of the **magnesium-fluorophosphate complex** effectively removes the available magnesium and blocks the enzyme's activity. **Analysis of Incorrect Options:** * **A. ATPase:** This enzyme is involved in ATP hydrolysis and energy transport, not the glycolytic pathway. While some ATPases are inhibited by substances like Ouabain, they are not the target of fluoride in glycolysis. * **C. Pyruvate kinase:** This is the final enzyme of glycolysis. While it is a regulated step, it is not inhibited by fluoride. * **D. Fructose-6-phosphatase:** This is a gluconeogenic enzyme (not part of the Embden-Meyerhof pathway) that converts Fructose-1,6-bisphosphate back to Fructose-6-phosphate. **Clinical Pearls & High-Yield Facts:** * **Blood Glucose Estimation:** In clinical practice, blood samples for glucose estimation are collected in **grey-topped vials** containing **Sodium Fluoride (NaF)**. This inhibits glycolysis in RBCs, preventing the artificial lowering of glucose levels before the sample reaches the lab. * **Anticoagulant Pair:** NaF is usually paired with **Potassium Oxalate**, which acts as the anticoagulant by chelating calcium. * **Reversibility:** The inhibition of Enolase by fluoride is reversible if the concentration of magnesium is significantly increased.

Q: Which of the following sugars is not synthesized or utilized in the human body?

L-fructose. **Explanation:** The human body primarily utilizes **D-isomers** of sugars (like D-glucose and D-fructose) for metabolism. While most sugars in our body belong to the D-series, there are specific exceptions where L-isomers are synthesized and utilized. **L-fructose**, however, is neither synthesized nor metabolized by human enzymes and has no physiological role in the body. **Analysis of Options:** * **L-fructose (Correct):** This is a synthetic sugar not found in human metabolic pathways. Human hexokinase and other glycolytic enzymes are stereospecific for D-fructose. * **L-fucose:** This is a crucial exception to the "D-sugar rule." It is an L-isomer sugar synthesized in the body and is a vital component of **glycoproteins** and **blood group substances** (H-substance). * **D-Glucose:** The primary metabolic fuel for the human body, especially for the brain and RBCs. * **D-Fructose:** A common dietary monosaccharide (found in fruits and honey) metabolized via the fructokinase pathway in the liver. **High-Yield Clinical Pearls for NEET-PG:** * **The "L-Sugar" Exceptions:** While most human sugars are D-isomers, remember these two L-isomers: **L-fucose** (found in glycoproteins) and **L-xylulose** (excreted in excess in Essential Pentosuria due to deficiency of L-xylulose reductase). * **Stereospecificity:** Enzymes are highly stereospecific; for example, glucose oxidase reacts only with D-glucose, not L-glucose. * **L-fucose Source:** It is derived from GDP-mannose. Deficiency in its transport leads to **Leukocyte Adhesion Deficiency Type II**.

Q: Which glucose transporter (GLUT) is primarily responsible for transporting glucose from the intestine to the liver?

GLUT2. **Explanation:** The correct answer is **GLUT2**. **Why GLUT2 is correct:** GLUT2 is a high-capacity, low-affinity (high $K_m$) bidirectional transporter. It is primarily located in the **basolateral membrane of intestinal mucosal cells**, the liver, pancreatic beta cells, and the renal tubular cells. After glucose is absorbed from the intestinal lumen via SGLT-1, it exits the enterocyte into the portal circulation via GLUT2. Because of its high $K_m$, GLUT2 acts as a "glucose sensor," ensuring that glucose uptake by the liver is proportional to blood glucose levels, especially in the postprandial (fed) state. **Why other options are incorrect:** * **GLUT1:** This is a high-affinity transporter found in almost all tissues, particularly **RBCs and the Blood-Brain Barrier**. It provides basal glucose uptake. * **GLUT3:** This is the primary transporter in **neurons**. It has a very low $K_m$, ensuring the brain receives glucose even during hypoglycemia. * **GLUT4:** This is the only **insulin-dependent** transporter. It is located in **skeletal muscle and adipose tissue**. In the absence of insulin, it remains sequestered in intracellular vesicles. **High-Yield Clinical Pearls for NEET-PG:** * **SGLT-1 vs. GLUT2:** Remember that SGLT-1 (Sodium-Glucose Co-transporter 1) handles the *active* uptake from the intestinal lumen, while GLUT2 handles the *facilitated* exit into the blood. * **Fanconi-Bickel Syndrome:** A rare glycogen storage disease caused by a congenital defect in **GLUT2**, leading to hepatomegaly and glucose intolerance. * **Bidirectionality:** GLUT2 is unique because it allows glucose to enter the liver for glycogenesis and exit the liver during gluconeogenesis/glycogenolysis.

Q: Which enzyme is responsible for the postprandial utilization of glucose?

Glucokinase. **Explanation:** The correct answer is **Glucokinase (Hexokinase IV)**. **Why Glucokinase is the correct answer:** Glucokinase is primarily located in the liver and pancreatic beta cells. Its unique kinetic properties make it ideal for the **postprandial (fed) state**: 1. **High $K_m$ (Low affinity):** It only becomes significantly active when blood glucose levels are high (e.g., after a meal). 2. **High $V_{max}$ (High capacity):** It can rapidly phosphorylate large amounts of glucose, allowing the liver to "clear" glucose from the portal blood and store it as glycogen, preventing postprandial hyperglycemia. 3. **Lack of Product Inhibition:** Unlike hexokinase, it is not inhibited by its product (Glucose-6-Phosphate), allowing continuous glucose uptake even when energy levels are high. **Why other options are incorrect:** * **Hexokinase (Types I-III):** These are found in extrahepatic tissues. They have a **low $K_m$ (high affinity)**, meaning they are already saturated at fasting glucose levels. Their role is to ensure tissues like the brain get glucose even during starvation, not to manage a postprandial surplus. * **Fructokinase:** This enzyme is specific to fructose metabolism (converting fructose to fructose-1-phosphate) and does not utilize glucose. **High-Yield Clinical Pearls for NEET-PG:** * **Molecular Sensor:** Glucokinase acts as the "glucose sensor" in the pancreas for insulin release. * **MODY Type 2:** Mutations in the glucokinase gene lead to Maturity-Onset Diabetes of the Young (MODY) type 2. * **Localization:** Glucokinase is regulated by the **Glucokinase Regulatory Protein (GKRP)**, which sequesters it in the nucleus during fasting. * **Inducibility:** Glucokinase is induced by **Insulin**, further enhancing its role in the fed state.

Q: Which of the following hormones stimulates glucose utilization?

Insulin. **Explanation:** The regulation of blood glucose is a balance between **anabolic (hypoglycemic)** and **catabolic (hyperglycemic)** hormones. **Why Insulin is Correct:** Insulin is the only major anabolic hormone that lowers blood glucose by stimulating **glucose utilization**. It achieves this through three primary mechanisms: 1. **Increased Uptake:** It promotes the translocation of **GLUT-4** transporters to the cell membranes of skeletal muscle and adipose tissue. 2. **Glycolysis:** It activates key enzymes like Phosphofructokinase-1 (PFK-1) and Glucokinase, favoring glucose breakdown. 3. **Glycogenesis:** It stimulates Glycogen Synthase to store glucose as glycogen in the liver and muscles. **Why Other Options are Incorrect:** Options B, C, and D are all **counter-regulatory (diabetogenic) hormones** that increase blood glucose levels: * **Glucagon:** Stimulates hepatic glycogenolysis and gluconeogenesis during fasting states. * **Corticosteroids (Cortisol):** Increase gluconeogenesis and decrease peripheral glucose uptake (insulin resistance) to ensure glucose availability during stress. * **Growth Hormone:** Inhibits glucose uptake in peripheral tissues (anti-insulin effect) and stimulates lipolysis. **High-Yield NEET-PG Pearls:** * **GLUT-4** is the only insulin-dependent glucose transporter (found in heart, skeletal muscle, and adipose tissue). * **Brain and Liver** do not require insulin for glucose uptake (GLUT-1 and GLUT-2 respectively). * **Rate-limiting enzyme of Glycolysis:** Phosphofructokinase-1 (PFK-1), which is induced by insulin via Fructose 2,6-bisphosphate. * **C-peptide** levels can be used to differentiate endogenous insulin production from exogenous insulin injection.

Q: All of the following are glycogen storage disorders except?

All of the above. **Explanation:** This question tests your knowledge of **Glycogen Storage Disorders (GSDs)**, which are a group of inherited metabolic disorders caused by enzyme deficiencies affecting glycogen synthesis or breakdown. **1. Why "All of the above" is correct:** All three options listed are classic examples of Glycogen Storage Disorders. Each represents a deficiency in a specific enzyme required for glycogen metabolism: * **Pompe Disease (GSD Type II):** Caused by a deficiency of **Acid α-1,4-glucosidase** (Acid Maltase). It is unique because it is also a lysosomal storage disorder, leading to glycogen accumulation in the heart and skeletal muscles. * **Anderson Disease (GSD Type IV):** Caused by a deficiency of the **Branching enzyme**. This leads to the formation of abnormal glycogen with long outer chains (amylopectin-like), which triggers an immune response leading to liver cirrhosis. * **Hers Disease (GSD Type VI):** Caused by a deficiency of **Liver Phosphorylase**. This results in the inability to break down glycogen in the liver, leading to hepatomegaly and mild hypoglycemia. **2. Clinical Pearls for NEET-PG:** * **Von Gierke Disease (Type I):** Most common GSD; deficiency of Glucose-6-Phosphatase. Presents with severe hypoglycemia, lactic acidosis, and hyperuricemia. * **Cori Disease (Type III):** Deficiency of Debranching enzyme; presents with milder hypoglycemia than Type I. * **McArdle Disease (Type V):** Deficiency of Muscle Phosphorylase; presents with exercise-induced cramps and myoglobinuria. * **Mnemonic for Types I-VI:** "**V**on **P**ompe **C**orrects **A**nderson's **M**uscular **H**ers" (Von Gierke, Pompe, Cori, Anderson, McArdle, Hers). **Conclusion:** Since Pompe, Anderson, and Hers are all recognized types of GSDs, the correct answer is "All of the above."

Question 1

Which enzyme is common to both glycogenolysis and glycogenesis?

Accepted Answer

Phosphoglucomutase

Answer

Glycogen phosphorylase

Answer

Glycogen synthase

Answer

Glucotransferase

Question 2

Which amino acid cannot be used for glycogen synthesis?

Accepted Answer

Brain

Answer

Muscle

Answer

RBCs

Answer

Kidney

Question 3

What is the main site of fluoride inhibition in the Embden-Meyerhof pathway?

Accepted Answer

Enolase

Answer

ATPase

Answer

Pyruvate kinase

Answer

Fructose-6-phosphatase

Question 4

Which of the following statements about the Hexose Monophosphate (HMP) pathway is FALSE?

Accepted Answer

Ribose-5-phosphate is produced during the oxidative phase of the pathway.

Answer

The oxidative phase of the pathway produces NADPH.

Answer

The pathway does not produce ATP.

Answer

It occurs in the testes, ovaries, placenta, and adrenal cortex.

Question 5

Which of the following sugars is not synthesized or utilized in the human body?

Accepted Answer

L-fructose

Answer

L-fucose

Answer

D-Glucose

Answer

D-Fructose

Question 6

Which glucose transporter (GLUT) is primarily responsible for transporting glucose from the intestine to the liver?

Accepted Answer

GLUT2

Answer

GLUT1

Answer

GLUT3

Answer

GLUT4

Question 7

Which enzyme is responsible for the postprandial utilization of glucose?

Accepted Answer

Glucokinase

Answer

Fructokinase

Answer

Hexokinase

Answer

All of the above

Question 8

ATP is an allosteric regulator of which enzymes?

Accepted Answer

PFK 1 and Pyruvate kinase

Answer

Hexokinase and PFK 1

Answer

PFK 1 and PFK 2

Answer

Glyceraldehyde 3 phosphate dehydrogenase and PFK 1

Question 9

Which of the following hormones stimulates glucose utilization?

Accepted Answer

Insulin

Answer

Growth hormone

Answer

Corticosteroid

Answer

Glucagon

Question 10

All of the following are glycogen storage disorders except?

Accepted Answer

All of the above

Answer

Pompe disease

Answer

Hers disease

Answer

Anderson disease

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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