Carbohydrate Metabolism Practice Questions

Q: Which of the following pairs represent epimers?

D-glucose & D-mannose. **Explanation** **1. Understanding the Concept: Epimers** Epimers are a specific type of diastereomer (isomers) that differ in configuration around only **one** specific carbon atom (other than the anomeric carbon). In the context of hexoses, this usually refers to the orientation of the hydroxyl (-OH) group. **2. Why D-glucose & D-mannose is correct** D-glucose and D-mannose are **C-2 epimers**. They are identical in every way except for the configuration at the second carbon (C2). In D-glucose, the -OH group at C2 is on the right, whereas in D-mannose, it is on the left. **3. Analysis of Incorrect Options** * **Option A: D-glucose & D-fructose:** These are **functional isomers**. Glucose is an aldose (contains an aldehyde group), while fructose is a ketose (contains a keto group). They have the same molecular formula but different functional groups. * **Option B: D-mannose & D-talose:** These are **C-4 epimers**. While they are epimers, they are not the classic pair usually tested in the context of glucose metabolism. (Note: D-glucose and D-galactose are the more high-yield C-4 epimers). **4. High-Yield Clinical Pearls for NEET-PG** * **C-2 Epimer:** Glucose and Mannose. * **C-4 Epimer:** Glucose and Galactose. * **Enzyme Note:** The interconversion of epimers is catalyzed by enzymes called **epimerases** (e.g., UDP-glucose 4-epimerase in galactose metabolism). * **Mnemonic:** Remember **"M2G4"**—**M**annose is **2**, **G**alactose is **4** (referring to the carbon position of epimerism relative to glucose).

Q: Fluoroacetate inhibits which of the following metabolic processes?

TCA cycle. **Explanation:** Fluoroacetate is a potent metabolic poison that acts as a competitive inhibitor of the **TCA cycle (Krebs cycle)**. **Why Option A is Correct:** Fluoroacetate itself is not toxic; however, it undergoes "lethal synthesis" within the mitochondria. It is converted into **fluoroacetyl-CoA**, which condenses with oxaloacetate to form **fluorocitrate**. Fluorocitrate is a powerful inhibitor of the enzyme **Aconitase**. This blockade prevents the conversion of citrate to isocitrate, leading to a buildup of citrate and a complete halt of the TCA cycle, resulting in cellular energy failure. **Why Other Options are Incorrect:** * **B. Glycolytic pathway:** Glycolysis is primarily inhibited by **Fluoride** (which inhibits Enolase) or **Iodoacetate** (which inhibits Glyceraldehyde-3-phosphate dehydrogenase), but not by fluoroacetate. * **C. Oxidative phosphorylation:** This refers to the synthesis of ATP via ATP synthase. Inhibitors include **Oligomycin**. * **D. Electron Transport Chain (ETC):** The ETC is inhibited by substances like **Rotenone** (Complex I), **Antimycin A** (Complex III), and **Cyanide/Carbon Monoxide** (Complex IV). **High-Yield Clinical Pearls for NEET-PG:** * **Lethal Synthesis:** This is the classic example of a non-toxic substance being converted into a toxic metabolite (Fluoroacetate → Fluorocitrate). * **Aconitase:** A non-heme iron-sulfur (Fe-S) protein. Its inhibition leads to **citrate accumulation**, which can further inhibit Phosphofructokinase-1 (PFK-1), indirectly slowing glycolysis. * **Source:** Fluoroacetate is found in certain plants and is used as a rodenticide (Compound 1080).

Q: Which of the following is NOT a component of the Citric acid cycle?

Malonate. ### Explanation The **Citric Acid Cycle (TCA cycle)** is a series of enzymatic reactions occurring in the mitochondrial matrix that oxidizes acetyl-CoA to CO₂ and H₂O. **Why Malonate is the correct answer:** **Malonate** is a potent **competitive inhibitor** of the enzyme succinate dehydrogenase. It is a structural analog of succinate; it binds to the enzyme's active site but cannot be oxidized, thereby halting the cycle. Because it acts as an inhibitor rather than a functional intermediate or enzyme within the pathway, it is not a component of the cycle itself. **Analysis of incorrect options:** * **Fumarase (Option A):** An enzyme that catalyzes the reversible hydration of fumarate to L-malate. * **Succinate dehydrogenase (Option C):** A unique enzyme that participates in both the TCA cycle and the Electron Transport Chain (Complex II). It converts succinate to fumarate. * **Alpha-ketoglutarate dehydrogenase (Option D):** A multi-enzyme complex that converts $\alpha$-ketoglutarate to succinyl-CoA. It requires five cofactors (Thiamine, Lipoic acid, CoA, FAD, and NAD+). **High-Yield Clinical Pearls for NEET-PG:** 1. **Competitive Inhibition:** Malonate inhibition of succinate dehydrogenase is the classic textbook example of competitive inhibition (increases $K_m$, no change in $V_{max}$). 2. **Fluoroacetate:** Known as a "suicide inhibitor," it is converted to fluorocitrate, which inhibits the enzyme **aconitase**. 3. **Arsenite Poisoning:** Arsenite inhibits the $\alpha$-ketoglutarate dehydrogenase complex by binding to the SH groups of **lipoic acid**, leading to a buildup of lactate and pyruvate. 4. **ATP Yield:** One turn of the TCA cycle generates **10 ATP** equivalents (3 NADH = 7.5, 1 FADH₂ = 1.5, 1 GTP = 1).

Q: UDP-Glucose is converted to UDP galactose by which enzyme?

Epimerase. **Explanation:** The conversion of **UDP-Glucose to UDP-Galactose** is a crucial step in galactose metabolism. This reaction is catalyzed by the enzyme **UDP-glucose-4-epimerase** (also known as GALE). **Why Epimerase is correct:** Glucose and Galactose are **C-4 epimers**, meaning they differ in configuration only at the fourth carbon atom. The enzyme epimerase facilitates the interconversion between these two sugars by shifting the hydroxyl group at the C-4 position. This reaction is reversible and is essential for: 1. Converting dietary galactose into glucose for energy. 2. Synthesizing UDP-galactose from UDP-glucose for the production of lactose, glycoproteins, and glycolipids when dietary galactose is unavailable. **Why other options are incorrect:** * **Lactose synthetase:** This enzyme complex (consisting of galactosyltransferase and α-lactalbumin) uses UDP-galactose and glucose to synthesize **Lactose** in the mammary glands. It does not convert UDP-glucose to UDP-galactose. * **Pyruvate kinase:** This is a key regulatory enzyme in **Glycolysis** that converts phosphoenolpyruvate (PEP) to pyruvate; it has no role in sugar nucleotide interconversion. **High-Yield Clinical Pearls for NEET-PG:** * **Galactosemia Type III:** A deficiency of UDP-glucose-4-epimerase leads to Epimerase Deficiency Galactosemia. * **Classic Galactosemia:** Caused by a deficiency of **GALT** (Galactose-1-phosphate uridyltransferase), which is the most common and severe form. * **Mnemonic:** "Glucose and Galactose are **4**-ever epimers" (to remember the C-4 position).

Q: Which of the following is normally present in the urine of a pregnant woman in the third trimester?

Glucose. **Explanation:** The correct answer is **Glucose (Option A)**. **Underlying Medical Concept:** During the third trimester of pregnancy, there is a physiological increase in the **Glomerular Filtration Rate (GFR)** by approximately 50%. Simultaneously, the renal threshold for glucose reabsorption decreases due to hormonal changes and increased load. This combination leads to **physiological glucosuria**, where glucose is excreted in the urine even in the presence of normal blood glucose levels. While it is common and often benign, it must be clinically differentiated from Gestational Diabetes Mellitus (GDM). **Analysis of Incorrect Options:** * **Lactose (Option B):** While "Lactosuria" can occur in the very late stages of pregnancy or during lactation due to milk production in the mammary glands, **Glucose** is the more consistent physiological finding in the urine during the third trimester due to renal hemodynamics. * **Galactose (Option C):** Galactosuria is typically associated with Galactosemia (an inborn error of metabolism) and is not a normal physiological finding in pregnancy. * **Fructose (Option D):** Fructosuria is usually seen in Essential Fructosuria (Fructokinase deficiency) or after excessive intake of fruits/honey; it is not a feature of normal pregnancy. **High-Yield Clinical Pearls for NEET-PG:** * **Renal Threshold for Glucose:** In non-pregnant adults, it is ~180 mg/dL. In pregnancy, this threshold significantly **drops**, making glucosuria a common finding. * **Benedict’s Test:** This test detects all reducing sugars (Glucose, Lactose, Galactose, Fructose). If a pregnant woman’s urine is positive for Benedict’s but negative for Glucose Oxidase (Dipstick), consider **Lactosuria**. * **GFR in Pregnancy:** Increases early in the first trimester and peaks in the third, leading to increased clearance of creatinine and urea.

Q: GLUT-2 is primarily expressed in which of the following tissues?

Pancreas. **Explanation:** **GLUT-2** is a high-capacity, low-affinity glucose transporter (high $K_m$). Its primary role is to act as a **glucose sensor**. It is expressed in the **Pancreatic beta-cells**, Liver, Kidney, and the basolateral membrane of the Small Intestine. In the pancreas, GLUT-2 allows glucose entry proportional to blood glucose levels, triggering insulin secretion. **Analysis of Options:** * **A. Pancreas (Correct):** GLUT-2 serves as the glucose sensor in beta-cells. Because of its high $K_m$, it only transports glucose into the cell when blood sugar levels are high, ensuring insulin is released only when needed. * **B & C. Adipose tissue and Skeletal muscles:** These tissues primarily express **GLUT-4**, which is the only **insulin-dependent** glucose transporter. Insulin triggers the translocation of GLUT-4 from intracellular vesicles to the plasma membrane. * **D. Brain:** The brain primarily utilizes **GLUT-1** and **GLUT-3**. These are high-affinity (low $K_m$) transporters that ensure a constant supply of glucose to neurons even during fasting or hypoglycemia. **High-Yield Clinical Pearls for NEET-PG:** * **GLUT-1:** Found in RBCs and Blood-Brain Barrier. Deficiency leads to De Vivo syndrome (infantile seizures). * **GLUT-2:** Involved in **Fanconi-Bickel Syndrome** (a glycogen storage disease caused by GLUT-2 mutations). * **GLUT-4:** The only insulin-responsive transporter; its recruitment is increased by **exercise** in skeletal muscles. * **GLUT-5:** Specifically functions as a **fructose** transporter, primarily located in the small intestine and spermatozoa. * **SGLT-1/2:** These are active transporters (sodium-dependent) found in the intestine and kidneys, unlike the GLUT family which facilitates passive diffusion.

Question 1

Which of the following pairs represent epimers?

Accepted Answer

D-glucose & D-mannose

Answer

D-glucose & D-fructose

Answer

D-mannose & D-talose

Answer

None of these

Question 2

Fluoroacetate inhibits which of the following metabolic processes?

Accepted Answer

TCA cycle

Answer

Glycolytic pathway

Answer

Oxidative phosphorylation

Answer

Electron Transport Chain (ETC)

Question 3

Which of the following is NOT a component of the Citric acid cycle?

Accepted Answer

Malonate

Answer

Fumarase

Answer

Succinate dehydrogenase

Answer

Alpha-ketoglutarate dehydrogenase

Question 4

Which of the following is an intermediate metabolite in the Tricarboxylic Acid (TCA) cycle?

Accepted Answer

Pyruvate

Answer

Isocitrate

Answer

Oxaloacetate

Answer

Malonate

Question 5

UDP-Glucose is converted to UDP galactose by which enzyme?

Accepted Answer

Epimerase

Answer

Lactose synthetase

Answer

Pyruvate kinase

Answer

All of the above

Question 6

The TCA cycle does not take place in which of the following cell types?

Accepted Answer

Erythrocytes

Answer

Hepatocytes

Answer

Osteocytes

Answer

Neurons

Question 7

Which of the following is normally present in the urine of a pregnant woman in the third trimester?

Accepted Answer

Glucose

Answer

Lactose

Answer

Galactose

Answer

Fructose

Question 8

A young man finds that every time he eats dairy products he feels very uncomfortable. His stomach becomes distended. He develops gas and diarrhea frequently. These symptoms do not appear when he eats foods other than dairy products. Which of the following is the most likely enzyme in which this young man is deficient?

Accepted Answer

sucrase

Answer

alpha-amylase

Answer

beta-galactosidase

Answer

alpha-glucosidase

Question 9

GLUT-2 is primarily expressed in which of the following tissues?

Accepted Answer

Pancreas

Answer

Adipose tissue

Answer

Skeletal muscles

Answer

Brain

Question 10

Glycosylated hemoglobin in a normal pregnant lady should be less than:

Accepted Answer

6%

Answer

4%

Answer

5%

Answer

7%

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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