Carbohydrate Metabolism Practice Questions

Q: Cane sugar is:

Sucrose. **Explanation:** **Cane sugar** is the common name for **Sucrose**, a disaccharide primarily derived from sugarcane and sugar beets. Chemically, sucrose is composed of one molecule of **α-D-glucose** and one molecule of **β-D-fructose** linked by an **α(1→2) glycosidic bond**. A critical biochemical feature of sucrose is that it is a **non-reducing sugar**. This is because the reducing groups (anomeric carbons) of both glucose and fructose are involved in the glycosidic linkage, leaving no free aldehyde or ketone group to reduce alkaline copper reagents (like Benedict’s or Fehling’s solution). **Analysis of Incorrect Options:** * **A. Glucose:** Known as **Grape sugar** or Dextrose. It is a monosaccharide and the primary fuel for the brain and RBCs. * **C. Fructose:** Known as **Fruit sugar** or Levulose. It is the sweetest naturally occurring sugar and is a ketohexose. * **D. Maltose:** Known as **Malt sugar**. It is a disaccharide of two glucose units (α1→4 linkage) produced during the digestion of starch by amylase. **High-Yield Clinical Pearls for NEET-PG:** * **Invert Sugar:** When sucrose is hydrolyzed (by the enzyme sucrase or acid), the optical rotation changes from dextrorotatory (+66.5°) to levorotatory (-19.7°) due to the strong levorotatory nature of the liberated fructose. This mixture is called "Invert Sugar." * **Hereditary Fructose Intolerance (HFI):** Patients with Aldolase B deficiency must avoid sucrose, as its hydrolysis releases fructose, which can lead to severe hypoglycemia and liver damage. * **Sucrose is the only common disaccharide that does not form osazones** because it lacks a free reducing group.

Q: The uronic acid pathway does not have a role in the formation of which glycosaminoglycan proteoglycan?

Keratan sulfate. **Explanation:** The **Uronic Acid Pathway** is an alternative oxidative pathway for glucose that serves two primary functions: the synthesis of **UDP-Glucuronic acid** and the production of Vitamin C (except in humans). UDP-glucuronic acid is the essential precursor for the synthesis of most Glycosaminoglycans (GAGs). **1. Why Keratan Sulfate is the correct answer:** Keratan sulfate is the **only** major GAG that does not contain a uronic acid (glucuronic or iduronic acid). Instead, it is composed of repeating units of **Galactose** and N-acetylglucosamine. Since the uronic acid pathway is responsible for providing the glucuronic acid building blocks, it plays no role in the synthesis of Keratan sulfate. **2. Why the other options are incorrect:** * **Chondroitin sulfate:** Composed of Glucuronic acid and N-acetylgalactosamine. * **Hyaluronic acid:** Composed of Glucuronic acid and N-acetylglucosamine. * **Heparan sulfate:** Composed of Glucuronic acid (or Iduronic acid) and N-acetylglucosamine. All three require UDP-glucuronic acid derived from the uronic acid pathway for their carbohydrate backbone. **High-Yield Clinical Pearls for NEET-PG:** * **Essential Pentosuria:** A deficiency of the enzyme **L-Xylulose Reductase** in the uronic acid pathway leads to the excretion of L-xylulose in urine. It is a benign condition but gives a positive Benedict’s test (reducing sugar). * **Drug Metabolism:** UDP-glucuronic acid is vital for **conjugation reactions** in the liver, making bilirubin and drugs (like morphine and steroids) more water-soluble for excretion. * **Vitamin C:** Humans cannot synthesize Vitamin C via this pathway due to the absence of the enzyme **L-gulonolactone oxidase**.

Q: Naf inhibits which of the following enzymes?

Enolase. **Explanation:** **1. Why Enolase is the Correct Answer:** Fluoride (specifically Sodium Fluoride, NaF) is a potent inhibitor of **Enolase**, the ninth enzyme in the glycolytic pathway. Enolase catalyzes the conversion of 2-phosphoglycerate to phosphoenolpyruvate (PEP). The inhibition occurs because fluoride ions, in the presence of inorganic phosphate, form a complex with magnesium ions (**Magnesium-Fluorophosphate complex**). Since Enolase requires $Mg^{2+}$ as a cofactor, this complex displaces the magnesium, effectively inactivating the enzyme and halting glycolysis. **2. Why the Other Options are Incorrect:** * **Glucokinase & Hexokinase:** These enzymes catalyze the first step of glycolysis (Glucose to Glucose-6-Phosphate). While they are regulatory enzymes, they are not inhibited by fluoride. Hexokinase is inhibited by its product, G-6-P. * **G-6 PD (Glucose-6-Phosphate Dehydrogenase):** This is the rate-limiting enzyme of the Hexose Monophosphate (HMP) Shunt. It is regulated by the $NADPH/NADP^+$ ratio, not by fluoride. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Blood Glucose Estimation:** In clinical practice, NaF is added to blood collection tubes (Grey-top tubes) used for glucose estimation. It prevents "in vitro" glycolysis by RBCs and WBCs, ensuring the measured glucose level reflects the patient's actual blood sugar at the time of draw. * **Potassium Oxalate:** NaF is usually combined with Potassium Oxalate, which acts as an anticoagulant by chelating calcium. * **Water Fluoridation:** At low concentrations, fluoride prevents dental caries by inhibiting bacterial enolase in oral plaque, reducing acid production. * **Reversibility:** The inhibition of Enolase by fluoride is competitive in the presence of phosphate.

Q: Which glucose transporter is present in the Beta cells of the Islets of Langerhans?

GLUT2. **Explanation:** The correct answer is **GLUT2**. In the Beta cells of the Islets of Langerhans, GLUT2 acts as a "glucose sensor." It has a **high Km** (low affinity) and a **high Vmax** (high capacity), meaning it only transports glucose into the cell when blood glucose levels are elevated. This ensures that insulin secretion is proportional to blood glucose concentrations, maintaining glycemic homeostasis. **Analysis of Options:** * **GLUT1:** Found primarily in **RBCs** and the **Blood-Brain Barrier**. It provides a basal level of glucose uptake required for cellular respiration. * **GLUT3:** Found in **Neurons** and the placenta. It has a very low Km (high affinity), allowing the brain to uptake glucose even during hypoglycemia. * **GLUT4:** The only **insulin-dependent** transporter. It is found in **Skeletal Muscle** and **Adipose Tissue**. In the presence of insulin, GLUT4 translocates from intracellular vesicles to the plasma membrane. **High-Yield Clinical Pearls for NEET-PG:** * **GLUT2 Locations:** Remember the mnemonic **"KILB"** — **K**idney (PCT), **I**ntestine (basolateral side), **L**iver, and **B**eta cells. * **Fanconi-Bickel Syndrome:** A rare glycogen storage disease caused by a congenital defect in the **GLUT2** transporter. * **SGLT vs. GLUT:** SGLT (Sodium-Glucose Linked Transporters) are active transporters (secondary active), whereas GLUTs are passive transporters (facilitated diffusion). * **GLUT5:** Specifically transports **Fructose** and is located in the small intestine and spermatozoa.

Q: In the Krebs cycle, which of the following enzymes catalyses the step in which the first CO2 molecule is released?

Isocitrate dehydrogenase. ### Explanation The Krebs cycle (TCA cycle) is the final common pathway for the oxidation of carbohydrates, lipids, and proteins. The correct answer is **Isocitrate dehydrogenase** because it catalyzes the first of two oxidative decarboxylation steps in the cycle. **1. Why Isocitrate Dehydrogenase is Correct:** In this step, Isocitrate (6C) undergoes oxidation and decarboxylation to form **$\alpha$-ketoglutarate (5C)**. This reaction requires $NAD^+$ as a cofactor and results in the release of the **first molecule of $CO_2$** and the production of NADH. This is also the rate-limiting step of the Krebs cycle. **2. Analysis of Incorrect Options:** * **Aconitase:** This enzyme catalyzes the isomerization of Citrate to Isocitrate via *cis*-aconitate. It is a rearrangement reaction; no carbon is lost as $CO_2$. * **Succinate thiokinase (Succinyl-CoA synthetase):** This enzyme converts Succinyl-CoA to Succinate. This step is significant for **substrate-level phosphorylation** (generating GTP/ATP), but no decarboxylation occurs. * **Succinate dehydrogenase:** This enzyme converts Succinate to Fumarate, reducing $FAD$ to $FADH_2$. It is unique because it is the only TCA enzyme embedded in the inner mitochondrial membrane (part of Complex II of the Electron Transport Chain). **3. High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Isocitrate dehydrogenase is inhibited by high ATP/NADH and stimulated by ADP and $Ca^{2+}$. * **Second $CO_2$ release:** Occurs in the next step, catalyzed by the **$\alpha$-ketoglutarate dehydrogenase complex** (converting 5C to 4C Succinyl-CoA). * **Cofactor Requirement:** The $\alpha$-ketoglutarate dehydrogenase complex requires five cofactors: Thiamine (B1), Riboflavin (B2), Niacin (B3), Pantothenic acid (B5), and Lipoic acid. * **Fluoroacetate:** A potent inhibitor of the TCA cycle that acts on the enzyme **Aconitase**.

Q: Which of the following is a debranching enzyme?

Amylo alpha-1,6-glucosidase. ### Explanation **Correct Option: C. Amylo alpha-1,6-glucosidase** Glycogen debranching is a two-step process required to mobilize glucose from branched glycogen chains. The **Debranching Enzyme** is a single polypeptide with two distinct catalytic activities: 1. **4-alpha-glucanotransferase:** Transfers a trisaccharide unit from a branch to a nearby linear chain. 2. **Amylo alpha-1,6-glucosidase:** Hydrolytically cleaves the remaining single glucose residue attached by an $\alpha(1\to6)$ linkage, releasing **free glucose**. This specific activity defines the "debranching" step. --- ### Why the other options are incorrect: * **A. Glycogen synthetase:** This is the rate-limiting enzyme for **glycogenesis** (glycogen synthesis). It catalyzes the formation of $\alpha(1\to4)$ glycosidic bonds. * **B. Glucose-6-phosphatase:** Found in the liver and kidneys, this enzyme converts Glucose-6-Phosphate to free glucose. It is the final step of both glycogenolysis and gluconeogenesis but does not act on the glycogen polymer itself. * **D. Amylo (1,4)-(1,6) transglycosylase:** Also known as the **Branching Enzyme**, it creates $\alpha(1\to6)$ linkages during glycogen synthesis. --- ### NEET-PG High-Yield Pearls: * **Cori’s Disease (GSD Type III):** Caused by a deficiency of the debranching enzyme. It presents with hepatomegaly, hypoglycemia, and accumulation of "limit dextrins" (abnormally short outer branches). * **Product Ratio:** Glycogenolysis yields **Glucose-1-Phosphate** (from Phosphorylase) and **Free Glucose** (from Debranching enzyme) in a ratio of approximately 10:1. * **Von Gierke’s Disease (GSD Type I):** Deficiency of Glucose-6-phosphatase; characterized by severe fasting hypoglycemia and hyperuricemia.

Q: Mucopolysaccharide hyaluronic acid is present in which of the following?

Vitreous humor. **Explanation:** Hyaluronic acid (Hyaluronan) is a unique **non-sulfated glycosaminoglycan (GAG)**. Unlike other GAGs, it is not covalently linked to a protein core and is not synthesized in the Golgi but at the plasma membrane. Its primary function is to serve as a lubricant and shock absorber due to its high water-binding capacity. * **Correct Answer (A):** Hyaluronic acid is found in high concentrations in the **vitreous humor** of the eye, where it maintains the gel-like consistency and optical clarity. It is also a major component of **synovial fluid** (joint lubrication) and the **umbilical cord** (Wharton’s jelly). * **Option B (Cornea):** While the cornea contains GAGs, the predominant types are **Keratan sulfate I** and **Dermatan sulfate**, which are essential for maintaining corneal transparency. * **Option C (Blood vessels):** The vascular wall primarily contains **Heparan sulfate** and **Dermatan sulfate**, which play roles in anticoagulation and structural integrity. * **Option D (Lens):** The lens is composed of specialized proteins called crystallins; it does not contain significant amounts of mucopolysaccharides like hyaluronic acid. **High-Yield Facts for NEET-PG:** 1. **Structure:** It consists of repeating units of **D-glucuronic acid** and **N-acetylglucosamine**. 2. **Enzyme:** It is degraded by **Hyaluronidase**, an enzyme found in high concentrations in mammalian sperm (to penetrate the ovum) and certain bacteria (to facilitate tissue spread). 3. **Clinical Link:** Hyaluronic acid is used clinically in intra-articular injections for osteoarthritis and as a dermal filler in cosmetic surgery. 4. **Tumor Marker:** Elevated levels are sometimes seen in Wilms' tumor and mesothelioma.

Q: Which enzyme is stimulated in gluconeogenesis during starvation?

Carboxylase. **Explanation:** In starvation, the body must maintain blood glucose levels through **gluconeogenesis**. The correct answer is **Pyruvate Carboxylase**, which is the first regulatory enzyme of this pathway. **1. Why Carboxylase is Correct:** During starvation, high levels of **Acetyl-CoA** (from fatty acid oxidation) act as an obligatory allosteric activator of **Pyruvate Carboxylase**. This enzyme converts pyruvate into oxaloacetate (OAA) in the mitochondria. This is a crucial "bypass" step to overcome the irreversible nature of the glycolytic enzyme pyruvate kinase, effectively shunting substrates toward glucose synthesis. **2. Why the Other Options are Incorrect:** * **Pyruvate Dehydrogenase (PDH):** This enzyme converts pyruvate to Acetyl-CoA for the TCA cycle. In starvation, PDH is **inhibited** by Acetyl-CoA and NADH to conserve three-carbon compounds for glucose production. * **Pyruvate Kinase:** This is a glycolytic enzyme. During starvation, it is **inhibited** by glucagon-mediated phosphorylation (via cAMP) to prevent the breakdown of phosphoenolpyruvate (PEP), ensuring PEP is used for gluconeogenesis instead. * **Glucokinase:** This enzyme functions in the liver to trap glucose during the well-fed state. In starvation, its activity is **decreased** (low insulin/high glucagon) to prevent the liver from consuming the glucose it is trying to produce. **High-Yield NEET-PG Pearls:** * **Biotin (Vitamin B7):** Pyruvate Carboxylase requires Biotin as a co-factor. Remember: "All carboxylases require Biotin." * **Subcellular Localization:** Pyruvate Carboxylase is located in the **mitochondria**, whereas the rest of gluconeogenesis occurs primarily in the cytosol. * **Key Activator:** Acetyl-CoA is the most important metabolic signal that switches the cell from glucose oxidation to glucose synthesis.

Question 1

Cane sugar is:

Accepted Answer

Sucrose

Answer

Glucose

Answer

Fructose

Answer

Maltose

Question 2

Deficiencies in the enzyme glucose-6-phosphatase are likely to lead to which of the following?

Accepted Answer

Decreased skeletal muscle glycogen accumulation

Answer

Decreased glucagon production

Answer

Hyperglycemia

Answer

Increased hepatic glycogen accumulation

Question 3

The uronic acid pathway does not have a role in the formation of which glycosaminoglycan proteoglycan?

Accepted Answer

Keratan sulfate

Answer

Chondroitin sulfate

Answer

Hyaluronic acid

Answer

Heparan sulfate

Question 4

Which of the following glycolytic enzymes is NOT used in gluconeogenesis?

Accepted Answer

Aldolase

Answer

Glucokinase

Answer

Pyruvate kinase

Answer

Phosphofructokinase

Question 5

Naf inhibits which of the following enzymes?

Accepted Answer

Enolase

Answer

Glucokinase

Answer

Hexokinase

Answer

G-6 PD

Question 6

Which glucose transporter is present in the Beta cells of the Islets of Langerhans?

Accepted Answer

GLUT2

Answer

GLUT1

Answer

GLUT3

Answer

GLUT4

Question 7

In the Krebs cycle, which of the following enzymes catalyses the step in which the first CO2 molecule is released?

Accepted Answer

Isocitrate dehydrogenase

Answer

Aconitase

Answer

Succinate thiokinase

Answer

Succinate dehydrogenase

Question 8

Which of the following is a debranching enzyme?

Accepted Answer

Amylo alpha-1,6-glucosidase

Answer

Glycogen synthetase

Answer

Glucose-6-phosphatase

Answer

Amylo (1,4)-(1,6) transglycosylase

Question 9

Mucopolysaccharide hyaluronic acid is present in which of the following?

Accepted Answer

Vitreous humor

Answer

Cornea

Answer

Blood vessels

Answer

Lens

Question 10

Which enzyme is stimulated in gluconeogenesis during starvation?

Accepted Answer

Carboxylase

Answer

Pyruvate dehydrogenase

Answer

Pyruvate kinase

Answer

Glucokinase

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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