Carbohydrate Metabolism Practice Questions

Q: Enzyme deficiency in Tarui disease is?

Muscle and erythrocyte phosphofructokinase 1. **Explanation:** **Tarui Disease (Glycogen Storage Disease Type VII)** is caused by a deficiency of the enzyme **Phosphofructokinase-1 (PFK-1)**. PFK-1 is the rate-limiting enzyme of glycolysis. In this disease, the M-isoform (found in muscles and erythrocytes) is affected. This leads to an inability to utilize glucose for energy in muscles, resulting in glycogen accumulation and exercise intolerance. **Analysis of Options:** * **Option B (Correct):** PFK-1 deficiency impairs the conversion of Fructose-6-Phosphate to Fructose-1,6-Bisphosphate. This blocks glycolysis, leading to muscle cramps and exercise-induced myoglobinuria. Hemolysis also occurs because erythrocytes rely solely on glycolysis for ATP. * **Option A (Incorrect):** Deficiency of **Glucose-6-phosphatase** causes **Von Gierke Disease (GSD Type I)**, characterized by severe fasting hypoglycemia and hepatomegaly. * **Option C (Incorrect):** Deficiency of **Lysosomal α-1,4 and α-1,6 glucosidase (Acid Maltase)** causes **Pompe Disease (GSD Type II)**, which primarily affects the heart (cardiomegaly). * **Option D (Incorrect):** Deficiency of **Liver phosphorylase kinase** causes **Hers Disease (GSD Type VI)** or **Type IX**, leading to hepatomegaly and growth retardation. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Similar to McArdle disease (Type V) but presents earlier in life and is accompanied by **hemolytic anemia** (due to erythrocyte PFK deficiency) and **hyperuricemia**. * **The "Out of Wind" Phenomenon:** Unlike McArdle disease (which has a "second wind" phenomenon), Tarui patients experience worsening symptoms after a high-carbohydrate meal because glucose further inhibits fatty acid oxidation, the only remaining energy source. * **Biochemical Marker:** Elevated levels of Fructose-6-phosphate and Glucose-6-phosphate in muscle biopsies.

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: 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: 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.

Q: Where is GLUT 4 primarily found?

Cardiac muscle. **Explanation:** The correct answer is **Cardiac muscle**. GLUT 4 is the only glucose transporter that is **insulin-dependent**. It is primarily sequestered in intracellular vesicles and translocates to the cell membrane only in the presence of insulin or during exercise. It is found in tissues that require rapid glucose uptake following a meal: **Skeletal muscle, Cardiac muscle, and Adipose tissue.** **Analysis of Options:** * **A. Endothelium:** The blood-brain barrier and vascular endothelium primarily utilize **GLUT 1**, which provides basal glucose uptake independent of insulin. * **B. Liver:** The liver (along with pancreatic beta cells and the kidney) utilizes **GLUT 2**. This is a high-capacity, low-affinity transporter that functions as a "glucose sensor." * **D. Lens:** The lens and cornea primarily utilize **GLUT 1** for insulin-independent glucose uptake. In states of hyperglycemia (Diabetes), excess glucose in the lens is converted to sorbitol via the polyol pathway, leading to cataracts. **High-Yield Clinical Pearls for NEET-PG:** * **GLUT 1:** Found in RBCs, Blood-Brain Barrier, and Retina. (Mnemonic: **1** is for **B**lood). * **GLUT 2:** Bidirectional transporter. Found in **L**iver, **I**slets (Beta cells), **K**idney, and **S**mall Intestine (basolateral membrane). (Mnemonic: **LIKS**). * **GLUT 3:** Found in **Neurons** (highest affinity for glucose to protect the brain during hypoglycemia). * **GLUT 4:** The only **Insulin-responsive** transporter. (Mnemonic: Muscle and Fat). * **GLUT 5:** Specifically a **Fructose** transporter found in the small intestine and spermatozoa. * **SGLT 1/2:** Sodium-dependent active transporters found in the small intestine and renal tubules respectively.

Q: Essential pentosuria is due to deficiency of which enzyme?

Xylulose reductase. **Explanation:** **Essential Pentosuria** is a rare, benign autosomal recessive metabolic disorder involving the **Uronic Acid Pathway**. 1. **Correct Answer: C. Xylulose reductase** In the uronic acid pathway, glucuronic acid is converted to L-xylulose. Under normal conditions, the enzyme **L-xylulose reductase** (also known as NADP-linked xylulose reductase) reduces L-xylulose to xylitol. A deficiency of this enzyme leads to the accumulation of L-xylulose in the blood and its subsequent excretion in the urine. Since L-xylulose is a reducing sugar, it gives a positive Benedict’s test, often leading to a misdiagnosis of diabetes mellitus. 2. **Why other options are incorrect:** * **A. Gulonolactone oxidase:** This enzyme converts L-gulonolactone to Vitamin C (Ascorbic acid). Humans lack this enzyme, making Vitamin C an essential dietary requirement (Evolutionary deficiency). * **B. Phosphoglucomutase:** This enzyme interconverts Glucose-1-Phosphate and Glucose-6-Phosphate. It is critical for glycogenesis and glycogenolysis, not the uronic acid pathway. * **D. Fructokinase:** Deficiency of this enzyme causes **Essential Fructosuria**, characterized by fructose in the urine. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Asymptomatic; usually an incidental finding of a reducing sugar in urine. * **Biochemical Marker:** High levels of **L-xylulose** in urine. * **Drug Interaction:** Administration of drugs like **Aminopyrine** or **Barbital** can increase the excretion of L-xylulose in patients with pentosuria by inducing the uronic acid pathway. * **Distinction:** Unlike Diabetes, blood glucose levels and Glucose Tolerance Tests (GTT) are normal in these patients.

Question 1

Enzyme deficiency in Tarui disease is?

Accepted Answer

Muscle and erythrocyte phosphofructokinase 1

Answer

Glucose-6-phosphatase

Answer

Lysosomal a1 - 4 and a1 - 6 glucosidase

Answer

Liver phosphorylase kinase

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

Which of the following prevents lens opacity by free radical scavenging?

Accepted Answer

Glutathione

Answer

Catalase

Answer

Vitamin A

Answer

Vitamin E

Question 4

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 5

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

Accepted Answer

Aldolase

Answer

Glucokinase

Answer

Pyruvate kinase

Answer

Phosphofructokinase

Question 6

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 7

Mucopolysaccharide hyaluronic acid is present in which of the following?

Accepted Answer

Vitreous humor

Answer

Cornea

Answer

Blood vessels

Answer

Lens

Question 8

Which enzyme is stimulated in gluconeogenesis during starvation?

Accepted Answer

Carboxylase

Answer

Pyruvate dehydrogenase

Answer

Pyruvate kinase

Answer

Glucokinase

Question 9

Where is GLUT 4 primarily found?

Accepted Answer

Cardiac muscle

Answer

Endothelium

Answer

Liver

Answer

Lens

Question 10

Essential pentosuria is due to deficiency of which enzyme?

Accepted Answer

Xylulose reductase

Answer

Gulonolactone oxidase

Answer

Phosphoglucomutase

Answer

Fructokinase

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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