Carbohydrate Metabolism Practice Questions

Q: In which of the following steps is ATP released?

Phosphoenolpyruvate to pyruvate. ### Explanation In glycolysis, ATP is generated via **substrate-level phosphorylation**. This process involves the direct transfer of a high-energy phosphate group from a metabolic intermediate to ADP. **1. Why Option A is Correct:** The conversion of **Phosphoenolpyruvate (PEP) to Pyruvate** is the final step of glycolysis, catalyzed by the enzyme **Pyruvate Kinase**. PEP contains a high-energy enol-phosphate bond. When this bond is cleaved, the energy released is sufficient to phosphorylate ADP to ATP. Since one glucose molecule produces two PEP molecules, this step yields **2 ATP** per glucose. This is an irreversible, rate-limiting step. **2. Why the Other Options are Incorrect:** * **Option B:** The conversion of Glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate (catalyzed by GAPDH) is an oxidation-reduction reaction. It generates **NADH**, not ATP. * **Option C & D:** These steps (catalyzed by **Phosphofructokinase-1** and **Hexokinase/Glucokinase** respectively) are the "investment phase" of glycolysis. They **consume ATP** rather than releasing it. **High-Yield NEET-PG Pearls:** * **Substrate-Level Phosphorylation (SLP):** In glycolysis, SLP occurs at two steps: 1,3-BPG to 3-Phosphoglycerate (Phosphoglycerate Kinase) and PEP to Pyruvate (Pyruvate Kinase). * **Pyruvate Kinase Deficiency:** This is the second most common cause of enzyme-deficient **hemolytic anemia** (after G6PD deficiency). Without ATP from this step, RBCs cannot maintain their Na+/K+ ATPase pumps, leading to swelling and lysis. * **Arsenic Poisoning:** Arsenate competes with inorganic phosphate in Step B, bypassing the first SLP step and resulting in **zero net ATP** production from glycolysis.

Q: Oxidation of galactose with a strong oxidizing agent produces which of the following?

Mucic acid. **Explanation:** The oxidation of monosaccharides depends on the strength of the oxidizing agent used. When a sugar is treated with a **strong oxidizing agent** (like concentrated Nitric Acid, $HNO_3$), both the aldehyde group at $C_1$ and the primary alcohol group at $C_6$ are oxidized to carboxylic acid groups. This results in the formation of a **dicarboxylic acid** (also known as an aldaric acid). 1. **Why Mucic Acid is correct:** When **Galactose** undergoes oxidation with concentrated $HNO_3$, it forms **Mucic acid** (Galactaric acid). Unlike other aldaric acids, Mucic acid is clinically significant because it is **insoluble in water** and forms crystals. This property is the basis of the "Mucic Acid Test" used to identify galactose in urine. 2. **Why other options are incorrect:** * **Gluconic acid:** This is a monocarboxylic acid formed by the oxidation of only the $C_1$ aldehyde group of Glucose (using a mild oxidizing agent). * **Galacturonic acid:** This is a uronic acid formed when only the $C_6$ primary alcohol of Galactose is oxidized (usually via enzymatic pathways). * **Saccharic acid (Glucaric acid):** This is the dicarboxylic acid produced when **Glucose** is treated with a strong oxidizing agent ($HNO_3$). **High-Yield Clinical Pearls for NEET-PG:** * **Mucic Acid Test:** Used to differentiate galactose from other sugars. The formation of insoluble, glass-like crystals confirms the presence of galactose or lactose. * **Galactosemia:** A deficiency in GALT (Galactose-1-phosphate uridyltransferase) leads to the accumulation of galactose, which can be detected in urine via the Mucic acid test. * **Sorbitol vs. Dulcitol:** Reduction of Glucose yields Sorbitol; reduction of Galactose yields **Dulcitol** (Galactitol), which is implicated in cataract formation in galactosemic patients.

Q: Cataract in galactosemia is caused by the accumulation of which substance?

Galactitol. **Explanation:** The correct answer is **Galactitol (Dulcitol)**. **1. Why Galactitol is correct:** In patients with Galactosemia (due to deficiency of GALT, GALK, or GALE enzymes), galactose levels rise in the blood and tissues. In the lens of the eye, the enzyme **Aldose Reductase** reduces excess galactose into its sugar alcohol form, **Galactitol**. Unlike galactose, galactitol cannot be further metabolized or easily diffuse out of the lens. It acts as an osmotically active substance, drawing water into the lens fibers. This leads to swelling, denaturation of lens proteins, and subsequent opacity, forming a **cataract**. **2. Why other options are incorrect:** * **Sorbitol:** While sorbitol also causes cataracts via the same osmotic mechanism, it is derived from **Glucose** (seen in Diabetes Mellitus), not galactose. * **Galactose:** While galactose is the substrate that accumulates systemically, it is not the direct cause of the cataract; its metabolic byproduct (galactitol) is the culprit due to its osmotic effect. **3. High-Yield Clinical Pearls for NEET-PG:** * **Classic Galactosemia:** Deficiency of **Galactose-1-phosphate uridyltransferase (GALT)**. It presents with "Oil-drop cataracts," hepatosplenomegaly, and jaundice. * **Galactokinase Deficiency (GALK):** A milder form where the primary clinical manifestation is early-onset cataracts without the severe systemic features of GALT deficiency. * **The Polyol Pathway:** This pathway (Aldose Reductase) is responsible for both diabetic cataracts (Glucose $\rightarrow$ Sorbitol) and galactosemic cataracts (Galactose $\rightarrow$ Galactitol). * **Key Enzyme:** Aldose Reductase has a high $K_m$ for glucose/galactose, meaning it only becomes active when sugar levels are significantly elevated.

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.

Question 1

In which of the following steps is ATP released?

Accepted Answer

Phosphoenolpyruvate to pyruvate

Answer

Glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate

Answer

Fructose-6-phosphate to fructose-1,6-bisphosphate

Answer

Glucose to glucose-6-phosphate

Question 2

Oxidation of galactose with a strong oxidizing agent produces which of the following?

Accepted Answer

Mucic acid

Answer

Gluconic acid

Answer

Galacturonic acid

Answer

Saccharic acid

Question 3

Cataract in galactosemia is caused by the accumulation of which substance?

Accepted Answer

Galactitol

Answer

Sorbitol

Answer

Galactose

Answer

None of the above

Question 4

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 5

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 6

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

Accepted Answer

Glutathione

Answer

Catalase

Answer

Vitamin A

Answer

Vitamin E

Question 7

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 8

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

Accepted Answer

Aldolase

Answer

Glucokinase

Answer

Pyruvate kinase

Answer

Phosphofructokinase

Question 9

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 10

Mucopolysaccharide hyaluronic acid is present in which of the following?

Accepted Answer

Vitreous humor

Answer

Cornea

Answer

Blood vessels

Answer

Lens

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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