Carbohydrate Metabolism Practice Questions

Q: Which of the following does NOT allosterically inhibit phosphofructokinase-1 (PFK-1)?

AMP/ADP. **Explanation:** Phosphofructokinase-1 (PFK-1) is the **rate-limiting and key committed step** of glycolysis, converting Fructose-6-phosphate to Fructose-1,6-bisphosphate. Its regulation is crucial for balancing cellular energy needs. **Why AMP/ADP is the correct answer:** AMP and ADP are indicators of a **low-energy state** in the cell. When ATP is consumed, AMP levels rise. AMP acts as a potent **allosteric activator** of PFK-1, signaling the cell to increase glycolytic flux to generate more ATP. Therefore, it does not inhibit the enzyme; it stimulates it. **Analysis of Incorrect Options (Inhibitors):** * **ATP:** Although a substrate, high levels of ATP act as an allosteric inhibitor. It binds to a regulatory site to decrease the enzyme's affinity for Fructose-6-phosphate, signaling that the cell has sufficient energy. * **Citrate:** An intermediate of the TCA cycle. High citrate levels indicate that the cycle is "saturated" and precursors for energy production are abundant, leading to the feedback inhibition of PFK-1. * **H+ (Low pH):** Accumulation of protons (acidosis) inhibits PFK-1. This is a protective mechanism, particularly in skeletal muscle, to prevent excessive lactic acid production and subsequent tissue damage during anaerobic glycolysis. **High-Yield NEET-PG Pearls:** * **Most Potent Activator:** Fructose-2,6-bisphosphate is the most powerful allosteric activator of PFK-1 (overcomes ATP inhibition). * **Insulin vs. Glucagon:** Insulin increases PFK-1 activity (via F-2,6-BP), while glucagon decreases it. * **Reciprocal Regulation:** Factors that activate PFK-1 (like F-2,6-BP and AMP) typically inhibit Fructose-1,6-bisphosphatase (gluconeogenesis), preventing a futile cycle.

Q: All of the following are enzymes of the TCA cycle, EXCEPT?

Malic enzyme. ### Explanation The **TCA cycle (Krebs cycle)** occurs in the mitochondrial matrix and consists of eight primary enzymatic steps. **Why Malic Enzyme is the correct answer:** **Malic enzyme** is not a component of the TCA cycle. It is an enzyme that catalyzes the oxidative decarboxylation of **Malate to Pyruvate**, simultaneously reducing NADP⁺ to **NADPH**. This reaction occurs in the cytosol and is considered an **anaplerotic** or shunt reaction. Its primary roles are providing NADPH for fatty acid synthesis and participating in the pyruvate-malate shuttle. It should not be confused with *Malate Dehydrogenase*, which converts Malate to Oxaloacetate within the TCA cycle. **Analysis of Incorrect Options:** * **A. Aconitase:** Catalyzes the isomerization of Citrate to Isocitrate via the intermediate *cis*-aconitate. It requires $Fe^{2+}$ as a cofactor. * **B. Fumarase (Fumarate Hydratase):** Catalyzes the hydration of Fumarate to L-Malate. * **D. Citrate Synthase:** The "pacemaker" or rate-limiting enzyme that condenses Acetyl-CoA and Oxaloacetate to form Citrate. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** All TCA enzymes are in the mitochondrial matrix except **Succinate Dehydrogenase**, which is located on the inner mitochondrial membrane (also acts as Complex II of the ETC). * **Energy Yield:** One turn of the TCA cycle produces **10 ATP** (3 NADH = 7.5, 1 $FADH_2$ = 1.5, 1 GTP = 1). * **Inhibitors:** Fluoroacetate inhibits Aconitase; Arsenite inhibits $\alpha$-Ketoglutarate Dehydrogenase. * **Malic Enzyme vs. Malate Dehydrogenase:** Remember, Malic enzyme produces **NADPH**, while Malate Dehydrogenase produces **NADH**.

Q: A 15-year-old type I diabetic patient faints after injecting himself with insulin. He is administered Glucagon and rapidly recovers consciousness. Glucagon induces activity of which enzyme?

Glycogen phosphorylase. ### Explanation **1. Why Glycogen Phosphorylase is Correct:** The patient is experiencing **hypoglycemia** due to insulin overdose. Glucagon is the counter-regulatory hormone secreted in response to low blood glucose. It acts primarily on the liver by binding to G-protein coupled receptors (GPCR), increasing intracellular **cAMP**, and activating **Protein Kinase A (PKA)**. PKA phosphorylates **Phosphorylase Kinase**, which in turn phosphorylates and activates **Glycogen Phosphorylase** (converting it from the inactive 'b' form to the active 'a' form). This triggers **glycogenolysis**, rapidly releasing glucose into the bloodstream to restore consciousness. **2. Why the Other Options are Incorrect:** * **A. Glycogen Synthase:** This enzyme is responsible for glycogenesis (glycogen synthesis). Glucagon-induced phosphorylation **inactivates** glycogen synthase to prevent a futile cycle during hypoglycemia. * **C & D. Glucokinase/Hexokinase:** These enzymes catalyze the first step of glycolysis (glucose to glucose-6-phosphate). Glucagon inhibits glycolysis in the liver (via inhibition of PFK-1 and Pyruvate Kinase) to conserve glucose for systemic circulation. Glucokinase expression is actually induced by insulin, not glucagon. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Action:** Glucagon $\rightarrow$ cAMP $\uparrow$ $\rightarrow$ PKA $\rightarrow$ Phosphorylase Kinase $\rightarrow$ Glycogen Phosphorylase. * **Key Regulatory Step:** Glycogen phosphorylase is the **rate-limiting enzyme** of glycogenolysis. * **Tissue Specificity:** Glucagon acts on the **liver** but has **no effect on muscle glycogen**, as muscle cells lack glucagon receptors. Muscle glycogen is used only for local energy during contraction. * **Clinical Use:** Glucagon is the emergency treatment of choice for severe hypoglycemia in diabetics when intravenous glucose access is unavailable.

Q: In which condition does hemolysis occur on oxidation?

G6PD deficiency. **Explanation:** **G6PD Deficiency (Correct Answer):** Glucose-6-Phosphate Dehydrogenase (G6PD) is the rate-limiting enzyme of the **Hexose Monophosphate (HMP) Shunt**. This pathway is the sole source of **NADPH** in mature erythrocytes. NADPH is essential for maintaining a pool of **reduced glutathione**, which acts as a cellular antioxidant by neutralizing reactive oxygen species (ROS) like hydrogen peroxide. In G6PD deficiency, oxidative stress (triggered by infections, fava beans, or drugs like Primaquine and Sulfonamides) leads to the oxidation of hemoglobin. This results in the formation of **Heinz bodies** (denatured hemoglobin), which damage the RBC membrane, leading to acute hemolysis. **Why other options are incorrect:** * **Hereditary Spherocytosis:** This is a **cytoskeletal defect** (deficiency of Spectrin or Ankyrin) causing RBCs to become spherical and trapped in the spleen. It is not primarily driven by oxidative stress. * **Sickle Cell Anemia:** This is a **qualitative hemoglobinopathy** (Glu → Val at 6th position of β-chain). Hemolysis occurs due to polymerization of HbS under hypoxic conditions, not oxidative stress. * **Hemophilia:** This is a **coagulation disorder** (deficiency of Factor VIII or IX) leading to bleeding tendencies; it does not involve intrinsic RBC defects or hemolysis. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** G6PD deficiency is **X-linked Recessive**. * **Morphology:** Look for **"Bite cells"** (Degmacytes) and **"Blister cells"** on a peripheral smear. * **Staining:** Heinz bodies are visualized using supravital stains like **Crystal Violet**. * **Protection:** G6PD deficiency provides a selective advantage against *Plasmodium falciparum* malaria.

Q: What is the primary purpose of carbohydrate loading practiced by endurance athletes?

To form glycogen with fewer branch points than normal. **Explanation:** **1. Why Option A is Correct:** Carbohydrate loading (glycogen supercompensation) involves depleting glycogen stores through intense exercise followed by a high-carbohydrate diet. During the rapid resynthesis phase, the activity of **Glycogen Synthase** (responsible for linear α-1,4 linkages) increases significantly more than the activity of the **Branching Enzyme** (amylo-1,4→1,6-transglucosidase). This results in the formation of glycogen molecules that are larger and have **fewer branch points** than normal. These "less branched" molecules are more compact, allowing the muscle to store a higher total mass of glucose per unit volume, providing a larger energy reservoir for endurance activities. **2. Why Other Options are Incorrect:** * **Option B:** More branch points would increase the solubility and rate of degradation (glycogenolysis) but would occupy more space, preventing the "packing" of extra glucose required for endurance. * **Option C:** Carbohydrate loading is specifically designed to maximize polysaccharide storage, not to facilitate gluconeogenesis or protein synthesis. * **Option D:** While blood glucose maintenance is a general physiological function of *liver* glycogen, the primary goal of carb-loading is to maximize *muscle* glycogen for local energy use during prolonged exertion. **3. High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme of Glycogenesis:** Glycogen Synthase (activated by Insulin/Glucose-6-P). * **Branching Enzyme:** Also known as *Glucosyl 4:6 transferase*. Deficiency leads to **Andersen’s Disease (GSD Type IV)**, characterized by long, unbranched glucose chains (amylopectin-like) that trigger an immune response. * **Muscle vs. Liver:** Muscle glycogen lacks **Glucose-6-Phosphatase**; therefore, it cannot contribute to blood glucose and is used exclusively for muscle contraction.

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

Fructokinase. **Explanation:** **Essential Fructosuria** is a benign, autosomal recessive metabolic disorder caused by a deficiency of the enzyme **Fructokinase** (also known as Ketohexokinase). 1. **Why Fructokinase is correct:** In the normal metabolic pathway, fructokinase converts fructose into fructose-1-phosphate in the liver. When this enzyme is deficient, fructose cannot be trapped inside the cell. Instead, it accumulates in the blood (fructosemia) and is excreted in the urine (fructosuria). Because fructose is a reducing sugar, it will give a positive result on a Benedict’s test but a negative result on a glucose oxidase dipstick. 2. **Why other options are incorrect:** * **Aldolase B:** Deficiency leads to **Hereditary Fructose Intolerance (HFI)**. This is a severe condition where fructose-1-phosphate accumulates, depleting intracellular phosphate and causing hypoglycemia and liver failure. * **Beta-galactosidase:** Deficiency is associated with **GM1 gangliosidosis** or Morquio syndrome Type B, involving the breakdown of keratan sulfate and gangliosides, not fructose. * **Aldose reductase:** This enzyme converts glucose to sorbitol. It is implicated in diabetic complications (cataracts, neuropathy) but its deficiency does not cause fructosuria. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Essential fructosuria is **asymptomatic**. It is often discovered incidentally during routine urine screening for reducing sugars. * **Alternative Pathway:** In the absence of fructokinase, **hexokinase** becomes the primary enzyme for fructose metabolism, converting it to fructose-6-phosphate (though this is a slow, minor pathway). * **Mnemonic:** "Fructokinase deficiency is **Friendly** (asymptomatic), Aldolase B deficiency is **Bad** (symptomatic/HFI)."

Q: A 3-year-old boy presented with several episodes of vomiting and lethargy, accompanied by hypoglycemia. His pediatrician suspected possible hepatic failure given the recurrent vomiting and lethargy. It was noted that these episodes occur after the ingestion of sweets or fruits. What is the most likely diagnosis?

Hereditary Fructose Intolerance. **Explanation:** The clinical presentation of vomiting, lethargy, and hypoglycemia triggered specifically by the ingestion of **fruits or sweets** is a classic hallmark of **Hereditary Fructose Intolerance (HFI)**. **1. Why Option A is Correct:** HFI is caused by a deficiency of **Aldolase B**. When fructose is ingested (found in fruits, honey, and sucrose/sweets), it is phosphorylated to **Fructose-1-Phosphate (F1P)** by fructokinase. Due to the absence of Aldolase B, F1P accumulates in the hepatocytes. This "trapping" of inorganic phosphate leads to: * **Inhibition of Glycogenolysis:** Phosphorylase enzyme is inhibited. * **Inhibition of Gluconeogenesis:** Due to lack of ATP and substrate availability. The result is profound **postprandial hypoglycemia** and liver dysfunction (vomiting/jaundice). **2. Why Other Options are Incorrect:** * **B. Glucose Homeostasis:** This is a physiological process, not a disease entity. * **C. Glycogen Storage Disease Type III (Cori Disease):** While it causes hypoglycemia and hepatomegaly, symptoms are typically triggered by fasting, not specifically by fructose/fruit ingestion. It is a deficiency of the debranching enzyme. * **D. Galactosemia:** Symptoms (cataracts, hepatomegaly, sepsis) usually appear in the neonatal period upon starting **milk** (lactose), not later with fruits/sweets. **NEET-PG High-Yield Pearls:** * **Enzyme Deficient:** Aldolase B (Chromosome 9). * **Key Trigger:** Introduction of weaning (fruit juices/sucrose). * **Biochemical Hallmark:** Intracellular depletion of inorganic phosphate ($P_i$) and ATP. * **Essential Fructosuria:** A benign condition caused by **Fructokinase** deficiency; unlike HFI, it does not cause hypoglycemia or liver damage as F1P does not accumulate.

Q: Non-ketotic hypoglycemia is seen in all of the following conditions except?

Glycogen storage disorders. **Explanation:** The hallmark of **non-ketotic hypoglycemia** is the absence of ketone bodies despite low blood glucose levels. This occurs when there is either an inhibition of fatty acid oxidation or a suppression of lipolysis. **1. Why Glycogen Storage Disorders (GSDs) are the exception:** In most GSDs (specifically Type I - von Gierke disease), hypoglycemia occurs due to the inability to release glucose from glycogen. However, the counter-regulatory hormones (glucagon and epinephrine) are activated. This triggers **intact fatty acid oxidation**, leading to the production of acetyl-CoA and subsequently **ketone bodies**. Therefore, GSDs typically present with **ketotic hypoglycemia**. **2. Analysis of Incorrect Options:** * **Hyperinsulinism:** Insulin is a potent inhibitor of lipolysis and fatty acid oxidation. High insulin levels prevent the formation of ketones, leading to profound non-ketotic hypoglycemia. * **Galactosemia & Hereditary Fructose Intolerance (HFI):** In these conditions, the accumulation of phosphorylated sugars (Galactose-1-P or Fructose-1-P) depletes intracellular inorganic phosphate. This inhibits both glycogenolysis and gluconeogenesis. The acute metabolic derangement often interferes with the transition to ketosis, typically presenting as non-ketotic hypoglycemia in the acute phase. **3. NEET-PG High-Yield Pearls:** * **Ketotic Hypoglycemia:** Seen in GSD Type I, Ketotic hypoglycemia of childhood (most common cause in toddlers), and Maple Syrup Urine Disease (MSUD). * **Non-Ketotic Hypoglycemia:** Think of **MCAD deficiency** (impaired beta-oxidation), **Hyperinsulinism**, and **Systemic Carnitine deficiency**. * **Key Diagnostic:** If a question mentions "hypoglycemia with absent ketones," always prioritize Fatty Acid Oxidation Disorders or Insulin excess.

Q: All of the following are true regarding proteoglycans, EXCEPT?

They hold a large amount of water.. **Explanation:** Proteoglycans are complex macromolecules consisting of a core protein covalently attached to long, unbranched chains of **Glycosaminoglycans (GAGs)**. **Why Option B is the "Except" (Correct Answer):** In the context of this specific question format, Option B is often marked as the "incorrect statement" if the examiner defines proteoglycans strictly by their chemical composition rather than their physical properties. However, physiologically, proteoglycans **do** hold large amounts of water. If this is the designated answer, it implies a technicality: Proteoglycans *attract* water due to their osmotic pressure, but the water is not a structural component of the molecule itself. *Note: In many standard textbooks, B is actually a true statement, making this a controversial "recall" question. If B is the keyed answer, it is likely because the examiner considers "holding water" a function of the GAG component, not the proteoglycan as a whole.* **Analysis of Other Options:** * **Option A:** **True.** Chondroitin sulfate is the most abundant GAG in the body, found in cartilage and bone. * **Option C:** **True.** They consist of a protein core (amino acids) and GAGs (long sugar chains). * **Option D:** **True.** GAGs contain sulfate and carboxyl groups, giving them a high **negative charge**. This causes the chains to repel each other, creating the "bottle-brush" appearance and allowing them to act as biological lubricants. **High-Yield NEET-PG Pearls:** * **Hyaluronic Acid:** The only GAG that is **not sulfated** and not covalently bound to a protein core. * **Heparin:** The GAG with the highest negative charge density; acts as an intracellular anticoagulant. * **Mucopolysaccharidoses (MPS):** Genetic disorders (e.g., Hurler, Hunter syndromes) caused by the deficiency of lysosomal enzymes that degrade GAGs. * **Aggrecan:** The major proteoglycan in cartilage.

Question 1

Which of the following does NOT allosterically inhibit phosphofructokinase-1 (PFK-1)?

Accepted Answer

AMP/ADP

Answer

Citrate

Answer

ATP

Answer

H+

Question 2

All of the following are enzymes of the TCA cycle, EXCEPT?

Accepted Answer

Malic enzyme

Answer

Aconitase

Answer

Fumarase

Answer

Citrate synthase

Question 3

A 15-year-old type I diabetic patient faints after injecting himself with insulin. He is administered Glucagon and rapidly recovers consciousness. Glucagon induces activity of which enzyme?

Accepted Answer

Glycogen phosphorylase

Answer

Glycogen synthase

Answer

Glucokinase

Answer

Hexokinase

Question 4

In which condition does hemolysis occur on oxidation?

Accepted Answer

G6PD deficiency

Answer

Hereditary spherocytosis

Answer

Sickle cell anemia

Answer

Hemophilia

Question 5

What is the primary purpose of carbohydrate loading practiced by endurance athletes?

Accepted Answer

To form glycogen with fewer branch points than normal

Answer

To form glycogen with more branch points than normal

Answer

To convert glucose to proteins

Answer

To maintain optimal blood glucose concentration

Question 6

Essential fructosuria is due to deficiency of which enzyme?

Accepted Answer

Fructokinase

Answer

beta-galactosidase

Answer

Aldolase B

Answer

Aldose reductase

Question 7

A 3-year-old boy presented with several episodes of vomiting and lethargy, accompanied by hypoglycemia. His pediatrician suspected possible hepatic failure given the recurrent vomiting and lethargy. It was noted that these episodes occur after the ingestion of sweets or fruits. What is the most likely diagnosis?

Accepted Answer

Hereditary Fructose Intolerance

Answer

Glucose Homeostasis

Answer

Glycogen storage disease type III

Answer

Galactosemia

Question 8

Non-ketotic hypoglycemia is seen in all of the following conditions except?

Accepted Answer

Glycogen storage disorders

Answer

Galactosemia

Answer

Hereditary fructose intolerance

Answer

Hyperinsulinism

Question 9

The immediate degradation of glycogen under normal conditions gives rise to which one of the following?

Accepted Answer

More glucose-1-phosphate than glucose

Answer

More glucose than glucose-1-phosphate

Answer

Equal amounts of glucose and glucose-1-phosphate

Answer

Neither glucose nor glucose-1-phosphate

Question 10

All of the following are true regarding proteoglycans, EXCEPT?

Accepted Answer

They hold a large amount of water.

Answer

Chondroitin sulfate is a glycosaminoglycan.

Answer

They are made up of sugar and amino acids.

Answer

They carry a negative charge.

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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