Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 551: Which enzyme of glycolysis is also used in gluconeogenesis?

A. Glucokinase
B. Phosphofructokinase (PFK)
C. Pyruvate kinase
D. Phosphotriose isomerase (Correct Answer)

Explanation: **Explanation:** The metabolic pathways of **Glycolysis** and **Gluconeogenesis** share several enzymes; however, they differ at three specific "irreversible" steps. **Why the correct answer is right:** **Phosphotriose isomerase** (also known as Triose Phosphate Isomerase) is a **reversible** enzyme. In glycolysis, it interconverts Dihydroxyacetone phosphate (DHAP) and Glyceraldehyde-3-phosphate (G3P). Because this reaction is near equilibrium, the same enzyme is utilized in gluconeogenesis to facilitate the reverse reaction. All reversible steps of glycolysis are shared with the gluconeogenic pathway. **Why the incorrect options are wrong:** Options A, B, and C represent the **three irreversible "bottleneck" steps** of glycolysis. These steps have high negative Gibbs free energy and must be bypassed in gluconeogenesis by different, specific enzymes: * **Glucokinase (A):** Bypassed by *Glucose-6-phosphatase* in gluconeogenesis. * **Phosphofructokinase-1 (B):** The rate-limiting step of glycolysis, bypassed by *Fructose-1,6-bisphosphatase*. * **Pyruvate kinase (C):** Bypassed by a two-step process involving *Pyruvate carboxylase* and *PEP carboxykinase (PEPCK)*. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** Gluconeogenesis occurs mainly in the **Liver** (90%) and Kidney (10%). * **Subcellular sites:** Gluconeogenesis is both mitochondrial and cytosolic, whereas glycolysis is purely cytosolic. * **Energy Requirement:** Gluconeogenesis is an energy-expensive process, requiring **6 ATP/GTP** equivalents to produce one molecule of glucose from two molecules of pyruvate. * **Key Regulatory Enzyme:** Fructose-1,6-bisphosphatase is the most important regulatory site for gluconeogenesis (inhibited by Fructose-2,6-bisphosphate).

Question 552: Which of the following is NOT typically observed in a patient with hereditary fructose intolerance?

A. Elevated glucose and fructose levels in blood after fructose load
B. Elevated fructose and galactose levels in blood after fructose load (Correct Answer)
C. Elevated fructose and galactose levels in urine after fructose load
D. Elevated fructose and maltose levels in blood after fructose load

Explanation: **Explanation:** **Hereditary Fructose Intolerance (HFI)** is an autosomal recessive disorder caused by a deficiency of **Aldolase B**. This enzyme is responsible for cleaving Fructose-1-Phosphate (F1P) into DHAP and glyceraldehyde. When deficient, F1P accumulates intracellularly, sequestering inorganic phosphate. This depletion of ATP inhibits both **gluconeogenesis** and **glycogenolysis**, leading to profound postprandial hypoglycemia. **Why Option B is the Correct Answer (The "NOT" observed):** In HFI, the primary metabolic derangement involves fructose. **Galactose metabolism** is an entirely independent pathway (Leloir pathway) occurring via Galactokinase and GALT. There is no biochemical basis for blood galactose levels to rise following a fructose load in an HFI patient. Therefore, the presence of elevated galactose in the blood is not a feature of this condition. **Analysis of Other Options:** * **Option A:** Elevated blood fructose occurs because it cannot be metabolized efficiently. Hypoglycemia (low glucose) is the hallmark; however, some distractors in exams use "elevated glucose" to test if you recognize the *fructose* part of the statement is correct but the *galactose* part in Option B is definitively wrong. * **Option C & D:** While maltose and galactose are not directly linked to the enzyme defect, these options are "incorrect" because they represent distractors that are less physiologically impossible than the specific association of galactosemia with HFI. In HFI, the main findings are **fructosemia** and **fructosuria**. **High-Yield Clinical Pearls for NEET-PG:** * **Enzyme Defect:** Aldolase B (Liver, Kidney, Small Intestine). * **Clinical Presentation:** Symptoms appear when the infant is weaned from breast milk and introduced to **fruit juices or honey** (sucrose/fructose). * **Key Findings:** Severe hypoglycemia, jaundice, vomiting, and hepatomegaly. * **Urine Test:** Positive for reducing sugars (Benedict’s test) but **negative** on Dipstick (which only detects glucose). * **Management:** Strict avoidance of Fructose, Sucrose, and Sorbitol.

Question 553: Which of the following is a reducing sugar?

A. Sucrose
B. Isomaltose (Correct Answer)
C. Trehalose
D. All of the above

Explanation: ### Explanation **Concept:** A sugar is classified as **reducing** if it possesses a free (unbound) anomeric carbon atom (aldehyde or ketone group). This allows the sugar to act as a reducing agent in tests like Benedict’s or Fehling’s. In disaccharides, if the glycosidic bond involves the anomeric carbons of both monosaccharides, the sugar becomes **non-reducing**. **Why Isomaltose is Correct:** Isomaltose is a disaccharide composed of two glucose units linked by an **α(1→6) glycosidic bond**. While the anomeric carbon (C1) of the first glucose is occupied in the bond, the anomeric carbon (C1) of the second glucose remains **free**. This free hemiacetal group allows it to reduce cupric ions, making it a reducing sugar. **Why Other Options are Incorrect:** * **Sucrose (Table Sugar):** It is formed by a linkage between the anomeric carbons of both monomers (**α1 → β2** linkage between Glucose and Fructose). Since both reducing groups are locked in the bond, it is a non-reducing sugar. * **Trehalose:** Found in mushrooms and insects, it consists of two glucose units linked by an **α1 → α1** bond. Both anomeric carbons are involved in the glycosidic linkage, making it non-reducing. **High-Yield Clinical Pearls for NEET-PG:** * **All Monosaccharides** (Glucose, Fructose, Galactose) are reducing sugars. * **Common Reducing Disaccharides:** Maltose, Isomaltose, and Lactose (Mnemonic: **MIL**). * **Non-reducing Disaccharides:** Sucrose and Trehalose. * **Clinical Correlation:** The **Benedict’s Test** is used to detect reducing sugars in urine (e.g., Glucosuria in Diabetes Mellitus or Galactosuria in Galactosemia). Sucrose will give a negative Benedict's test unless it is first hydrolyzed by acid.

Question 554: Which disorder of carbohydrate metabolism typically involves cardiac involvement?

A. Glycogen Storage Disease Type II (Pompe) (Correct Answer)
B. Galactosemia
C. Glycogen Storage Disease Type I (Von-Gierke)
D. Hereditary fructose intolerance

Explanation: ### Explanation **Correct Answer: A. Glycogen Storage Disease Type II (Pompe)** **Why it is correct:** Glycogen Storage Disease Type II (Pompe disease) is unique among GSDs because it is a **lysosomal storage disorder**. It is caused by a deficiency of **Acid α-1,4-glucosidase (Acid Maltase)**, the enzyme responsible for breaking down glycogen within lysosomes. Unlike other GSDs that primarily affect the liver or skeletal muscle, Pompe disease leads to the massive accumulation of glycogen in the **cardiac muscle**, skeletal muscle, and liver. The infantile-onset form typically presents with **hypertrophic cardiomyopathy**, leading to early heart failure and death. **Why the other options are incorrect:** * **B. Galactosemia:** Caused by GALT deficiency. It primarily presents with cataracts, hepatosplenomegaly, and intellectual disability, but does not involve the heart. * **C. GSD Type I (Von Gierke):** Caused by Glucose-6-Phosphatase deficiency. It affects the liver and kidneys, presenting with severe hypoglycemia, lactic acidosis, and "doll-like" facies. Cardiac involvement is absent. * **D. Hereditary Fructose Intolerance:** Caused by Aldolase B deficiency. It leads to hypoglycemia and jaundice after fructose ingestion but has no cardiac manifestations. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** "Pompe trashes the **Pump** (Heart)." * **ECG Finding:** Characteristically shows **short PR interval** and giant QRS complexes. * **Enzyme:** Acid Maltase (Lysosomal enzyme). * **Histology:** PAS-positive material (glycogen) in vacuoles. * **Key differentiator:** Pompe is the only GSD that is also a Lysosomal Storage Disease. Blood glucose levels are typically **normal** in Pompe disease, unlike Type I or III.

Question 555: In the Krebs cycle, from which step is CO2 released, catalyzed by which of the following enzymes?

A. Isocitrate dehydrogenase (Correct Answer)
B. Succinate dehydrogenase
C. Aconitase
D. Succinate thiokinase

Explanation: In the Krebs cycle (TCA cycle), carbon dioxide ($CO_2$) is released during **oxidative decarboxylation** reactions. There are two such steps in the cycle: 1. **Isocitrate to $\alpha$-Ketoglutarate:** Catalyzed by **Isocitrate Dehydrogenase**. 2. **$\alpha$-Ketoglutarate to Succinyl-CoA:** Catalyzed by the $\alpha$-Ketoglutarate Dehydrogenase complex. **Isocitrate dehydrogenase** is the correct answer as it catalyzes the first decarboxylation step, converting a 6-carbon molecule (isocitrate) into a 5-carbon molecule ($\alpha$-ketoglutarate) while reducing $NAD^+$ to $NADH$. ### Analysis of Incorrect Options: * **B. Succinate dehydrogenase:** This enzyme converts succinate to fumarate. It is a redox reaction that produces $FADH_2$, not $CO_2$. It is unique because it is the only TCA enzyme located in the inner mitochondrial membrane (part of Complex II of the ETC). * **C. Aconitase:** This enzyme catalyzes the isomerization of citrate to isocitrate via the intermediate *cis*-aconitate. No $CO_2$ or energy equivalents are produced here. * **D. Succinate thiokinase (Succinyl-CoA synthetase):** This enzyme converts Succinyl-CoA to Succinate. This is a **substrate-level phosphorylation** step, producing GTP (or ATP), not $CO_2$. ### High-Yield Clinical Pearls for NEET-PG: * **Rate-Limiting Step:** Isocitrate dehydrogenase is the **rate-limiting enzyme** of the TCA cycle. It is allosterically activated by ADP and $Ca^{2+}$, and inhibited by ATP and NADH. * **Carbon Count:** The TCA cycle begins with a 6-carbon citrate and ends with a 4-carbon oxaloacetate; the 2 carbons lost are those released as $CO_2$ by Isocitrate DH and $\alpha$-Ketoglutarate DH. * **Fluoroacetate Poisoning:** Aconitase is inhibited by fluoroacetate (rat poison), which is converted to fluorocitrate, halting the cycle.

Question 556: The HMP shunt is of great importance in cellular metabolism because it produces which of the following?

A. ATP
B. ADP
C. Acetyl CoA
D. NADPH (Correct Answer)

Explanation: **Explanation:** The **Hexose Monophosphate (HMP) Shunt**, also known as the Pentose Phosphate Pathway (PPP), is a unique alternative pathway to glycolysis. Unlike most metabolic pathways, its primary purpose is not the production of energy (ATP) but the generation of specialized molecules for biosynthesis and antioxidant defense. **1. Why NADPH is Correct:** The HMP shunt is the body’s primary source of **NADPH**. This molecule is essential for two main reasons: * **Reductive Biosynthesis:** It provides the reducing power for synthesizing fatty acids, steroids, and cholesterol (highly active in the liver, lactating mammary glands, and adrenal cortex). * **Antioxidant Defense:** It acts as a cofactor for *Glutathione Reductase*, which regenerates reduced glutathione. This is critical in Red Blood Cells (RBCs) to neutralize reactive oxygen species and prevent hemolysis. **2. Why Other Options are Incorrect:** * **ATP & ADP:** The HMP shunt is an **energy-neutral** pathway. No ATP is consumed or produced during the reactions. * **Acetyl CoA:** This is the end product of the Pyruvate Dehydrogenase complex and is a precursor for the TCA cycle, not the HMP shunt. **Clinical Pearls for NEET-PG:** * **Rate-limiting Enzyme:** Glucose-6-Phosphate Dehydrogenase (G6PD). * **G6PD Deficiency:** The most common enzymopathy worldwide. Lack of NADPH leads to an inability to maintain reduced glutathione, resulting in oxidative stress, **Heinz bodies**, and **Bite cells** on peripheral smears. * **Non-oxidative Phase:** Produces **Ribose-5-Phosphate**, which is essential for nucleotide and DNA/RNA synthesis. * **Thiamine (B1):** Acts as a cofactor for **Transketolase**, an enzyme in the non-oxidative phase. Measuring erythrocyte transketolase activity is used to diagnose B1 deficiency.

Question 557: Prolonged carbohydrate deficiency leads to?

A. Metabolic alkalosis
B. Ketoacidosis (Correct Answer)
C. Vitamin C deficiency
D. Respiratory acidosis

Explanation: **Explanation:** **Why Ketoacidosis is the correct answer:** When there is a prolonged deficiency of carbohydrates (starvation or low-carb diets), the body’s glucose stores (glycogen) are depleted. To maintain energy production, the body shifts to **lipolysis** (breakdown of fats). This process releases large amounts of free fatty acids, which undergo **beta-oxidation** in the liver to produce **Acetyl-CoA**. Under normal conditions, Acetyl-CoA enters the TCA cycle by combining with oxaloacetate. However, in carbohydrate deficiency, oxaloacetate is diverted toward **gluconeogenesis** to maintain blood glucose levels. This results in an accumulation of Acetyl-CoA, which is then shunted into the **ketogenesis** pathway. The resulting ketone bodies (acetoacetate and β-hydroxybutyrate) are acidic. Their accumulation lowers blood pH, leading to **metabolic acidosis**, specifically **ketoacidosis**. **Why other options are incorrect:** * **Metabolic Alkalosis:** This involves an increase in blood pH (e.g., due to persistent vomiting or antacid overuse). Carbohydrate deficiency produces acids, causing the opposite effect. * **Vitamin C Deficiency:** While Vitamin C is a carbohydrate derivative (ascorbic acid), its deficiency (Scurvy) is caused by a lack of dietary intake of fresh fruits/vegetables, not by a general lack of macronutrient carbohydrates. * **Respiratory Acidosis:** This is caused by CO₂ retention due to lung disease or hypoventilation, not by metabolic shifts in fuel utilization. **High-Yield NEET-PG Pearls:** * **Ketone bodies:** Acetoacetate, β-hydroxybutyrate, and Acetone (non-metabolizable, excreted via breath). * **Rate-limiting enzyme of ketogenesis:** HMG-CoA Synthase (mitochondrial). * **Brain Adaptation:** During prolonged starvation, the brain adapts to use ketone bodies for up to 75% of its energy needs. * **Key distinction:** Diabetic Ketoacidosis (DKA) occurs due to insulin deficiency, whereas starvation ketoacidosis occurs due to glucose unavailability; both share the same underlying biochemical mechanism of oxaloacetate depletion.

Question 558: Phospho-dephosphorylation of phosphofructokinase and fructose 1, 6-bisphosphatase by fructose 2, 6-bisphosphate regulation is seen in which of the following?

A. Brain
B. Liver (Correct Answer)
C. Adrenal cortex
D. RBC

Explanation: ### Explanation The regulation of glycolysis and gluconeogenesis is primarily controlled by the bifunctional enzyme **PFK-2/FBPase-2**, which determines the levels of **Fructose 2,6-bisphosphate (F2,6-BP)**. **Why Liver is the Correct Answer:** The liver plays a central role in maintaining systemic blood glucose levels. In the liver, the bifunctional enzyme is regulated by **cAMP-dependent phosphorylation** (via Protein Kinase A). * **In the fasting state:** Glucagon increases cAMP, activating Protein Kinase A, which **phosphorylates** the enzyme. This activates the phosphatase domain (FBPase-2) and inactivates the kinase domain (PFK-2), leading to decreased F2,6-BP levels, thereby inhibiting glycolysis and promoting gluconeogenesis. * **In the fed state:** Insulin promotes **dephosphorylation**, activating PFK-2, increasing F2,6-BP, and stimulating glycolysis. **Why Other Options are Incorrect:** * **Brain:** The brain lacks the machinery for gluconeogenesis and does not regulate glucose metabolism via the hormonal phospho-dephosphorylation of F2,6-BP; it relies on a continuous supply of glucose. * **Adrenal Cortex:** While metabolically active, it does not serve as a primary glucose-regulating organ like the liver. * **RBC:** Red blood cells lack mitochondria and gluconeogenic enzymes. Their glycolysis is regulated primarily by the ATP/AMP ratio and 2,3-BPG levels, not by the hormonal regulation of F2,6-BP seen in the liver. **High-Yield Clinical Pearls for NEET-PG:** 1. **Fructose 2,6-bisphosphate** is the most potent allosteric activator of **PFK-1** (the rate-limiting enzyme of glycolysis). 2. **Muscle Isoenzyme:** Unlike the liver, the muscle isoform of PFK-2 is **not** inhibited by phosphorylation; instead, it is activated by epinephrine to ensure rapid energy production during exercise. 3. **Reciprocal Regulation:** F2,6-BP simultaneously activates PFK-1 and inhibits Fructose 1,6-bisphosphatase, preventing a "futile cycle."

Question 559: A bodybuilder who consumes raw eggs for protein develops fatigue on moderate exercise. The prescribing physician suspects a vitamin deficiency. Which enzyme is likely deficient in him?

A. Glucose 6 Phosphatase
B. Pyruvate Carboxylase (Correct Answer)
C. PEPCK
D. Glycogen Phosphorylase

Explanation: ### Explanation **1. Why Pyruvate Carboxylase is the Correct Answer:** The clinical presentation describes **Biotin (Vitamin B7) deficiency**. Raw egg whites contain a glycoprotein called **avidin**, which binds tightly to biotin and prevents its absorption in the gut. Biotin is a mandatory co-enzyme for **carboxylation reactions** (mnemonic: "ABC" enzymes – ATP, Biotin, and CO₂). **Pyruvate Carboxylase** is a biotin-dependent enzyme that converts pyruvate to oxaloacetate (OAA). This reaction is the first step of **gluconeogenesis** and is also "anaplerotic," meaning it replenishes OAA for the TCA cycle. A deficiency leads to impaired glucose production and reduced ATP generation during exercise, resulting in fatigue and lactic acidosis. **2. Analysis of Incorrect Options:** * **A. Glucose 6 Phosphatase:** This enzyme is deficient in **Von Gierke Disease** (GSD Type I). While it affects gluconeogenesis, it is not biotin-dependent and is not affected by raw egg consumption. * **C. PEPCK (Phosphoenolpyruvate Carboxykinase):** This is the second step of gluconeogenesis. It requires **GTP**, not biotin. * **D. Glycogen Phosphorylase:** This enzyme is involved in glycogenolysis (deficient in **McArdle Disease**). It requires **Pyridoxal Phosphate (Vitamin B6)** as a cofactor, not biotin. **3. Clinical Pearls for NEET-PG:** * **Biotin-Dependent Enzymes:** 1. **Pyruvate Carboxylase** (Gluconeogenesis) 2. **Acetyl-CoA Carboxylase** (Fatty acid synthesis) 3. **Propionyl-CoA Carboxylase** (Odd-chain fatty acid metabolism) * **Avidin-Biotin Interaction:** This is one of the strongest non-covalent bonds in nature. Cooking denatures avidin, making cooked eggs safe. * **Key Symptoms:** Biotin deficiency presents with dermatitis, alopecia, enteritis, and neurological symptoms (lethargy/hypotonia).

Question 560: UDP Glucose is formed from which precursor molecule?

A. Glucose-1-phosphate (Correct Answer)
B. Glycogen
C. Lactose phosphate
D. Starch

Explanation: **Explanation:** **1. Why Glucose-1-phosphate is correct:** UDP-Glucose (Uridine diphosphate glucose) is the activated form of glucose required for glycogen synthesis, galactose metabolism, and the synthesis of glycoproteins. It is synthesized by the enzyme **UDP-glucose pyrophosphorylase**. This enzyme catalyzes the reaction between **Glucose-1-phosphate (G1P)** and **UTP** (Uridine triphosphate). During this process, the phosphate on G1P and the innermost phosphate of UTP form the UDP-linkage, releasing inorganic pyrophosphate (PPi). The subsequent hydrolysis of PPi by pyrophosphatase makes this reaction irreversible, driving glycogen synthesis forward. **2. Why the other options are incorrect:** * **Glycogen:** This is the storage polymer of glucose. UDP-glucose is a *precursor* used by glycogen synthase to add glucose units to an existing glycogen chain; glycogen is the end product, not the precursor of UDP-glucose. * **Lactose phosphate:** Lactose is a disaccharide of glucose and galactose. While UDP-galactose is involved in lactose synthesis in mammary glands, "lactose phosphate" is not a standard intermediate in the formation of UDP-glucose. * **Starch:** This is the storage form of glucose in plants. Human metabolism breaks down starch into free glucose in the gut, which must then be phosphorylated to Glucose-6-P and converted to Glucose-1-P before it can form UDP-glucose. **Clinical Pearls for NEET-PG:** * **Galactosemia:** Deficiency of *Galactose-1-phosphate uridyltransferase* (GALT) prevents the conversion of Galactose-1-P and UDP-Glucose into UDP-Galactose and Glucose-1-P, leading to Classic Galactosemia. * **Glycogenesis:** UDP-glucose is the immediate donor of glucose residues for the enzyme **Glycogen Synthase**. * **Bilirubin Conjugation:** UDP-glucuronic acid (derived from UDP-glucose) is essential for conjugating bilirubin in the liver via the enzyme *UDP-glucuronosyltransferase*.

Practice by Chapter

Carbohydrate Chemistry and Classification

Practice Questions

Glycolysis: Reactions and Regulation

Practice Questions

Gluconeogenesis: Reactions and Regulation

Practice Questions

Glycogen Metabolism: Synthesis and Breakdown

Practice Questions

Glycogen Storage Diseases

Practice Questions

Pentose Phosphate Pathway

Practice Questions

Metabolism of Fructose and Galactose

Practice Questions

Disorders of Fructose and Galactose Metabolism

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Blood Glucose Regulation

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Diabetes Mellitus: Biochemical Aspects

Practice Questions

Glycosylation and Glycoproteins

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Lactose Intolerance and Galactosemia

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