Carbohydrate Metabolism Practice Questions

Q: Which enzyme of glycolysis is also used in gluconeogenesis?

Phosphotriose isomerase. **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).

Q: Which of the following is a reducing sugar?

Isomaltose. ### 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.

Q: Which disorder of carbohydrate metabolism typically involves cardiac involvement?

Glycogen Storage Disease Type II (Pompe). ### 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.

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

Isocitrate dehydrogenase. 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.

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

NADPH. **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.

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

Liver. ### 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."

Q: 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?

Pyruvate Carboxylase. ### 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).

Q: UDP Glucose is formed from which precursor molecule?

Glucose-1-phosphate. **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*.

Question 1

Which enzyme of glycolysis is also used in gluconeogenesis?

Accepted Answer

Phosphotriose isomerase

Answer

Glucokinase

Answer

Phosphofructokinase (PFK)

Answer

Pyruvate kinase

Question 2

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

Accepted Answer

Elevated fructose and galactose levels in blood after fructose load

Answer

Elevated glucose and fructose levels in blood after fructose load

Answer

Elevated fructose and galactose levels in urine after fructose load

Answer

Elevated fructose and maltose levels in blood after fructose load

Question 3

Which of the following is a reducing sugar?

Accepted Answer

Isomaltose

Answer

Sucrose

Answer

Trehalose

Answer

All of the above

Question 4

Which disorder of carbohydrate metabolism typically involves cardiac involvement?

Accepted Answer

Glycogen Storage Disease Type II (Pompe)

Answer

Galactosemia

Answer

Glycogen Storage Disease Type I (Von-Gierke)

Answer

Hereditary fructose intolerance

Question 5

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

Accepted Answer

Isocitrate dehydrogenase

Answer

Succinate dehydrogenase

Answer

Aconitase

Answer

Succinate thiokinase

Question 6

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

Accepted Answer

NADPH

Answer

ATP

Answer

ADP

Answer

Acetyl CoA

Question 7

Prolonged carbohydrate deficiency leads to?

Accepted Answer

Ketoacidosis

Answer

Metabolic alkalosis

Answer

Vitamin C deficiency

Answer

Respiratory acidosis

Question 8

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

Accepted Answer

Liver

Answer

Brain

Answer

Adrenal cortex

Answer

RBC

Question 9

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?

Accepted Answer

Pyruvate Carboxylase

Answer

Glucose 6 Phosphatase

Answer

PEPCK

Answer

Glycogen Phosphorylase

Question 10

UDP Glucose is formed from which precursor molecule?

Accepted Answer

Glucose-1-phosphate

Answer

Glycogen

Answer

Lactose phosphate

Answer

Starch

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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