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: 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: 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*.

Q: What is the Pasteur effect?

Inhibition of glycolysis by oxygen. **Explanation:** The **Pasteur Effect** refers to the observation that the rate of glycolysis is significantly decreased in the presence of oxygen compared to anaerobic conditions. **Why Option A is Correct:** In the presence of oxygen (aerobic conditions), cells can utilize the **Electron Transport Chain (ETC)** and oxidative phosphorylation. This process is far more efficient, yielding **30–32 ATP** per glucose molecule, compared to only **2 ATP** produced during anaerobic glycolysis. Because the cell’s energy requirements are met more efficiently, high levels of ATP and citrate act as allosteric inhibitors of **Phosphofructokinase-1 (PFK-1)**, the rate-limiting enzyme of glycolysis. Thus, oxygen "inhibits" the rapid consumption of glucose. **Why Other Options are Incorrect:** * **Option B:** The absence of oxygen actually *stimulates* glycolysis (the opposite of the Pasteur effect) to compensate for low ATP yields. * **Option C:** Glycolysis contains three irreversible steps (Hexokinase, PFK-1, and Pyruvate Kinase); the Pasteur effect is a regulatory phenomenon, not a description of reversibility. * **Option D:** Oxygen decelerates glycolysis; the *acceleration* of glycolysis in the presence of oxygen is known as the **Warburg Effect**, typically seen in cancer cells. **NEET-PG High-Yield Pearls:** * **Key Enzyme:** The Pasteur effect primarily targets **PFK-1**. * **Warburg Effect:** This is the "clinical opposite" where cancer cells prefer glycolysis even when oxygen is plentiful (aerobic glycolysis), allowing them to use glycolytic intermediates for cell growth. * **Crabtree Effect:** The inhibition of oxygen consumption by high concentrations of glucose (seen in yeast and some tumor cells).

Q: Alpha-amylase acts on which type of glycosidic bond?

Alpha 1-4 glycosidic bond. **Explanation:** **Alpha-amylase** (both salivary and pancreatic) is an **endo-glycosidase** that specifically hydrolyzes the internal **alpha 1-4 glycosidic bonds** of polysaccharides like starch and glycogen. By breaking these linear bonds, it converts complex carbohydrates into smaller units like maltose, maltotriose, and alpha-limit dextrins. **Analysis of Options:** * **Option A (Correct):** Alpha-amylase acts exclusively on alpha 1-4 bonds. It requires a pH of 6.7–7.0 and chloride ions ($Cl^-$) for optimal activity. * **Option B (Incorrect):** Alpha 1-6 glycosidic bonds are the "branch points" in amylopectin and glycogen. Alpha-amylase **cannot** hydrolyze these bonds. These are broken by "debranching enzymes" (like isomaltase in the gut). * **Option C (Incorrect):** Beta 1-4 bonds are found in **cellulose**. Humans lack the enzyme (cellulase) to break these bonds, which is why cellulose serves as dietary fiber. * **Option D (Incorrect):** Beta 1-6 bonds are not typically found in major dietary starches or glycogen. **High-Yield Clinical Pearls for NEET-PG:** * **Chloride Activation:** Alpha-amylase is a metalloenzyme that requires **Calcium ($Ca^{2+}$)** for stability and **Chloride ($Cl^-$)** for activation. * **Limit Dextrins:** Because amylase cannot bypass or break alpha 1-6 branch points, the remaining branched fragments are called **alpha-limit dextrins**. * **Diagnostic Marker:** Serum amylase rises in **Acute Pancreatitis** (within 2–12 hours), though lipase is considered more specific. * **Isoenzymes:** Salivary amylase (Ptyalin) is inactivated by gastric HCl, while Pancreatic amylase continues digestion in the duodenum.

Q: Which of the following is NOT seen in lactose intolerance?

Alkaline urine. **Explanation:** Lactose intolerance occurs due to a deficiency of the **lactase enzyme** (Option D) in the brush border of the small intestine. When lactose is not hydrolyzed into glucose and galactose, it remains in the intestinal lumen, acting as an osmotic agent that draws water in, leading to osmotic diarrhea. **Why "Alkaline Urine" is the correct answer:** In lactose intolerance, the unabsorbed lactose reaches the colon, where bacteria ferment it into **Short-Chain Fatty Acids (SCFAs)** (like lactic, acetic, and propionic acid) and gases ($H_2, CO_2$). These acids are absorbed into the bloodstream, leading to a mild **metabolic acidosis**. Consequently, the kidneys compensate by excreting excess hydrogen ions, resulting in **acidic urine**, not alkaline urine. **Analysis of other options:** * **Acidic Stool (Option C):** The bacterial fermentation of lactose produces lactic acid and other SCFAs, which significantly lowers the stool pH (typically **< 5.5**). * **Reducing Substances in Stool (Option A):** Lactose is a reducing sugar. Since it remains unabsorbed, it can be detected in the stool using Benedict’s reagent or Clinitest tablets. * **Lactase Enzyme Deficiency (Option B):** This is the primary pathophysiology, whether congenital, primary (developmental), or secondary (post-gastroenteritis). **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard Diagnosis:** Hydrogen Breath Test (increased $H_2$ due to bacterial fermentation). * **Stool pH:** Characteristically < 5.5. * **Clinical Presentation:** Explosive, watery diarrhea, abdominal distension, and perianal excoriation (due to acidic stools). * **Secondary Lactose Intolerance:** Most commonly occurs after a viral rotavirus infection due to sloughing of the villi tips where lactase is located.

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

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 7

UDP Glucose is formed from which precursor molecule?

Accepted Answer

Glucose-1-phosphate

Answer

Glycogen

Answer

Lactose phosphate

Answer

Starch

Question 8

What is the Pasteur effect?

Accepted Answer

Inhibition of glycolysis by oxygen

Answer

Inhibition of glycolysis due to absence of oxygen

Answer

Reversible reaction of glycolysis

Answer

Acceleration of glycolysis by oxygen

Question 9

Alpha-amylase acts on which type of glycosidic bond?

Accepted Answer

Alpha 1-4 glycosidic bond

Answer

Alpha 1-6 glycosidic bond

Answer

Beta 1-4 glycosidic bond

Answer

Beta 1-6 glycosidic bond

Question 10

Which of the following is NOT seen in lactose intolerance?

Accepted Answer

Alkaline urine

Answer

Reducing substances present in stools

Answer

Acidic stool

Answer

Lactase enzyme deficiency

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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