Carbohydrate Metabolism Practice Questions

Q: What is the cofactor for Glycogen phosphorylase in Glycogenolysis?

Pyridoxal phosphate. **Explanation:** **1. Why Pyridoxal Phosphate (PLP) is correct:** Glycogen phosphorylase is the rate-limiting enzyme of glycogenolysis. It catalyzes the phosphorolytic cleavage of glycogen to produce glucose-1-phosphate. Unlike most enzymes that use PLP for transamination or decarboxylation, glycogen phosphorylase requires **Pyridoxal Phosphate (Vitamin B6)** as an essential cofactor for its catalytic activity. Specifically, the **phosphate group** of PLP acts as a general acid-base catalyst, promoting the attack of inorganic phosphate on the glycosidic bond. This is a unique mechanism where the aldehyde group of PLP (usually the active site) is not directly involved in the catalysis. **2. Why other options are incorrect:** * **Thiamine pyrophosphate (TPP):** This is the active form of Vitamin B1. it is a cofactor for oxidative decarboxylation reactions (e.g., Pyruvate Dehydrogenase, $\alpha$-ketoglutarate dehydrogenase) and the HMP shunt (Transketolase). * **Citrate:** This is an intermediate of the TCA cycle and acts as an allosteric **inhibitor** of Phosphofructokinase-1 (PFK-1) in glycolysis, not a cofactor. * **FAD:** Flavin adenine dinucleotide (Vitamin B2 derivative) acts as an electron carrier in redox reactions, such as those catalyzed by Succinate dehydrogenase. **3. High-Yield Clinical Pearls for NEET-PG:** * **McArdle Disease (GSD Type V):** Caused by a deficiency of muscle glycogen phosphorylase. Patients present with exercise intolerance, muscle cramps, and myoglobinuria. * **Hers Disease (GSD Type VI):** Caused by a deficiency of liver glycogen phosphorylase, leading to hepatomegaly and mild fasting hypoglycemia. * **Regulation:** Glycogen phosphorylase is activated by **phosphorylation** (via phosphorylase kinase) and allosterically activated by **AMP** in the muscle. * **B6 Fact:** Over 80% of the body's Vitamin B6 is stored in the muscle, bound to glycogen phosphorylase.

Q: Which of the following is NOT an example of a glycosaminoglycan?

None of the above. **Explanation:** The correct answer is **None of the above** because all three options listed (Hyaluronic acid, Chondroitin sulfate, and Heparin) are classic examples of **Glycosaminoglycans (GAGs)**. **Understanding Glycosaminoglycans (GAGs):** GAGs, formerly known as mucopolysaccharides, are long, unbranched heteropolysaccharides consisting of repeating disaccharide units. These units typically contain an **amino sugar** (D-glucosamine or D-galactosamine) and an **uronic acid** (glucuronic or iduronic acid). Due to their high negative charge, they attract water, providing lubrication and shock absorption in the extracellular matrix. * **Option A: Hyaluronic Acid** is a GAG. It is unique because it is the only GAG that is **non-sulfated**, not covalently bound to a protein core, and is found in the vitreous humor and synovial fluid. * **Option B: Chondroitin Sulfate** is the **most abundant GAG** in the body, found prominently in cartilage, bone, and heart valves. * **Option C: Heparin** is an intracellular GAG found in mast cells. It acts as a potent anticoagulant by activating antithrombin III. (Note: Heparan sulfate is its extracellular counterpart found in basement membranes). **High-Yield Clinical Pearls for NEET-PG:** 1. **Keratan Sulfate:** The only GAG that **lacks uronic acid** (contains galactose instead). 2. **Mucopolysaccharidoses (MPS):** Genetic deficiencies of lysosomal enzymes that degrade GAGs (e.g., **Hurler Syndrome** - α-L-iduronidase deficiency; **Hunter Syndrome** - Iduronate sulfatase deficiency, which is X-linked recessive). 3. **Specific Locations:** * **Dermatan sulfate:** Skin, blood vessels, heart valves. * **Heparan sulfate:** Basement membranes; determines charge selectivity in the glomerular filtration barrier.

Q: Which enzyme, when absent, would impair the rate-limiting step of glycogenolysis?

Glycogen phosphorylase. **Explanation:** **Glycogenolysis** is the biochemical breakdown of glycogen into glucose-1-phosphate and glucose. The correct answer is **Glycogen phosphorylase** because it catalyzes the **rate-limiting and regulatory step** of this pathway. 1. **Why Glycogen Phosphorylase is correct:** This enzyme breaks the $\alpha(1\to4)$ glycosidic bonds by adding an inorganic phosphate (phosphorolysis), releasing glucose-1-phosphate. It acts sequentially from the non-reducing ends of glycogen chains until it reaches a point four glucose residues away from a branch point. Its activity is tightly regulated by hormonal signals (glucagon/epinephrine) via covalent modification (phosphorylation). 2. **Why other options are incorrect:** * **1,4-Glucanotransferase:** This is part of the "debranching enzyme" complex. It moves a trisaccharide unit from one branch to another but is not the rate-limiting step. * **Glycogen synthetase:** This is the rate-limiting enzyme for **glycogenesis** (glycogen synthesis), not glycogenolysis. * **Phosphoglucomutase:** This enzyme catalyzes the reversible conversion of glucose-1-phosphate to glucose-6-phosphate. It is an equilibrium enzyme and not a regulatory bottleneck. **High-Yield Clinical Pearls for NEET-PG:** * **Cofactor:** Glycogen phosphorylase requires **Pyridoxal Phosphate (Vitamin B6)** as an essential cofactor. * **Clinical Correlation:** A deficiency of liver glycogen phosphorylase leads to **Von Gierke's Type VI (Hers Disease)**, while a deficiency in muscle leads to **McArdle Disease (Type V)**. * **Limit Dextrin:** Glycogen phosphorylase cannot break $\alpha(1\to6)$ bonds; the remaining branched structure it leaves behind is called "limit dextrin."

Q: A genetic disorder renders fructose 1,6-biphosphatase in the liver less sensitive to regulation by fructose 2,6-biphosphate. All of the following metabolic changes are observed in this disorder except?

Level of fructose 1,6-biphosphate is higher than normal. **Explanation:** The core of this question lies in understanding the reciprocal regulation of Glycolysis and Gluconeogenesis by **Fructose 2,6-bisphosphate (F2,6-BP)**. **1. Why Option A is the correct answer (The "Except" statement):** Fructose 2,6-BP is the most potent **allosteric inhibitor** of Fructose 1,6-bisphosphatase (F1,6-BPase), the rate-limiting enzyme of gluconeogenesis. If F1,6-BPase becomes **less sensitive** to this inhibition, the enzyme remains constitutively active. This leads to an accelerated conversion of Fructose 1,6-bisphosphate into Fructose 6-phosphate. Consequently, the steady-state levels of **Fructose 1,6-bisphosphate will be lower than normal**, making Option A false and thus the correct answer. **2. Analysis of Incorrect Options:** * **Option B:** As explained above, increased F1,6-BPase activity depletes the substrate, leading to lower levels of Fructose 1,6-bisphosphate. * **Option C:** Fructose 1,6-bisphosphate is a potent **feed-forward activator** of Pyruvate Kinase. Since its levels are low (Option B), Pyruvate Kinase activity decreases, resulting in less pyruvate formation. * **Option D:** With decreased glycolytic flux (due to low F1,6-BP levels and inhibited Pyruvate Kinase), the net production of ATP from the glycolytic pathway is significantly reduced. **NEET-PG High-Yield Pearls:** * **F2,6-BP Dual Role:** It is a potent **activator of PFK-1** (Glycolysis) and a potent **inhibitor of F1,6-BPase** (Gluconeogenesis). * **Bifunctional Enzyme:** F2,6-BP levels are controlled by the PFK-2/FBPase-2 complex. Insulin increases F2,6-BP (promoting glycolysis), while Glucagon decreases it (promoting gluconeogenesis). * **Clinical Correlation:** Deficiency of F1,6-BPase leads to fasting hypoglycemia and lactic acidosis, as the liver cannot generate glucose from non-carbohydrate precursors.

Q: Which of the following is NOT a true statement about the pentose phosphate pathway?

All of the above statements are true.. The **Pentose Phosphate Pathway (PPP)**, also known as the Hexose Monophosphate (HMP) Shunt, is a unique alternative route for glucose oxidation that occurs in the cytosol. Unlike glycolysis, its primary purpose is not energy production but the generation of **NADPH** and **Pentoses** (Ribose-5-phosphate). ### **Analysis of Statements:** * **Statement A (True):** During the oxidative phase, Glucose-6-phosphate undergoes decarboxylation. Specifically, the enzyme **6-phosphogluconate dehydrogenase** converts 6-phosphogluconate to Ribulose-5-phosphate, releasing **CO₂**. * **Statement B (True):** The oxidative phase is irreversible and utilizes **NADP⁺** as the electron acceptor. The key rate-limiting enzyme, **Glucose-6-phosphate dehydrogenase (G6PD)**, reduces NADP⁺ to NADPH. * **Statement C (True):** The HMP shunt is unique because **no ATP is directly consumed or produced** during the cycle. It is an anabolic-supporting pathway rather than a catabolic energy-yielding one. Since all statements (A, B, and C) are biochemically accurate, **Option D** is the correct choice. ### **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** G6PD (induced by Insulin). * **Tissues involved:** Occurs in tissues requiring NADPH for fatty acid/steroid synthesis (Adrenal cortex, Liver, Lactating mammary glands) or for maintaining reduced glutathione (Erythrocytes). * **Clinical Correlation:** **G6PD Deficiency** leads to hemolytic anemia due to the inability to neutralize free radicals in RBCs, often triggered by fava beans or oxidative drugs (e.g., Primaquine). * **Transketolase:** This enzyme in the non-oxidative phase requires **Thiamine (Vitamin B1)** as a cofactor; measuring its activity is used to diagnose Thiamine deficiency.

Q: After oral glucose feeding of 50 gm, what is the expected metabolic response?

All of these. This question tests your understanding of the **Well-Fed State (Absorptive Phase)**, which is primarily mediated by an increased **Insulin-to-Glucagon ratio**. ### **Explanation of the Correct Answer** When 50g of glucose is ingested, blood glucose levels rise, triggering the release of insulin from pancreatic beta cells. This shifts the body into an anabolic state: * **Decreased Ketone Body Production:** Insulin inhibits **Hormone-Sensitive Lipase (HSL)**, reducing the breakdown of adipose tissue into free fatty acids (FFAs). Since FFAs are the precursors for ketogenesis in the liver, ketone production drops significantly. * **Increased Lactate Production upon Exercise:** In the fed state, muscle glycogen stores are replenished. During subsequent exercise, the abundance of glucose and glycogen leads to increased glycolytic flux. This results in higher pyruvate levels, which are converted to lactate via anaerobic glycolysis. * **Decreased Gluconeogenesis:** Insulin suppresses the expression of key gluconeogenic enzymes (e.g., PEPCK, Fructose-1,6-bisphosphatase) and decreases the availability of substrates like glycerol and amino acids. The body prioritizes using exogenous glucose rather than synthesizing it. Since all three physiological changes occur simultaneously following glucose intake, **Option D** is correct. ### **High-Yield Clinical Pearls for NEET-PG** * **The "Insulin Effect":** Insulin is the only hypoglycemic hormone. It promotes glycolysis, glycogenesis, and lipogenesis while inhibiting gluconeogenesis, glycogenolysis, and ketogenesis. * **Key Enzyme Regulation:** Insulin activates **Phosphofructokinase-1 (PFK-1)** via Fructose-2,6-bisphosphate, making it the "pacemaker" of glycolysis in the fed state. * **Metabolic Switch:** The transition from fasting to fed state is characterized by the brain switching from using ketone bodies (during prolonged fasts) back to exclusive glucose utilization.

Q: All the following enzymes catalyze physiologically irreversible reactions of glycolysis except?

Enolase. ### Explanation In glycolysis, reactions are categorized as either **reversible** or **irreversible**. Irreversible reactions are the key regulatory points of the pathway because they involve a significant decrease in free energy ($\Delta G$), making them "one-way" valves under physiological conditions. **Why Enolase is the Correct Answer:** Enolase catalyzes the conversion of 2-phosphoglycerate to phosphoenolpyruvate (PEP). This is a **reversible** reaction. Unlike the regulatory steps of glycolysis, this step can easily proceed in the opposite direction during **gluconeogenesis** using the same enzyme. **Analysis of Incorrect Options (Irreversible Enzymes):** The three irreversible steps of glycolysis are often referred to as the "bottlenecks" or "regulatory valves": * **Hexokinase (Step 1):** Converts Glucose to Glucose-6-Phosphate. It is irreversible to "trap" glucose inside the cell. * **Phosphofructokinase-1 (PFK-1) (Step 3):** Converts Fructose-6-Phosphate to Fructose-1,6-Bisphosphate. This is the **rate-limiting step** of glycolysis. * **Pyruvate Kinase (Step 10):** Converts PEP to Pyruvate. This is the final irreversible step and produces ATP via substrate-level phosphorylation. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Fluoride Inhibition:** Enolase is inhibited by **Fluoride**. This is why fluoride is added to gray-top vacutainers (sodium fluoride) when collecting blood for glucose estimation—it stops glycolysis to ensure accurate sugar levels. * **Gluconeogenesis Bypass:** Because Hexokinase, PFK-1, and Pyruvate Kinase are irreversible, gluconeogenesis must use four different enzymes (Pyruvate carboxylase, PEP carboxykinase, Fructose-1,6-bisphosphatase, and Glucose-6-phosphatase) to bypass these steps. * **Mnemonic:** Remember the irreversible steps as **"HPP"** (Hexokinase, PFK, Pyruvate Kinase).

Q: Lactose on hydrolysis yields which of the following?

One molecule of glucose and one molecule of galactose. **Explanation:** Lactose, commonly known as "milk sugar," is a disaccharide found exclusively in mammalian milk. It consists of two monosaccharides—**D-glucose and D-galactose**—joined by a **$\beta$(1→4) glycosidic linkage**. During digestion, the enzyme **Lactase** (a brush-border disaccharidase) hydrolyzes this bond, releasing one molecule of glucose and one molecule of galactose into the bloodstream. **Analysis of Options:** * **Option A (2 molecules of fructose):** This does not occur in human carbohydrate metabolism. * **Option B (2 molecules of glucose):** This is the result of the hydrolysis of **Maltose** (by maltase) or **Isomaltose** (by isomaltase). * **Option C (One glucose and one fructose):** This is the result of the hydrolysis of **Sucrose** (table sugar) by the enzyme sucrase. * **Option D (Correct):** Lactose is specifically composed of glucose and galactose. **High-Yield Clinical Pearls for NEET-PG:** 1. **Lactose Intolerance:** Caused by a deficiency of the enzyme lactase. It leads to osmotic diarrhea, bloating, and flatulence due to the bacterial fermentation of undigested lactose in the colon. 2. **Galactosemia:** A deficiency in enzymes like **GALT** (Galactose-1-phosphate uridyltransferase) prevents the metabolism of galactose (derived from lactose), leading to cataracts, liver damage, and intellectual disability in infants. 3. **Reducing Sugar:** Lactose is a reducing sugar because it retains a free anomeric carbon on the glucose residue. 4. **Source:** It is the least sweet of all common sugars and is the only carbohydrate of animal origin in the human diet.

Q: Which enzyme plays an important role in regulating blood glucose levels after feeding?

Glucokinase. **Explanation** **Glucokinase (Hexokinase IV)** is the correct answer because it acts as a "glucose sensor" in the liver and pancreatic beta cells. Unlike Hexokinase I-III, Glucokinase has a **high Km** (low affinity) and a **high Vmax** (high capacity). This means it only becomes significantly active when blood glucose levels are high (postprandial/after feeding). It rapidly phosphorylates glucose to glucose-6-phosphate, trapping it in the liver for glycogen synthesis and glycolysis, thereby lowering blood glucose levels. **Analysis of Incorrect Options:** * **B. Glucose-6-phosphatase:** This enzyme is involved in **gluconeogenesis and glycogenolysis** (fasting state). It converts glucose-6-phosphate back to free glucose to be released into the blood. It increases blood glucose, rather than regulating it after feeding. * **C. Phosphofructokinase-1 (PFK-1):** While it is the rate-limiting enzyme of glycolysis, its primary role is regulating the *flux* of carbohydrate breakdown within the cell to meet energy needs, not the systemic regulation of blood glucose levels. * **D. Pyruvate kinase:** This is the final enzyme of glycolysis. While regulated by insulin, it does not serve as the primary sensor or gatekeeper for blood glucose entry into metabolic pathways. **High-Yield NEET-PG Pearls:** * **Inducibility:** Glucokinase is induced by **Insulin**, whereas Hexokinase is not. * **Inhibition:** Glucokinase is **not** inhibited by its product (Glucose-6-Phosphate), allowing it to continue clearing glucose even when levels are high. * **Clinical Correlation:** Mutations in the Glucokinase gene lead to **MODY type 2** (Maturity-Onset Diabetes of the Young), characterized by a higher threshold for insulin release. * **Localization:** Glucokinase is sequestered in the nucleus by Glucokinase Regulatory Protein (GKRP) during fasting and released into the cytosol after a meal.

Question 1

What is the cofactor for Glycogen phosphorylase in Glycogenolysis?

Accepted Answer

Pyridoxal phosphate

Answer

Thiamine pyrophosphate

Answer

Citrate

Answer

FAD

Question 2

Which of the following is NOT an example of a glycosaminoglycan?

Accepted Answer

None of the above

Answer

Hyaluronic acid

Answer

Chondroitin sulfate

Answer

Heparin

Question 3

Which enzyme, when absent, would impair the rate-limiting step of glycogenolysis?

Accepted Answer

Glycogen phosphorylase

Answer

1,4-Glucanotransferase

Answer

Glycogen synthetase

Answer

Phosphoglucomutase

Question 4

A genetic disorder renders fructose 1,6-biphosphatase in the liver less sensitive to regulation by fructose 2,6-biphosphate. All of the following metabolic changes are observed in this disorder except?

Accepted Answer

Level of fructose 1,6-biphosphate is higher than normal

Answer

Level of fructose 1,6-biphosphate is lower than normal

Answer

Less pyruvate is formed

Answer

Less ATP is generated

Question 5

Which of the following is NOT a true statement about the pentose phosphate pathway?

Accepted Answer

All of the above statements are true.

Answer

Carbon dioxide is produced during the pathway.

Answer

The oxidative phase utilizes NADP+.

Answer

No ATP is directly generated by the pathway.

Question 6

After oral glucose feeding of 50 gm, what is the expected metabolic response?

Accepted Answer

All of these

Answer

Decreased ketone body production

Answer

Increased lactate production upon exercise

Answer

Decreased gluconeogenesis

Question 7

All the following enzymes catalyze physiologically irreversible reactions of glycolysis except?

Accepted Answer

Enolase

Answer

Hexokinase

Answer

Phosphofructokinase

Answer

Pyruvate kinase

Question 8

Lactose on hydrolysis yields which of the following?

Accepted Answer

One molecule of glucose and one molecule of galactose

Answer

2 molecules of fructose

Answer

2 molecules of glucose

Answer

One molecule of glucose and one molecule of fructose

Question 9

Which enzyme plays an important role in regulating blood glucose levels after feeding?

Accepted Answer

Glucokinase

Answer

Glucose-6-phosphatase

Answer

Phosphofructokinase

Answer

Pyruvate kinase

Question 10

In muscles, how many ATP molecules are produced from the conversion of one glucose residue in the linear chain of glycogen to lactic acid via anaerobic glycolysis?

Accepted Answer

3

Answer

1

Answer

2

Answer

4

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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