Carbohydrate Metabolism Practice Questions

Q: Arrange the steps of glycogenolysis in sequence.

Breakdown of alpha(1-6) bond. **Explanation:** Glycogenolysis is the biochemical breakdown of glycogen into glucose-1-phosphate and glucose. The process follows a specific sequential order to navigate the branched structure of glycogen. **Sequential Steps of Glycogenolysis:** 1. **Step 1 (Option C):** **Glycogen Phosphorylase** cleaves $\alpha(1\to4)$ glycosidic bonds from the non-reducing ends. It stops when four glucose residues remain before a branch point. 2. **Step 2 (Option A):** The resulting structure, with short four-residue branches, is called a **Limit Dextrin**. 3. **Step 3 (Option B):** The **Debranching enzyme** (specifically the **4:4 transferase** activity) moves the outer three of the four glucose residues from the branch to a nearby straight chain. 4. **Step 4 (Option D):** Finally, the **$\alpha(1\to6)$ glucosidase** activity of the debranching enzyme hydrolyzes the single remaining glucose residue at the branch point. **Why Option D is the "Final" Step:** In a sequential arrangement, the breakdown of the $\alpha(1\to6)$ bond is the terminal step of the debranching process. While Phosphorylase initiates the process (Option C), it cannot complete it without the debranching enzyme first creating limit dextrins (Option A) and transferring residues (Option B). Thus, the $\alpha(1\to6)$ cleavage represents the completion of a "cycle" of debranching. **Clinical Pearls for NEET-PG:** * **Von Gierke Disease (Type I GSD):** Deficiency of Glucose-6-Phosphatase; presents with severe hypoglycemia and hepatomegaly. * **Cori Disease (Type III GSD):** Deficiency of **Debranching Enzyme**; results in the accumulation of **Limit Dextrins**. * **McArdle Disease (Type V GSD):** Deficiency of **Muscle Glycogen Phosphorylase**; presents with exercise-induced cramps and myoglobinuria. * **Rate-limiting enzyme:** Glycogen Phosphorylase (activated by cAMP-mediated phosphorylation).

Q: In glycolysis, which enzyme catalyzes the first committed step?

Phosphofructokinase. **Explanation:** The correct answer is **Phosphofructokinase-1 (PFK-1)**. In biochemistry, a "committed step" is an irreversible reaction that is unique to a specific pathway and effectively "commits" the substrate to that pathway's completion. 1. **Why PFK-1 is correct:** While Hexokinase catalyzes the first step of glycolysis, its product (Glucose-6-Phosphate) can enter other pathways like the Pentose Phosphate Pathway (PPP) or Glycogenesis. PFK-1 catalyzes the conversion of Fructose-6-Phosphate to **Fructose-1,6-bisphosphate**. This is the first irreversible reaction unique to glycolysis, making it the primary rate-limiting and committed step. 2. **Why other options are incorrect:** * **Hexokinase/Glucokinase:** These catalyze the first step of glycolysis, but not the *committed* step, as G6P has multiple metabolic fates. * **2,3-diphosphoglycerate (2,3-DPG):** This is a bypass product of glycolysis (Rapoport-Luebering shunt) found in RBCs; it is not an enzyme. **High-Yield Clinical Pearls for NEET-PG:** * **Regulation:** PFK-1 is allosterically inhibited by **ATP and Citrate** (energy-rich state) and activated by **AMP and Fructose-2,6-bisphosphate**. * **Fructose-2,6-bisphosphate:** This is the most potent allosteric activator of PFK-1, produced by the bifunctional enzyme PFK-2. * **Insulin vs. Glucagon:** Insulin increases PFK-1 activity (promoting glycolysis), while Glucagon decreases it. * **Rate-limiting enzymes:** Always distinguish between the "first step" (Hexokinase) and the "rate-limiting/committed step" (PFK-1).

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: A person undergoes two fine needle muscle biopsies to obtain small amounts of tissue for biochemical analysis. The first is taken at the beginning of the experiment while the subject is at rest. The second is taken at the end of 30 minutes of aerobic exercise on a stationary bicycle. The activity of the muscle pyruvate dehydrogenase complexes is found to be much higher in the second measurement. Which of the following biochemical changes would be most likely to produce this effect?

Increased pyruvate concentration. **Explanation:** The **Pyruvate Dehydrogenase (PDH) complex** is the key regulatory bridge between glycolysis and the TCA cycle. Its activity is primarily regulated by **reversible phosphorylation**: * **PDH Kinase:** Phosphorylates and **inactivates** PDH. * **PDH Phosphatase:** Dephosphorylates and **activates** PDH. **Why Option D is Correct:** During aerobic exercise, the rate of glycolysis increases to meet energy demands, leading to an **increased pyruvate concentration**. Pyruvate acts as a potent **allosteric inhibitor of PDH Kinase**. By inhibiting the kinase, pyruvate prevents the inactivation of the PDH complex, thereby maintaining it in its active (dephosphorylated) state to facilitate aerobic respiration. **Analysis of Incorrect Options:** * **A. Decreased ADP:** High ADP (low energy signal) actually stimulates PDH by inhibiting PDH kinase. Decreased ADP would imply high ATP, which inhibits the complex. * **B. Decreased intracellular Ca²⁺:** Calcium is a powerful **activator** of PDH phosphatase. During muscle contraction, intracellular Ca²⁺ increases, which activates the phosphatase and subsequently the PDH complex. Decreased Ca²⁺ would result in lower PDH activity. * **C. Increased acetyl CoA:** Acetyl CoA is a product of the PDH reaction. It exerts **feedback inhibition** by activating PDH kinase, which inactivates the complex. **NEET-PG High-Yield Pearls:** * **PDH Activators:** NAD+, ADP, Pyruvate, and Ca²⁺ (especially in skeletal muscle). * **PDH Inhibitors:** NADH, ATP, and Acetyl CoA (products of the reaction). * **Clinical Correlation:** PDH deficiency is the most common cause of congenital lactic acidosis. It presents with neurological symptoms and is managed with a **ketogenic diet** (high fat, low carb) to bypass the PDH block.

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: 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.

Q: An infant presents with hepatosplenomegaly, hypoglycemia, hyperlipidemia, acidosis, and normally structured glycogen deposition in the liver. What is the diagnosis?

Von Gierke's disease. **Explanation:** The clinical presentation of hepatosplenomegaly, severe fasting hypoglycemia, hyperlipidemia, and lactic acidosis, combined with the deposition of **normally structured glycogen**, is the classic triad for **Von Gierke’s Disease (GSD Type I)**. **1. Why Von Gierke’s Disease is correct:** This condition is caused by a deficiency of **Glucose-6-Phosphatase**. This enzyme is the final step in both glycogenolysis and gluconeogenesis. Because the liver cannot convert Glucose-6-Phosphate into free glucose, patients suffer from profound fasting hypoglycemia. The accumulation of G-6-P shunts into alternative pathways, leading to: * **Hyperlipidemia:** Increased VLDL and triglycerides. * **Acidosis:** Specifically lactic acidosis (due to impaired gluconeogenesis) and hyperuricemia (Gout). * **Normal Glycogen Structure:** Unlike debranching enzyme defects, the glycogen molecule itself is synthesized normally but cannot be broken down into glucose. **2. Why other options are incorrect:** * **Her’s Disease (Type VI):** Deficiency of liver phosphorylase. It presents similarly but is much milder; lactic acid and uric acid levels are typically normal. * **Cori’s Disease (Type III):** Deficiency of Debranching enzyme. While it causes hypoglycemia, the glycogen deposited has **abnormal structure** (short outer chains/limit dextrins). * **Andersen’s Disease (Type IV):** Deficiency of Branching enzyme. It presents with liver cirrhosis and failure in early infancy, and the glycogen has **abnormally long, unbranched chains** (amylopectin-like). **NEET-PG High-Yield Pearls:** * **Von Gierke’s** is associated with "doll-like facies" and xanthomas. * **Key Lab Finding:** Hyperuricemia (due to increased PPP shunt and decreased renal clearance of urate). * **Treatment:** Frequent cornstarch feeds to maintain glucose levels.

Q: How many molecules of ATP are produced by substrate-level phosphorylation during glycolysis?

4. **Explanation:** In biochemistry, **substrate-level phosphorylation (SLP)** refers to the direct synthesis of ATP from ADP (or GTP from GDP) using the energy released from a high-energy intermediate, without the involvement of the electron transport chain or oxygen. In the payoff phase of glycolysis, SLP occurs at two specific enzymatic steps for every molecule of glucose: 1. **Phosphoglycerate Kinase:** Conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate (produces 1 ATP). 2. **Pyruvate Kinase:** Conversion of phosphoenolpyruvate (PEP) to pyruvate (produces 1 ATP). Since one glucose molecule splits into two triose phosphates (Glyceraldehyde-3-phosphate), these steps occur twice per glucose molecule. Therefore, **2 + 2 = 4 ATP** molecules are produced via SLP. **Analysis of Options:** * **Option A (5) & B (6):** These numbers do not correspond to any standard ATP yield in glycolysis. * **Option D (3):** This is incorrect. While the *net* gain of ATP in anaerobic glycolysis is 2 (4 produced minus 2 consumed in the preparatory phase), the question specifically asks for the total produced by SLP. **High-Yield Clinical Pearls for NEET-PG:** * **Net ATP Yield:** In aerobic glycolysis, the net yield is 7 or 8 ATP (depending on the shuttle used). In anaerobic glycolysis, the net yield is always 2 ATP. * **Arsenate Poisoning:** Arsenate competes with inorganic phosphate in the GAPDH reaction, bypassing the first SLP step. This results in **zero net ATP** production, though glycolysis continues. * **Rapoport-Luebering Cycle:** In RBCs, 1,3-BPG can be converted to 2,3-BPG. This bypasses the first SLP step, sacrificing ATP production to facilitate oxygen delivery to tissues.

Question 1

Arrange the steps of glycogenolysis in sequence.

Accepted Answer

Breakdown of alpha(1-6) bond

Answer

Formation of limit dextrins

Answer

Transfer of glucose residues from branched chain to neighboring straight chain (Glucan Transferase)

Answer

Breakdown of alpha(1-4) bond from non-reducing end

Question 2

In glycolysis, which enzyme catalyzes the first committed step?

Accepted Answer

Phosphofructokinase

Answer

2, 3-diphosphoglycerate

Answer

Glucokinase

Answer

Hexokinase

Question 3

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 4

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 5

A person undergoes two fine needle muscle biopsies to obtain small amounts of tissue for biochemical analysis. The first is taken at the beginning of the experiment while the subject is at rest. The second is taken at the end of 30 minutes of aerobic exercise on a stationary bicycle. The activity of the muscle pyruvate dehydrogenase complexes is found to be much higher in the second measurement. Which of the following biochemical changes would be most likely to produce this effect?

Accepted Answer

Increased pyruvate concentration

Answer

Decreased ADP

Answer

Decreased intracellular Ca2+

Answer

Increased acetyl CoA

Question 6

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

Accepted Answer

Enolase

Answer

Hexokinase

Answer

Phosphofructokinase

Answer

Pyruvate kinase

Question 7

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 8

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

Question 9

An infant presents with hepatosplenomegaly, hypoglycemia, hyperlipidemia, acidosis, and normally structured glycogen deposition in the liver. What is the diagnosis?

Accepted Answer

Von Gierke's disease

Answer

Her's disease

Answer

Cor's disease

Answer

Anderson's disease

Question 10

How many molecules of ATP are produced by substrate-level phosphorylation during glycolysis?

Accepted Answer

4

Answer

5

Answer

6

Answer

3

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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