Carbohydrate Metabolism Practice Questions

Q: Which enzyme catalyzes the irreversible step in the citric acid cycle?

α-ketoglutarate dehydrogenase. ### Explanation The Citric Acid Cycle (TCA cycle) is regulated primarily at its **irreversible steps**, which have a large negative Gibbs free energy ($\Delta G$). **Why α-ketoglutarate dehydrogenase is correct:** The conversion of α-ketoglutarate to Succinyl-CoA by the **α-ketoglutarate dehydrogenase complex** is a key regulatory, irreversible oxidative decarboxylation step. This multienzyme complex requires five cofactors (Thiamine, Lipoic acid, CoA, FAD, and NAD+). It is inhibited by its products (Succinyl-CoA and NADH) and activated by $Ca^{2+}$. **Analysis of Incorrect Options:** * **Aconitase:** Catalyzes the reversible isomerization of Citrate to Isocitrate via cis-aconitate. * **Succinate thiokinase (Succinyl-CoA Synthetase):** Catalyzes the reversible conversion of Succinyl-CoA to Succinate. This step is unique as it involves **substrate-level phosphorylation** (generating GTP/ATP). * **Isocitrate dehydrogenase:** While this is the **rate-limiting step** and is physiologically irreversible, in the context of standard NEET-PG questions, α-ketoglutarate dehydrogenase and Citrate Synthase are the classic examples of committed, highly exergonic irreversible steps. (Note: If both are present, Isocitrate dehydrogenase is the rate-limiting enzyme, but α-ketoglutarate dehydrogenase is the most "irreversible" in terms of equilibrium). **High-Yield Clinical Pearls for NEET-PG:** 1. **Three Irreversible Steps:** Citrate Synthase, Isocitrate Dehydrogenase, and α-ketoglutarate Dehydrogenase. 2. **Cofactor Dependency:** α-ketoglutarate dehydrogenase requires the same five cofactors as the Pyruvate Dehydrogenase (PDH) complex. **Thiamine deficiency** (B1) inhibits these enzymes, leading to impaired ATP production and Wernicke-Korsakoff syndrome. 3. **Arsenic Poisoning:** Arsenite inhibits α-ketoglutarate dehydrogenase by binding to the -SH groups of **Lipoic acid**, leading to a buildup of upstream metabolites.

Q: Which of the following enzymes is a constituent of the HMP shunt?

G-6-P dehydrogenase. ### Explanation **Correct Answer: C. G-6-P dehydrogenase (G6PD)** The **Hexose Monophosphate (HMP) Shunt**, also known as the Pentose Phosphate Pathway (PPP), occurs in the cytosol. **Glucose-6-phosphate dehydrogenase (G6PD)** is the **rate-limiting and committed enzyme** of the oxidative phase of this pathway. It catalyzes the conversion of Glucose-6-phosphate to 6-phosphogluconolactone, simultaneously reducing NADP+ to **NADPH**. This pathway is crucial for generating NADPH (used in reductive biosynthesis and maintaining reduced glutathione) and Ribose-5-phosphate (for nucleotide synthesis). **Why the other options are incorrect:** * **A. Glucose-6-phosphatase:** This enzyme is involved in **Gluconeogenesis** and **Glycogenolysis**. It converts Glucose-6-phosphate back to free Glucose, primarily in the liver and kidneys. * **B. Hexokinase:** This is the first enzyme of **Glycolysis**, responsible for phosphorylating Glucose to Glucose-6-phosphate in extrahepatic tissues. * **D. Phosphorylase (Glycogen Phosphorylase):** This is the rate-limiting enzyme of **Glycogenolysis**, which breaks down glycogen into Glucose-1-phosphate. **High-Yield Clinical Pearls for NEET-PG:** * **G6PD Deficiency:** The most common enzymopathy worldwide. It leads to **hemolytic anemia** under oxidative stress (e.g., Fava beans, Primaquine, Infection) because RBCs cannot generate enough NADPH to maintain reduced glutathione, leading to **Heinz bodies** and **Bite cells**. * **Tissue Distribution:** The HMP shunt is highly active in tissues requiring NADPH for fatty acid or steroid synthesis (Adrenal cortex, Liver, Lactating mammary glands) and in RBCs to combat oxidative stress. * **Non-oxidative Phase:** Uses **Transketolase**, which requires **Thiamine (Vitamin B1)** as a cofactor—a common point of integration with nutrition questions.

Q: Which of the following glycosaminoglycans plays a significant role in wound healing?

Hyaluronic acid. ### Explanation **Correct Answer: C. Hyaluronic acid** **Why Hyaluronic acid is correct:** Hyaluronic acid (HA) is a unique glycosaminoglycan (GAG) because it is non-sulfated and not bound to a protein core. It plays a pivotal role in **wound healing** by creating a highly hydrated, porous extracellular matrix. During the early inflammatory phase, HA levels rise, facilitating the migration and proliferation of fibroblasts and keratinocytes. Its "molecular sieve" property allows for the diffusion of nutrients and signaling molecules, which is essential for tissue repair and remodeling. **Why the other options are incorrect:** * **A. Keratan sulfate:** Found primarily in the **cornea** (maintaining transparency) and cartilage. It does not play a primary role in the dynamic process of wound repair. * **B. Dermatan sulfate:** Predominantly found in the **skin, blood vessels, and heart valves**. While it is present in the skin, its primary function relates to structural integrity and anticoagulation (via interaction with Heparin Cofactor II) rather than the active healing phase. * **C. Chondroitin sulfate:** The most abundant GAG in the body, found in **cartilage and bone**. It provides tensile strength and resistance to compression but is not the primary mediator of wound healing. **High-Yield NEET-PG Pearls:** * **Hyaluronic Acid:** Only GAG that is **not sulfated**, not covalently linked to protein (not a proteoglycan), and contains **Glucuronic acid + N-acetylglucosamine**. * **Dermatan Sulfate:** Associated with **Hurler Syndrome** (accumulation due to alpha-L-iduronidase deficiency). * **Heparin:** The only GAG that is **intracellular** (found in mast cells) and acts as a potent natural anticoagulant. * **Chondroitin Sulfate:** Most abundant GAG in the human body.

Q: Which of the following statements regarding the TCA cycle is true?

It is amphibolic in nature.. ### Explanation **1. Why the Correct Answer is Right:** The TCA cycle (Krebs cycle) is described as **amphibolic** because it plays a dual role in metabolism: * **Catabolic role:** It involves the oxidation of Acetyl-CoA into $CO_2$ and $H_2O$ to generate energy in the form of ATP, NADH, and $FADH_2$. * **Anabolic role:** Its intermediates serve as precursors for various biosynthetic pathways. For example, **Succinyl-CoA** is used for heme synthesis, and **$\alpha$-ketoglutarate** and **Oxaloacetate** are used for the synthesis of non-essential amino acids. **2. Why the Other Options are Incorrect:** * **Option A:** The TCA cycle is strictly **aerobic**. While oxygen isn't used directly in the cycle, it is required to regenerate $NAD^+$ and $FAD$ via the Electron Transport Chain (ETC). Without oxygen, the cycle halts. * **Option B:** The TCA cycle occurs in the **mitochondrial matrix**. The only exception is the enzyme *Succinate Dehydrogenase*, which is located on the inner mitochondrial membrane. Glycolysis occurs in the cytosol. * **Option C:** The TCA cycle provides several intermediates for gluconeogenesis. Most notably, **Oxaloacetate** is a direct precursor for phosphoenolpyruvate (PEP) synthesis. **3. NEET-PG High-Yield Clinical Pearls:** * **Rate-limiting enzyme:** Isocitrate Dehydrogenase. * **Substrate-level phosphorylation:** Occurs at the step where Succinyl-CoA is converted to Succinate (catalyzed by Succinate thiokinase), producing **GTP**. * **Inhibitors to remember:** **Fluoroacetate** (inhibits Aconitase), **Arsenite** (inhibits $\alpha$-ketoglutarate dehydrogenase), and **Malonate** (competitive inhibitor of Succinate dehydrogenase). * **Vitamins required:** The $\alpha$-ketoglutarate dehydrogenase complex requires five cofactors: Thiamine ($B_1$), Riboflavin ($B_2$), Niacin ($B_3$), Pantothenic acid ($B_5$), and Lipoic acid.

Q: What is the end product of anaerobic glycolysis?

2 ATP + 2 NAD. ### Explanation **Concept:** In anaerobic glycolysis (occurring in RBCs or exercising muscle), the primary goal is to generate energy in the absence of oxygen. The process follows the standard glycolytic pathway until the formation of **Pyruvate**. However, to keep glycolysis running, the cell must regenerate **NAD+** from **NADH**. This is achieved by the enzyme **Lactate Dehydrogenase (LDH)**, which reduces Pyruvate to **Lactate** while simultaneously oxidizing NADH back to NAD+. **Why Option A is Correct:** 1. **Net ATP:** Glycolysis consumes 2 ATP and produces 4 ATP, resulting in a **net gain of 2 ATP**. 2. **NAD+ Regeneration:** The 2 NADH molecules produced during the glyceraldehyde-3-phosphate dehydrogenase step are consumed by LDH to produce **2 NAD+**. Therefore, the "end products" in terms of coenzymes are 2 NAD+. **Why Other Options are Incorrect:** * **Option B:** 2 NADH is the product of *aerobic* glycolysis (before the electron transport chain). In anaerobic conditions, NADH is used up to create lactate. * **Option C:** FADH2 is produced in the TCA cycle (Succinate Dehydrogenase step), not in glycolysis. * **Option D:** While 4 ATP are produced in the payoff phase, the *net* yield is 2 ATP because 2 were invested in the preparatory phase. **NEET-PG High-Yield Pearls:** * **RBCs:** Always undergo anaerobic glycolysis because they lack mitochondria. * **Key Enzyme:** **Lactate Dehydrogenase (LDH)** is the marker for anaerobic metabolism. * **Rapoport-Luebering Cycle:** A shunt in RBC glycolysis that produces 2,3-BPG, which shifts the oxygen dissociation curve to the right (facilitating O2 unloading). * **Arsenite Poisoning:** Inhibits enzymes requiring lipoic acid, but **Arsenate** competes with inorganic phosphate in glycolysis, resulting in **zero net ATP** production.

Q: Activity of which of the following enzymes is NOT affected by insulin?

Hexokinase. ### Explanation The correct answer is **Hexokinase (Option C)**. **1. Why Hexokinase is the correct answer:** Insulin is an anabolic hormone that regulates glucose metabolism primarily through **transcriptional regulation** (inducing/repressing enzyme synthesis) and **covalent modification** (dephosphorylation). **Hexokinase (specifically Type I, II, and III)** is a constitutive enzyme found in most extrahepatic tissues. It has a very low $K_m$ (high affinity) for glucose, allowing it to function at maximum velocity even during fasting. Its activity is primarily regulated by **allosteric inhibition** by its product, **Glucose-6-Phosphate**, and it is **not** induced or activated by insulin. **2. Why the other options are incorrect:** * **Glucokinase (Hexokinase IV):** Unlike other hexokinases, Glucokinase (found in liver and pancreatic $\beta$-cells) is **induced by insulin** at the transcriptional level. It has a high $K_m$ and is not inhibited by G6P. * **Pyruvate Kinase:** Insulin promotes the **dephosphorylation** (activation) of the L-type Pyruvate Kinase in the liver via protein phosphatase-1. It also induces its synthesis to promote glycolysis. * **Glycogen Synthase:** This is the rate-limiting enzyme of glycogenesis. Insulin triggers a signaling cascade that inactivates Glycogen Synthase Kinase-3 (GSK3) and activates protein phosphatase-1, keeping Glycogen Synthase in its **active (dephosphorylated) state**. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** Insulin **dephosphorylates** to activate (Exceptions: Glycogen phosphorylase and Hormone-sensitive lipase are *inactivated* by dephosphorylation). * **Glucokinase vs. Hexokinase:** Glucokinase acts as a "glucose sensor" in the pancreas; mutations in the Glucokinase gene lead to **MODY type 2** (Maturity-Onset Diabetes of the Young). * **Key Insulin-Induced Enzymes:** Glucokinase, PFK-1, Pyruvate Kinase, Acetyl-CoA Carboxylase, and HMG-CoA Reductase.

Q: Which of the following is an allosteric stimulator of glycogen phosphorylase?

AMP. **Explanation:** The regulation of **Glycogen Phosphorylase**, the rate-limiting enzyme of glycogenolysis, is a high-yield topic for NEET-PG. This enzyme exists in two forms: *Phosphorylase a* (active, phosphorylated) and *Phosphorylase b* (inactive, dephosphorylated). **Why AMP is the Correct Answer:** In muscle tissue, **AMP** acts as a potent **allosteric activator**. A high concentration of AMP signifies a low-energy state in the cell. AMP binds to glycogen phosphorylase *b*, inducing a conformational change that activates the enzyme even without covalent phosphorylation. This ensures that glycogen is broken down rapidly to provide glucose for ATP production during vigorous exercise. **Analysis of Incorrect Options:** * **ATP (Option A):** High levels of ATP indicate an energy-rich state. ATP competes with AMP for the same allosteric site, acting as an **inhibitor** to prevent unnecessary glycogen breakdown. * **Insulin (Option C):** Insulin is a hormone, not an allosteric modulator. It regulates the enzyme **hormonally** by promoting dephosphorylation (via Protein Phosphatase-1), which converts the active form to the inactive *phosphorylase b*. * **Glucose-6-phosphate (Option D):** This is a downstream product of glycogenolysis. High levels signal that the cell's energy needs are met; thus, it acts as an **allosteric inhibitor**. **High-Yield Clinical Pearls for NEET-PG:** * **Muscle vs. Liver:** In the **liver**, glycogen phosphorylase is primarily inhibited by **Free Glucose**, which acts as a sensor for blood sugar levels. AMP does *not* significantly stimulate the liver isoform. * **Calcium Regulation:** During muscle contraction, **Ca²⁺** binds to the calmodulin subunit of Phosphorylase Kinase, activating it and further stimulating glycogenolysis. * **McArdle Disease (GSD Type V):** Caused by a deficiency of skeletal muscle glycogen phosphorylase, leading to exercise intolerance and "second wind" phenomenon.

Q: Deficiency of lysosomal maltase causes which condition?

Pompe disease. **Explanation:** **Correct Answer: D. Pompe disease** Pompe disease (Glycogen Storage Disease Type II) is unique among glycogen storage diseases because it is also a **lysosomal storage disorder**. It is caused by a deficiency of the enzyme **lysosomal alpha-1,4-glucosidase** (also known as **Acid Maltase**). While most glycogen breakdown occurs in the cytosol, about 1–3% of glycogen is degraded within lysosomes. In Pompe disease, glycogen accumulates within the lysosomes of all organs, most significantly affecting cardiac and skeletal muscle, leading to massive cardiomegaly and respiratory failure. **Analysis of Incorrect Options:** * **A. McArdle's disease (GSD Type V):** Caused by a deficiency of **muscle glycogen phosphorylase**. It presents with exercise intolerance, muscle cramps, and myoglobinuria, but lacks cardiac involvement. * **B. Andersen disease (GSD Type IV):** Caused by a deficiency of the **branching enzyme**. It leads to the accumulation of abnormal glycogen with long outer chains (amylopectin-like), resulting in early-onset liver cirrhosis. * **C. Cori disease (GSD Type III):** Caused by a deficiency of the **debranching enzyme**. It results in the accumulation of "limit dextrin" and presents with hepatomegaly and hypoglycemia, similar to Von Gierke disease but milder. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** "Pompe trashes the **Pump** (heart)." * **Key Feature:** It is the only GSD that is a lysosomal storage disease. * **Clinical Presentation:** Infantile form presents with "floppy baby" syndrome (hypotonia) and **massive cardiomegaly**. * **Biochemical Marker:** Normal blood glucose levels (unlike Type I, III, and VI) because cytosolic glycogenolysis remains intact.

Q: McArdle's disease causes muscle cramps and muscle fatigue with increased muscle glycogen. Which of the following enzymes is deficient?

Muscle phosphorylase. **Explanation:** **McArdle’s Disease (Glycogen Storage Disease Type V)** is caused by a deficiency of **Muscle Phosphorylase** (myophosphorylase). This enzyme is responsible for the rate-limiting step of glycogenolysis in skeletal muscle, breaking down glycogen into glucose-1-phosphate. Without it, muscles cannot mobilize glucose during strenuous exercise, leading to the characteristic clinical triad of muscle cramps, exercise intolerance, and increased muscle glycogen stores. **Analysis of Options:** * **Muscle Phosphorylase (Correct):** Deficiency prevents glycogen breakdown in muscles. A hallmark finding is the **failure of blood lactate to rise** after ischemic exercise, as glycogen cannot be converted to lactate. * **Hepatic Hexokinase:** This enzyme is involved in glucose phosphorylation in the liver. Its deficiency would affect glycolysis, not glycogen storage, and would not present with muscle-specific symptoms. * **Muscle Glycogen Synthetase:** Deficiency of this enzyme (GSD Type 0b) would lead to a *decrease* in glycogen levels (hypoglycemia/fatigue), whereas McArdle’s presents with *increased* glycogen. * **Muscle Hexokinase:** While hexokinase is vital for glucose entry into glycolysis, its deficiency does not lead to the massive accumulation of glycogen seen in GSDs. **High-Yield Clinical Pearls for NEET-PG:** * **"Second Wind" Phenomenon:** Patients often experience improved exercise tolerance after a few minutes of activity once the body switches to using free fatty acids and blood glucose. * **Myoglobinuria:** Intense exercise can lead to rhabdomyolysis, resulting in "burgundy-colored" urine (myoglobinuria) and potential renal failure. * **Diagnosis:** Ischemic forearm exercise test (flat lactate curve) and muscle biopsy showing subsarcolemmal glycogen deposits.

Q: The enzyme alpha-amylase secreted by the pancreas digests starch into which of the following major products?

Limit dextrins, maltose, and maltotriose. ### Explanation **1. Why Option D is Correct:** Pancreatic alpha-amylase is an **endoamylase** that specifically hydrolyzes internal **alpha-1,4 glycosidic linkages** in starch. However, it cannot break alpha-1,6 linkages (branch points) or the alpha-1,4 bonds immediately adjacent to these branch points. As a result, starch (amylopectin) is not broken down into individual glucose molecules in the lumen. Instead, the digestion stops at three major products: * **Maltose:** A disaccharide (2 glucose units). * **Maltotriose:** A trisaccharide (3 glucose units). * **Alpha-limit dextrins:** Branched oligosaccharides containing the alpha-1,6 bonds that amylase cannot cleave. **2. Why Other Options are Incorrect:** * **Option A:** Amylose and amylopectin are the *substrates* (complex starches), not the products of digestion. * **Option B:** Glucose, galactose, and fructose are **monosaccharides**. Amylase does not produce monosaccharides; these are the final products of digestion by **brush border enzymes** (maltase, lactase, sucrase). * **Option C:** Sucrose is a dietary disaccharide (table sugar), not a breakdown product of starch. **3. NEET-PG High-Yield Pearls:** * **Site of Action:** Digestion begins in the mouth (Salivary Amylase/Ptyalin) but the majority occurs in the small intestine via Pancreatic Amylase. * **pH Requirement:** Amylase requires an alkaline pH (~7.1) to function optimally, which is provided by pancreatic bicarbonate. * **Final Step:** The products (maltose, maltotriose, limit dextrins) are converted to glucose by brush border enzymes (**Maltase** and **Isomaltase/Alpha-dextrinase**) before absorption via **SGLT-1**. * **Clinical Link:** In acute pancreatitis, serum amylase levels rise rapidly, serving as a common diagnostic marker.

Question 1

Which enzyme catalyzes the irreversible step in the citric acid cycle?

Accepted Answer

α-ketoglutarate dehydrogenase

Answer

Aconitase

Answer

Succinate thiokinase

Answer

Isocitrate dehydrogenase

Question 2

Which of the following enzymes is a constituent of the HMP shunt?

Accepted Answer

G-6-P dehydrogenase

Answer

Glucose-6-phosphatase

Answer

Hexokinase

Answer

Phosphorylase

Question 3

Which of the following glycosaminoglycans plays a significant role in wound healing?

Accepted Answer

Hyaluronic acid

Answer

Keratan sulfate

Answer

Dermatan sulfate

Answer

Chondroitin sulfate

Question 4

Which of the following statements regarding the TCA cycle is true?

Accepted Answer

It is amphibolic in nature.

Answer

It is an anaerobic process.

Answer

It occurs in the cytosol.

Answer

It contains no intermediates for gluconeogenesis.

Question 5

What is the end product of anaerobic glycolysis?

Accepted Answer

2 ATP + 2 NAD

Answer

2 ATP + 2 NADH

Answer

2 ATP + 2 FADH2

Answer

4 ATP + 2 NAD

Question 6

Activity of which of the following enzymes is NOT affected by insulin?

Accepted Answer

Hexokinase

Answer

Pyruvate kinase

Answer

Glycogen synthase

Answer

Glucokinase

Question 7

Which of the following is an allosteric stimulator of glycogen phosphorylase?

Accepted Answer

AMP

Answer

ATP

Answer

Insulin

Answer

Glucose-6-phosphate

Question 8

Deficiency of lysosomal maltase causes which condition?

Accepted Answer

Pompe disease

Answer

McArdle's disease

Answer

Andersen disease

Answer

Cori disease

Question 9

McArdle's disease causes muscle cramps and muscle fatigue with increased muscle glycogen. Which of the following enzymes is deficient?

Accepted Answer

Muscle phosphorylase

Answer

Hepatic hexokinase

Answer

Muscle glycogen synthetase

Answer

Muscle hexokinase

Question 10

The enzyme alpha-amylase secreted by the pancreas digests starch into which of the following major products?

Accepted Answer

Limit dextrins, maltose, and maltotriose

Answer

Amylose, amylopectin, and maltose

Answer

Glucose, galactose, and fructose

Answer

Glucose, sucrose, and maltotriose

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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