Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 411: The availability of which of the following is essential for the functioning of the TCA cycle?

A. Acetyl CoA
B. Oxaloacetate (Correct Answer)
C. Insulin
D. Glucagon

Explanation: **Explanation:** The TCA cycle (Krebs cycle) begins with the condensation of **Acetyl CoA** (2C) and **Oxaloacetate** (4C) to form **Citrate** (6C), catalyzed by the enzyme *Citrate Synthase*. While Acetyl CoA provides the fuel, **Oxaloacetate (OAA)** is considered the "limiting factor" and the "catalytic member" of the cycle. 1. **Why Oxaloacetate is correct:** For the cycle to continue, OAA must be regenerated at the end of each turn. If OAA levels are depleted (e.g., during starvation or gluconeogenesis), the cycle slows down significantly, regardless of how much Acetyl CoA is available. This is why OAA is often called the "primer" of the TCA cycle. 2. **Why Acetyl CoA is incorrect:** Although it is the primary substrate, its availability alone cannot drive the cycle if OAA is absent. In states of high Acetyl CoA but low OAA (like uncontrolled Diabetes Mellitus), Acetyl CoA is diverted toward **ketogenesis** instead of the TCA cycle. 3. **Why Insulin/Glucagon are incorrect:** These are hormones that regulate metabolic pathways (like glycolysis or gluconeogenesis) via phosphorylation or gene expression. They do not act as direct substrates or essential co-factors for the enzymatic reactions within the mitochondrial matrix where the TCA cycle occurs. **High-Yield Clinical Pearls for NEET-PG:** * **Anaplerotic Reaction:** The most important reaction to replenish OAA is the carboxylation of pyruvate by **Pyruvate Carboxylase** (requires Biotin and ATP). * **"Fats burn in the flame of carbohydrates":** This classic adage refers to the fact that OAA is derived from glucose (via pyruvate); without glucose-derived OAA, the Acetyl CoA from fatty acid oxidation cannot enter the TCA cycle. * **Location:** All TCA enzymes are in the mitochondrial matrix except **Succinate Dehydrogenase**, which is located on the inner mitochondrial membrane (Complex II of ETC).

Question 412: Regarding proteoglycans, which statement is false?

A. Chondroitin sulfate is a component of proteoglycans.
B. They hold a small amount of water. (Correct Answer)
C. They are composed of sugars.
D. They carry an electrical charge.

Explanation: **Explanation** Proteoglycans are complex macromolecules consisting of a core protein with one or more covalently attached **glycosaminoglycan (GAG)** chains. **Why Option B is the correct (false) statement:** Proteoglycans are highly hydrophilic. Due to the presence of sulfate and carboxyl groups, they carry a high density of negative charges. these charges repel each other and attract a large cloud of cations (like $Na^+$), which creates high osmotic pressure. Consequently, proteoglycans **hold a large amount of water**, forming a hydrated "gel" that provides structural support and shock absorption in tissues like cartilage and the vitreous humor. **Analysis of other options:** * **Option A:** Chondroitin sulfate is the most abundant GAG in the body and is a primary component of proteoglycans found in cartilage and bone. * **Option C:** They are composed of sugars (specifically repeating disaccharide units of GAGs) which make up about 95% of their weight. * **Option D:** They carry a strong **negative electrical charge** (polyanionic) due to the sulfate and uronic acid groups. **High-Yield NEET-PG Pearls:** * **Hyaluronic Acid:** The only GAG that is **not sulfated** and not covalently attached to a protein core (it exists as a free carbohydrate chain). * **Aggrecan:** The major proteoglycan in cartilage; it associates with hyaluronic acid to form massive aggregates. * **Clinical Correlation:** Deficiencies in enzymes that degrade GAGs lead to **Mucopolysaccharidoses** (e.g., Hurler and Hunter Syndromes), characterized by the accumulation of these molecules in lysosomes.

Question 413: What is the net generation of ATP up to pyruvate in aerobic glycolysis?

A. 2
B. 7 (Correct Answer)
C. 15
D. 35

Explanation: **Explanation:** In aerobic glycolysis, the net ATP yield is calculated by accounting for both direct substrate-level phosphorylation and the oxidative phosphorylation of reduced coenzymes. **The Calculation:** 1. **ATP Consumption:** 2 ATP are consumed in the preparatory phase (Hexokinase and Phosphofructokinase-1 reactions). 2. **ATP Production (Substrate-level):** 4 ATP are produced (2 from Phosphoglycerate kinase and 2 from Pyruvate kinase). 3. **NADH Production:** 2 NADH are generated at the Glyceraldehyde-3-phosphate dehydrogenase step. 4. **The Yield:** In aerobic conditions, these 2 NADH enter the electron transport chain via the **Malate-Aspartate shuttle**, yielding **2.5 ATP per NADH** (Total = 5 ATP). 5. **Net Total:** (4 Substrate ATP + 5 Oxidative ATP) – 2 Consumed ATP = **7 ATP**. *Note: If the Glycerol-3-phosphate shuttle is used (common in muscle/brain), the yield is 1.5 ATP per NADH, totaling 5 net ATP. However, 7 is the standard "high-yield" answer for aerobic glycolysis in most medical exams.* **Analysis of Incorrect Options:** * **Option A (2):** This is the net ATP yield for **anaerobic glycolysis**, where NADH is consumed to reduce pyruvate to lactate, leaving only substrate-level ATP. * **Option C (15):** This represents the ATP yield for one molecule of Acetyl-CoA entering the TCA cycle plus its associated oxidative phosphorylation. * **Option D (35):** This is closer to the total ATP yield for the complete oxidation of one glucose molecule to $CO_2$ and $H_2O$ (approx. 30–32 ATP). **Clinical Pearls for NEET-PG:** * **Rate-limiting step:** Phosphofructokinase-1 (PFK-1). * **Rapoport-Luebering Cycle:** In RBCs, glycolysis can bypass the phosphoglycerate kinase step to produce 2,3-BPG, resulting in **zero net ATP** production for that molecule of glucose. * **Arsenic Poisoning:** Inhibits ATP production in glycolysis by competing with inorganic phosphate at the GAPDH step.

Question 414: A chronic alcoholic has recently developed trouble with their ability to balance, becomes easily confused, and displays nystagmus. An assay of which of the following enzymes can determine a biochemical reason for these symptoms?

A. Isocitrate dehydrogenase
B. Transaldolase
C. Glyceraldehyde-3-phosphate dehydrogenase
D. Transketolase (Correct Answer)

Explanation: ### Explanation **1. Why Transketolase is the Correct Answer:** The patient is presenting with the classic triad of **Wernicke Encephalopathy** (Ataxia, Confusion, and Ophthalmoplegia/Nystagmus), which is caused by a deficiency of **Thiamine (Vitamin B1)**. Thiamine pyrophosphate (TPP) is a vital cofactor for several enzymes, including **Transketolase** (HMP Shunt), Pyruvate Dehydrogenase, and α-ketoglutarate dehydrogenase. In a clinical setting, the diagnosis is confirmed by measuring **Erythrocyte Transketolase Activity**. If the enzyme activity increases significantly upon the addition of TPP in vitro, it confirms a thiamine deficiency. This is the most reliable biochemical marker for B1 status. **2. Why the Other Options are Incorrect:** * **Isocitrate Dehydrogenase (A):** This is a rate-limiting enzyme of the TCA cycle. While it is essential for aerobic metabolism, it does not require Thiamine as a cofactor and is not used to assay B1 levels. * **Transaldolase (B):** Like transketolase, this enzyme is part of the non-oxidative phase of the HMP shunt. However, unlike transketolase, it does **not** require Thiamine (TPP) for its catalytic activity. * **Glyceraldehyde-3-phosphate dehydrogenase (C):** This is a key enzyme in glycolysis. It requires NAD+ as a cofactor, not Thiamine. **3. High-Yield Clinical Pearls for NEET-PG:** * **The "ATP" Mnemonic:** Thiamine is a cofactor for **A**lpha-ketoglutarate dehydrogenase, **T**ransketolase, and **P**yruvate dehydrogenase. * **Wernicke-Korsakoff Syndrome:** If untreated, Wernicke’s progresses to Korsakoff psychosis (irreversible confabulations and anterograde amnesia) involving damage to the **mammillary bodies**. * **Glucose Administration:** Never give IV glucose to a suspected alcoholic before Thiamine. Glucose oxidation consumes remaining Thiamine stores, potentially precipitating acute Wernicke encephalopathy.

Question 415: Which enzyme catalyzes the rate-limiting step of the Citric Acid Cycle?

A. Citrate synthase
B. Isocitrate dehydrogenase
C. Alpha-ketoglutarate dehydrogenase
D. All of the above (Correct Answer)

Explanation: In the Citric Acid Cycle (TCA cycle), the "rate-limiting step" is not confined to a single enzyme but is regulated by three highly exergonic, irreversible reactions. While many textbooks traditionally highlight **Isocitrate Dehydrogenase** as the primary pacemaker, for the NEET-PG exam, it is crucial to recognize that the overall flux of the cycle is controlled by the collective activity of three enzymes: 1. **Citrate Synthase:** The first committed step where Acetyl-CoA joins Oxaloacetate. It is inhibited by ATP, NADH, and Succinyl-CoA. 2. **Isocitrate Dehydrogenase:** Often cited as the "main" rate-limiting step, it catalyzes the oxidative decarboxylation of isocitrate to $\alpha$-ketoglutarate. It is strongly activated by ADP and inhibited by ATP/NADH. 3. **$\alpha$-Ketoglutarate Dehydrogenase:** This multienzyme complex catalyzes the conversion of $\alpha$-ketoglutarate to Succinyl-CoA. It is inhibited by its products (Succinyl-CoA and NADH). **Why "All of the above" is correct:** In metabolic regulation, "rate-limiting" refers to the points where the pathway can be turned on or off. Since all three enzymes are irreversible and subject to allosteric regulation based on the cell's energy status (ATP/ADP ratio), they collectively dictate the cycle's velocity. **High-Yield NEET-PG Pearls:** * **Co-factors:** $\alpha$-Ketoglutarate dehydrogenase requires five co-factors: TPP, Lipoate, CoA, FAD, and NAD+ (Same as Pyruvate Dehydrogenase). * **Inhibitor:** Fluoroacetate inhibits Aconitase (suicide inhibition). * **Arsenite Poisoning:** Inhibits $\alpha$-Ketoglutarate dehydrogenase by binding to the -SH groups of Lipoic acid. * **ATP Yield:** One turn of the TCA cycle produces **10 ATP** (3 NADH = 7.5, 1 FADH₂ = 1.5, 1 GTP = 1).

Question 416: How is fructose transported from the intestine into enterocytes?

A. GLUT 1
B. GLUT 4
C. GLUT 5 (Correct Answer)
D. GLUT 7

Explanation: ### Explanation **Correct Option: C (GLUT 5)** Fructose is unique among dietary monosaccharides because its absorption is independent of sodium. It is transported from the intestinal lumen into the enterocytes via **facilitated diffusion** using the **GLUT 5** transporter. Unlike glucose and galactose, which use the SGLT-1 (Sodium-Glucose Co-transporter 1) for active transport, fructose does not require ATP. Once inside the enterocyte, fructose (along with glucose and galactose) exits into the portal circulation via the **GLUT 2** transporter located on the basolateral membrane. **Why Incorrect Options are Wrong:** * **GLUT 1:** This is a high-affinity glucose transporter found primarily in **erythrocytes (RBCs)** and the **blood-brain barrier**. It provides a basal level of glucose uptake. * **GLUT 4:** This is the only **insulin-dependent** glucose transporter. It is found in **skeletal muscle** and **adipose tissue**. It is sequestered in intracellular vesicles and moves to the cell surface only in the presence of insulin. * **GLUT 7:** This transporter is located on the **endoplasmic reticulum membrane** of hepatocytes. It transports free glucose (produced by glucose-6-phosphatase) into the cytosol before it is released into the blood. **High-Yield Clinical Pearls for NEET-PG:** * **SGLT-1 vs. GLUT 5:** Remember that SGLT-1 is for Glucose/Galactose (Active), while GLUT 5 is specific for **Fructose** (Passive/Facilitated). * **GLUT 2:** Known as the "bidirectional" or "high-capacity, low-affinity" transporter. It is found in the **Liver, Pancreas (beta cells), Kidney, and Small Intestine**. * **Fructose Malabsorption:** A deficiency or downregulation of GLUT 5 can lead to fructose malabsorption, resulting in osmotic diarrhea and bloating.

Question 417: Which of the following is a substrate for gluconeogenesis?

A. Acetyl-CoA
B. Fatty acid
C. Pyruvic acid (Correct Answer)
D. All of the above

Explanation: **Explanation:** **Gluconeogenesis** is the metabolic pathway that results in the generation of glucose from non-carbohydrate substrates. This process occurs primarily in the liver and kidneys during periods of fasting or intense exercise. **Why Pyruvic Acid is Correct:** Pyruvic acid (Pyruvate) is a major substrate for gluconeogenesis. It is converted into **Oxaloacetate** by the enzyme *Pyruvate Carboxylase* (the first regulatory step). Oxaloacetate is then converted to Phosphoenolpyruvate (PEP) by *PEP Carboxykinase*, effectively bypassing the irreversible step of glycolysis. Other major substrates include glucogenic amino acids (like Alanine), Lactate, and Glycerol. **Why Other Options are Incorrect:** * **Acetyl-CoA:** In humans, Acetyl-CoA cannot be converted back into glucose. The reaction catalyzed by the *Pyruvate Dehydrogenase Complex* (Pyruvate → Acetyl-CoA) is irreversible. Furthermore, for every two carbons of Acetyl-CoA that enter the TCA cycle, two carbons are lost as $CO_2$, resulting in no net gain of carbon for glucose synthesis. * **Fatty Acids:** Even-chain fatty acids undergo $\beta$-oxidation to produce Acetyl-CoA. As established above, Acetyl-CoA cannot be used for gluconeogenesis. (Note: Only the small glycerol backbone of a triglyceride and odd-chain fatty acids—which yield Propionyl-CoA—are glucogenic). **High-Yield NEET-PG Pearls:** * **Key Enzymes:** Pyruvate Carboxylase (requires **Biotin**), PEP Carboxykinase, Fructose-1,6-bisphosphatase, and Glucose-6-phosphatase. * **Acetyl-CoA's Role:** While not a substrate, Acetyl-CoA is an **obligatory activator** of Pyruvate Carboxylase. * **Location:** Gluconeogenesis occurs in both the **Mitochondria** and **Cytosol**. * **Leucine and Lysine:** These are the only two amino acids that are purely ketogenic and cannot serve as substrates for gluconeogenesis.

Question 418: Insulin causes a decrease in the activity of which enzyme?

A. PFK-1 (Phosphofructokinase-1)
B. Glucokinase
C. Pyruvate Carboxylase (Correct Answer)
D. Acetyl CoA Carboxylase

Explanation: ### Explanation The metabolic role of **Insulin** is to promote energy storage and glucose utilization (anabolism) while inhibiting the production of new glucose (gluconeogenesis). **Why Pyruvate Carboxylase is the Correct Answer:** Pyruvate Carboxylase is the first rate-limiting enzyme of **gluconeogenesis**, converting pyruvate to oxaloacetate in the mitochondria. Insulin acts as a metabolic "switch" that turns off gluconeogenesis to prevent the liver from producing excess glucose when blood sugar is already high. It decreases the activity and expression of Pyruvate Carboxylase (and PEPCK) to ensure that the flux of carbon atoms is directed toward glycolysis and storage rather than glucose synthesis. **Analysis of Incorrect Options:** * **PFK-1 (Option A):** This is the rate-limiting enzyme of glycolysis. Insulin **increases** its activity indirectly by increasing levels of Fructose-2,6-bisphosphate, its most potent allosteric activator. * **Glucokinase (Option B):** Insulin **induces** the synthesis of Glucokinase in the liver. This allows the liver to effectively trap glucose from the portal blood after a meal. * **Acetyl CoA Carboxylase (Option C):** This is the rate-limiting enzyme for fatty acid synthesis. Insulin **activates** this enzyme via dephosphorylation, promoting the conversion of excess glucose into fat. **NEET-PG High-Yield Pearls:** * **The "Fed State" Rule:** Insulin activates enzymes involved in Glycolysis, Glycogenesis, and Lipogenesis. It inhibits enzymes involved in Gluconeogenesis and Glycogenolysis. * **Key Inhibited Enzymes:** Insulin decreases the activity/expression of the "Big Four" gluconeogenic enzymes: **P**yruvate Carboxylase, **P**EPCK, **F**ructose-1,6-bisphosphatase, and **G**lucose-6-phosphatase (Mnemonic: **P**ush **P**ill **F**or **G**lucose). * **Mechanism:** Insulin often regulates these enzymes through **dephosphorylation** (via Protein Phosphatase 1) or by altering gene transcription via the FOXO1 transcription factor.

Question 419: In an embryo with a complete deficiency of pyruvate kinase, how many net moles of ATP are generated in the conversion of 1 mole of glucose through the glycolytic pathway?

A. 0 (Correct Answer)
B. 1
C. 2
D. 3

Explanation: ### Explanation **1. Why the Correct Answer (A) is Right:** In the normal glycolytic pathway, there is an initial **investment phase** where 2 ATPs are consumed (by Hexokinase and Phosphofructokinase-1). This is followed by a **payoff phase** where 4 ATPs are generated: * 2 ATPs at the **Phosphoglycerate Kinase** step. * 2 ATPs at the **Pyruvate Kinase** step. **Pyruvate Kinase (PK)** catalyzes the final irreversible step of glycolysis: the conversion of Phosphoenolpyruvate (PEP) to Pyruvate. In a **complete deficiency** of PK, the second payoff step is blocked. Consequently, the cell only generates the 2 ATPs from the Phosphoglycerate Kinase step. * **Net ATP Calculation:** (2 ATP generated) - (2 ATP invested) = **0 Net ATP.** **2. Why the Incorrect Options are Wrong:** * **Option B (1 ATP):** This does not correspond to any standard stage of glycolysis. * **Option C (2 ATP):** This is the net yield of normal anaerobic glycolysis in a healthy cell (4 generated - 2 invested). * **Option D (3 ATP):** This is the net yield of glycolysis starting from **Glycogen** (1 ATP saved as the Hexokinase step is bypassed), but only if PK is functional. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **RBC Vulnerability:** Mature RBCs lack mitochondria and depend entirely on glycolysis for energy. PK deficiency is the **second most common** cause of enzyme-deficient hemolytic anemia (after G6PD deficiency). * **Mechanism of Anemia:** Without ATP, the Na+/K+ ATPase pump fails, leading to loss of ion balance, cell dehydration (echinocytes), and premature destruction in the spleen. * **Biochemical Marker:** PK deficiency leads to an accumulation of upstream metabolites, most notably **2,3-Bisphosphoglycerate (2,3-BPG)**. This causes a **right shift** in the Oxygen Dissociation Curve, helping oxygen delivery to tissues despite the anemia. * **Inheritance:** Autosomal Recessive.

Question 420: Essential pentosuria is caused by a defect in which metabolic pathway?

A. Hexose Monophosphate (HMP) shunt
B. Uronic acid pathway (Correct Answer)
C. Tricarboxylic acid (TCA) cycle
D. Gluconeogenesis

Explanation: **Explanation:** **Essential Pentosuria** is a rare, benign autosomal recessive metabolic disorder caused by a deficiency of the enzyme **L-xylulose reductase**. This enzyme is a key component of the **Uronic Acid Pathway** (also known as the Glucuronic Acid Pathway). 1. **Why Option B is correct:** In the normal uronic acid pathway, glucuronic acid is converted to L-xylulose. Under normal conditions, L-xylulose reductase then reduces L-xylulose to **xylitol**, which eventually enters the HMP shunt. In essential pentosuria, the deficiency of this enzyme leads to the accumulation of L-xylulose in the blood and its subsequent excretion in the urine. 2. **Why other options are incorrect:** * **HMP Shunt (A):** While the products of the uronic acid pathway eventually enter the HMP shunt, the primary defect lies in the uronic acid pathway itself. * **TCA Cycle (C):** This is the final common pathway for the oxidation of carbohydrates, fats, and proteins; it is unrelated to pentose metabolism. * **Gluconeogenesis (D):** This pathway involves the synthesis of glucose from non-carbohydrate precursors and does not involve L-xylulose. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** It is a **"benign"** condition. Patients are asymptomatic, and it is usually discovered incidentally during routine urine testing. * **Diagnostic Pitfall:** L-xylulose is a **reducing sugar**. Therefore, urine tests using Benedict’s reagent will be positive, which can lead to a **misdiagnosis of Diabetes Mellitus**. However, the glucose oxidase test (dipstick) will be negative. * **Drug Interaction:** Administration of drugs like **Aminopyrine** or **Barbital** can increase the excretion of L-xylulose in these patients by inducing the uronic acid pathway.

Practice by Chapter

Carbohydrate Chemistry and Classification

Practice Questions

Glycolysis: Reactions and Regulation

Practice Questions

Gluconeogenesis: Reactions and Regulation

Practice Questions

Glycogen Metabolism: Synthesis and Breakdown

Practice Questions

Glycogen Storage Diseases

Practice Questions

Pentose Phosphate Pathway

Practice Questions

Metabolism of Fructose and Galactose

Practice Questions

Disorders of Fructose and Galactose Metabolism

Practice Questions

Blood Glucose Regulation

Practice Questions

Diabetes Mellitus: Biochemical Aspects

Practice Questions

Glycosylation and Glycoproteins

Practice Questions

Lactose Intolerance and Galactosemia

Practice Questions

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