Carbohydrate Metabolism Practice Questions

Q: Which of the following molecules can participate in gluconeogenesis?

Propionyl CoA. **Explanation:** Gluconeogenesis is the metabolic pathway that results in the generation of glucose from non-carbohydrate substrates. **Why Propionyl CoA is Correct:** Propionyl CoA is produced during the oxidation of **odd-chain fatty acids** and the catabolism of certain amino acids (Valine, Isoleucine, Methionine, Threonine). It is converted into **Succinyl CoA** (a TCA cycle intermediate) via a Vitamin B12-dependent pathway. Succinyl CoA eventually forms Oxaloacetate, which enters the gluconeogenic pathway. This makes Propionyl CoA the only part of a fatty acid that can contribute to a net gain of glucose. **Why Other Options are Incorrect:** * **Acetyl CoA:** In humans, the Pyruvate Dehydrogenase (PDH) reaction is irreversible. Acetyl CoA cannot be converted back to Pyruvate. Furthermore, for every two carbons of Acetyl CoA entering the TCA cycle, two are lost as $CO_2$; thus, there is no net synthesis of glucose from Acetyl CoA. * **Muscle Glycogen:** While muscle glycogen breaks down into Glucose-1-Phosphate, it cannot contribute to blood glucose levels because muscle lacks the enzyme **Glucose-6-Phosphatase**. It is used exclusively for local energy production via glycolysis. **High-Yield NEET-PG Pearls:** * **Key Substrates:** Lactate (Cori Cycle), Glycerol (from TAGs), Glucogenic amino acids (mainly Alanine), and Propionate. * **Rate-limiting enzyme:** Fructose-1,6-bisphosphatase. * **Clinical Link:** Deficiency of **Propionyl-CoA carboxylase** (requires Biotin) leads to Propionic Acidemia, presenting with vomiting, ketosis, and developmental delay. * **Odd vs. Even:** Even-chain fatty acids are **never** gluconeogenic; only odd-chain fatty acids are.

Q: Which of the following is a non-reducing sugar?

Sucrose. ### Explanation **Concept Overview:** A sugar is classified as **reducing** if it has a free or potentially free aldehyde or ketone group. This group allows the sugar to act as a reducing agent in chemical tests (like Benedict’s or Fehling’s). In disaccharides, if the **anomeric carbons** of both monosaccharides are involved in a glycosidic bond, the sugar becomes **non-reducing**. **Why Sucrose is the Correct Answer:** Sucrose is a disaccharide composed of **$\alpha$-D-glucose** and **$\beta$-D-fructose**. The glycosidic linkage occurs between the **C1 of glucose** (aldehyde group) and the **C2 of fructose** (keto group). Since both functional groups are locked in the bond, there is no free reactive group available to reduce copper or silver ions. **Analysis of Incorrect Options:** * **A. Glucose:** A monosaccharide with a free aldehyde group at C1; it is a classic reducing sugar. * **B. Lactose:** A disaccharide (Galactose + Glucose) with a **$\beta$(1$\rightarrow$4) linkage**. The anomeric carbon (C1) of the glucose unit remains free. * **C. Maltose:** A disaccharide (Glucose + Glucose) with an **$\alpha$(1$\rightarrow$4) linkage**. The anomeric carbon of the second glucose unit is free. **High-Yield Clinical Pearls for NEET-PG:** * **Benedict’s Test:** Used to detect reducing sugars in urine (e.g., glucosuria in Diabetes Mellitus). * **Invert Sugar:** Sucrose is dextrorotatory, but upon hydrolysis, it produces a levorotatory mixture of glucose and fructose; hence, hydrolyzed sucrose is called "invert sugar." * **Trehalose:** Another high-yield non-reducing sugar (found in mushrooms) where two glucose units are linked via an **$\alpha$1$\rightarrow$1 bond**. * **Seliwanoff’s Test:** Sucrose gives a positive result (cherry red color) because it contains fructose (a ketose).

Q: Which of the following enzymes found in the liver is involved in gluconeogenesis during the postabsorptive state?

Glucose-6-phosphatase. ### Explanation **Correct Answer: C. Glucose-6-phosphatase** **Why it is correct:** Gluconeogenesis is the metabolic pathway that generates glucose from non-carbohydrate precursors (like lactate, glycerol, and amino acids) during fasting or the postabsorptive state. **Glucose-6-phosphatase** is one of the four "key" or "bottleneck" enzymes of gluconeogenesis. It catalyzes the final step: the hydrolysis of Glucose-6-phosphate into free glucose and inorganic phosphate. This enzyme is primarily located in the lumen of the endoplasmic reticulum of the liver and kidneys, allowing these organs to release free glucose into the bloodstream to maintain glycemic levels. **Why the other options are incorrect:** * **A & B (G6PD and 6-phosphogluconate dehydrogenase):** These are the rate-limiting enzymes of the **Pentose Phosphate Pathway (Hexose Monophosphate Shunt)**. They are involved in generating NADPH and ribose-5-phosphate, not in the synthesis of glucose. * **D (Glucokinase):** This enzyme is involved in **glycolysis** and glycogenesis. It catalyzes the conversion of glucose to glucose-6-phosphate. In the postabsorptive state, glucokinase activity is low to prevent a futile cycle, as its action is the exact opposite of Glucose-6-phosphatase. **High-Yield Clinical Pearls for NEET-PG:** * **The Four Key Gluconeogenic Enzymes:** Pyruvate carboxylase, PEP carboxykinase, Fructose-1,6-bisphosphatase, and Glucose-6-phosphatase. * **Von Gierke Disease (GSD Type I):** Caused by a deficiency of Glucose-6-phosphatase. It presents with severe fasting hypoglycemia, hepatomegaly, and lactic acidosis because the liver cannot export glucose. * **Muscle Metabolism:** Muscle lacks Glucose-6-phosphatase; therefore, muscle glycogen cannot contribute directly to blood glucose levels.

Q: Gluconeogenesis does not occur significantly from which of the following in humans?

Fatty acids. **Explanation:** The correct answer is **Fatty acids**. In humans, even-chain fatty acids cannot be converted into glucose because the breakdown of fatty acids yields **Acetyl-CoA**. The conversion of Pyruvate to Acetyl-CoA (via the Pyruvate Dehydrogenase complex) is **irreversible**. Humans lack the enzymes (Isocitrate lyase and Malate synthase) of the **Glyoxylate cycle**, which would allow the conversion of Acetyl-CoA into oxaloacetate for gluconeogenesis. Consequently, Acetyl-CoA enters the TCA cycle and is completely oxidized to $CO_2$. **Why the other options are incorrect:** * **Lactate:** Produced during anaerobic glycolysis (Cori Cycle), lactate is transported to the liver where it is converted back to pyruvate by LDH and enters gluconeogenesis. * **Pyruvate:** This is a primary substrate. It is carboxylated to Oxaloacetate by Pyruvate Carboxylase, the first regulatory step of gluconeogenesis. * **Amino Acids:** Glucogenic amino acids (e.g., Alanine) are deaminated to form pyruvate or TCA cycle intermediates (like $\alpha$-ketoglutarate), which are then converted to glucose. **High-Yield Clinical Pearls for NEET-PG:** * **Exception:** While even-chain fatty acids are non-glucogenic, **Odd-chain fatty acids** are glucogenic because their terminal 3-carbon unit, **Propionyl-CoA**, is converted to Succinyl-CoA. * **Glycerol:** The glycerol backbone of triglycerides *is* glucogenic; it enters the pathway as Dihydroxyacetone phosphate (DHAP). * **Key Regulatory Enzyme:** Fructose-1,6-bisphosphatase is the rate-limiting enzyme of gluconeogenesis. * **Location:** Gluconeogenesis occurs primarily in the **Liver** (90%) and **Kidney** (10%).

Q: A 24-year-old graduate student presents with complaints of severe muscle cramps and weakness with even mild exercise. Muscle biopsy demonstrates glycogen accumulation, but hepatic biopsy is unremarkable. Which of the following is the most likely diagnosis?

McArdle's disease. **Explanation:** The clinical presentation of exercise-induced muscle cramps and weakness, combined with glycogen accumulation isolated to the muscle (and not the liver), is the hallmark of **McArdle’s Disease (GSD Type V)**. **1. Why McArdle’s Disease is Correct:** McArdle’s disease is caused by a deficiency of **myophosphorylase** (muscle glycogen phosphorylase). This enzyme is essential for breaking down glycogen into glucose-1-phosphate in skeletal muscle. Because the liver isoform of the enzyme is coded by a different gene, hepatic function remains normal. Patients typically experience the "second wind" phenomenon, where symptoms improve after a few minutes of exercise as the body switches to using free fatty acids and blood glucose for energy. **2. Why the Other Options are Incorrect:** * **Pompe Disease (GSD Type II):** Caused by acid alpha-glucosidase deficiency. It involves systemic glycogen accumulation in lysosomes, primarily affecting the heart (cardiomegaly) and muscles, but it presents in infancy with severe hypotonia. * **Krabbe’s Disease:** A sphingolipidosis caused by galactocerebrosidase deficiency. It presents with neurological symptoms (demyelination, irritability, seizures) rather than exercise-induced muscle cramps. * **Niemann-Pick Disease:** A lysosomal storage disorder (sphingomyelinase deficiency) characterized by hepatosplenomegaly and neurodegeneration (Cherry-red spot on macula), not primary muscle pathology. **High-Yield Clinical Pearls for NEET-PG:** * **Ischemic Forearm Exercise Test:** In McArdle’s, this test shows a **failure of blood lactate to rise** (since glycogen cannot be converted to lactate) but a significant rise in ammonia. * **Myoglobinuria:** Severe exercise can lead to rhabdomyolysis and "burgundy-colored" urine. * **Mnemonic:** **M**cArdle = **M**uscle only; **v**ery (Type **V**) painful cramps.

Q: Which cell in the body primarily utilizes the pentose phosphate pathway?

Red Blood Cells (RBCs). The Pentose Phosphate Pathway (PPP), also known as the Hexose Monophosphate (HMP) Shunt, is the primary source of **NADPH** in the body. **Why Red Blood Cells (RBCs) are the correct answer:** RBCs are uniquely dependent on the PPP because they lack mitochondria. The PPP is their **only source of NADPH**. NADPH is essential for maintaining a pool of **reduced glutathione**, which neutralizes reactive oxygen species (ROS) like hydrogen peroxide. Without this pathway, hemoglobin would undergo oxidative damage, leading to the formation of Heinz bodies and subsequent hemolysis. **Analysis of Incorrect Options:** * **Hepatocytes:** While the liver is a major site for the PPP (used for fatty acid and cholesterol synthesis), hepatocytes have alternative metabolic pathways and mitochondria to manage oxidative stress. * **Muscle cells:** Muscles primarily utilize glucose for ATP production via glycolysis and the TCA cycle. They have very low PPP activity because they do not perform significant lipid synthesis. * **Neurons:** Neurons rely almost exclusively on aerobic glycolysis and the TCA cycle for high energy demands. While they use some NADPH for antioxidant defense, it is not their primary metabolic characteristic compared to RBCs. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Glucose-6-Phosphate Dehydrogenase (G6PD). * **G6PD Deficiency:** The most common enzymopathy worldwide. It leads to neonatal jaundice and drug-induced hemolytic anemia (e.g., after taking Primaquine or eating Fava beans) due to the inability of RBCs to produce NADPH. * **Tissues with high PPP activity:** Adrenal cortex, Liver, Testes/Ovaries (for steroid synthesis), and Lactating mammary glands (for fatty acid synthesis). * **Products:** NADPH (for reductive biosynthesis) and Ribose-5-phosphate (for nucleotide synthesis).

Q: All of the following are enzymes of the TCA cycle located in the mitochondrial matrix, EXCEPT?

Succinate dehydrogenase. ### Explanation **Core Concept:** The Citric Acid Cycle (TCA cycle) occurs primarily in the **mitochondrial matrix**. However, **Succinate Dehydrogenase (Option C)** is the unique exception. It is the only enzyme of the TCA cycle that is **integral to the inner mitochondrial membrane**. This enzyme serves a dual role: 1. **In the TCA Cycle:** It catalyzes the oxidation of succinate to fumarate. 2. **In the Electron Transport Chain (ETC):** It is also known as **Complex II**. It transfers electrons from succinate via FADH₂ directly into the Coenzyme Q pool. **Analysis of Incorrect Options:** * **A. Alpha-ketoglutarate dehydrogenase:** A multi-enzyme complex located in the mitochondrial matrix. It requires five cofactors (Thiamine, Lipoic acid, CoA, FAD, NAD). * **B. Isocitrate dehydrogenase:** The rate-limiting enzyme of the TCA cycle, located within the mitochondrial matrix. * **D. Malate dehydrogenase:** Catalyzes the final step of the cycle (Malate to Oxaloacetate) and is located in the mitochondrial matrix. (Note: A cytosolic isoenzyme also exists for the malate-aspartate shuttle). **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for TCA Enzymes:** "**C**an **I** **K**eep **S**elling **S**ubstances **F**or **M**oney?" (**C**itrate synthase, **I**socitrate DH, alpha-**K**etoglutarate DH, **S**uccinyl-CoA synthetase, **S**uccinate DH, **F**umarase, **M**alate DH). * **Inhibitor:** Succinate dehydrogenase is competitively inhibited by **Malonate** (a classic example of competitive inhibition). * **FADH₂ Linkage:** Succinate dehydrogenase is the only TCA enzyme that produces FADH₂ instead of NADH. * **Riboflavin Connection:** Since it uses FAD as a prosthetic group, its activity is impaired in Vitamin B2 deficiency.

Question 1

In glycogen metabolism, a metabolically active enzyme found in the liver is converted from its inactive dephosphorylated state to its active phosphorylated state. Which of the following is true about this enzyme?

Accepted Answer

More commonly seen in the fasting state than in the fed state.

Answer

Phosphorylation sometimes activates the enzyme.

Answer

Catecholamines directly stimulate it.

Answer

Always activated by cAMP-dependent protein kinase.

Question 2

Ascorbate and alpha-tocopherol are maintained in a reduced state in the lens by:

Accepted Answer

Glutathione

Answer

Glucose

Answer

Glycoprotein

Answer

Fatty acid

Question 3

Which of the following molecules can participate in gluconeogenesis?

Accepted Answer

Propionyl CoA

Answer

Acetyl CoA

Answer

Muscle glycogen

Answer

All of the above

Question 4

Which of the following is a non-reducing sugar?

Accepted Answer

Sucrose

Answer

Glucose

Answer

Lactose

Answer

Maltose

Question 5

Which of the following enzymes found in the liver is involved in gluconeogenesis during the postabsorptive state?

Accepted Answer

Glucose-6-phosphatase

Answer

Glucose-6-phosphate dehydrogenase

Answer

6-phosphogluconate dehydrogenase

Answer

Glucokinase

Question 6

Gluconeogenesis does not occur significantly from which of the following in humans?

Accepted Answer

Fatty acids

Answer

Lactate

Answer

Pyruvate

Answer

Amino acids

Question 7

A 24-year-old graduate student presents with complaints of severe muscle cramps and weakness with even mild exercise. Muscle biopsy demonstrates glycogen accumulation, but hepatic biopsy is unremarkable. Which of the following is the most likely diagnosis?

Accepted Answer

McArdle's disease

Answer

Pompe disease

Answer

Krabbe's disease

Answer

Niemann-Pick disease

Question 8

Which cell in the body primarily utilizes the pentose phosphate pathway?

Accepted Answer

Red Blood Cells (RBCs)

Answer

Hepatocytes

Answer

Muscle cells

Answer

Neurons

Question 9

A 42-year-old man presented with symptoms of weakness, fatigue, shortness of breath, and dizziness. His hemoglobin level was less than 7 g/dl (normal for a male being greater than 13.5 g/dl). Red blood cells of the patient showed abnormally low levels of lactate production. Heinz bodies were not found in peripheral blood film. Deficiency of which one of the following enzymes would be the most likely cause of this patient's anemia?

Accepted Answer

Pyruvate kinase

Answer

Phosphoglucose isomerase

Answer

Glucose-6-phosphate dehydrogenase (G6PD)

Answer

Hexokinase

Question 10

All of the following are enzymes of the TCA cycle located in the mitochondrial matrix, EXCEPT?

Accepted Answer

Succinate dehydrogenase

Answer

Alpha-ketoglutarate dehydrogenase

Answer

Isocitrate dehydrogenase

Answer

Malate dehydrogenase

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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