Carbohydrate Metabolism — MCQs
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Which of the following steps is specific for gluconeogenesis?
Which of the following is an aldose sugar?
Which of the following enzymes participates exclusively in glycolysis?
Which of the following is not a step in gluconeogenesis?
In the oxidative phase of pentose phosphate pathway, NADPH is produced in?
Which enzyme converts glucose to sorbitol?
When the insulin to glucagon ratio decreases, which enzyme is primarily active?
Which of the following dietary fibers is most characteristically insoluble in water?
An adolescent male patient presents to you with exercise intolerance. He gives a history of developing cramps on exertion. Which of the following enzyme deficiencies could be the cause?
Classical galactosemia (Type I) is due to deficiency of which enzyme?
Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs
Question 721: Which of the following steps is specific for gluconeogenesis?
- A. Oxaloacetate to PEP (Correct Answer)
- B. Oxaloacetate to citrate
- C. Oxaloacetate to glucose
- D. Pyruvate to acetyl CoA
Explanation: ***Oxaloacetate to PEP*** - This step, catalyzed by **PEP carboxykinase (PEPCK)**, is a bypass reaction necessary to overcome the irreversible pyruvate kinase step in glycolysis. - It is a key regulatory point in **gluconeogenesis**, allowing the synthesis of glucose from non-carbohydrate precursors. *Oxaloacetate to citrate* - This reaction is part of the **Krebs cycle (citric acid cycle)**, where oxaloacetate combines with acetyl-CoA to form citrate. - It does not directly lead to **glucose synthesis** and is not unique to gluconeogenesis. *Oxaloacetate to glucose* - This is an **overly broad statement** and not a direct, single enzymatic step in gluconeogenesis. - While oxaloacetate is an intermediate in the gluconeogenic pathway, it must first be converted to **PEP** and then proceed through several more steps to become glucose. *Pyruvate to acetyl CoA* - This reaction is catalyzed by the **pyruvate dehydrogenase complex** and represents a committed step into oxidative metabolism, primarily the Krebs cycle. - This step is **irreversible** in mammals and prevents the direct conversion of acetyl-CoA back to pyruvate or glucose, making it not relevant for gluconeogenesis.
Question 722: Which of the following is an aldose sugar?
- A. Ribulose
- B. Fructose
- C. Glyceraldehyde (Correct Answer)
- D. None of the options
Explanation: ***Glyceraldehyde*** - **Glyceraldehyde** is the simplest **aldose**, a monosaccharide with an **aldehyde group** (CHO) at one end of its carbon chain. - Its chemical structure is a three-carbon chain with the aldehyde group on the first carbon, making it an **aldo sugar**. *Ribulose* - **Ribulose** is a **ketose**, specifically a **ketopentose**, meaning it is a five-carbon sugar with a **ketone group** (C=O) in its structure. - The ketone group in ribulose is typically located on the second carbon, distinguishing it from aldoses. *Fructose* - **Fructose** is another example of a **ketose**, specifically a **ketohexose**, as it is a six-carbon sugar containing a **ketone group**. - Its ketone group is usually found on the second carbon atom, which differentiates ketoses from aldoses structurally. *None of the options* - This option is incorrect because **glyceraldehyde** is indeed an aldose sugar, fitting the definition of a monosaccharide with an aldehyde functional group. - As **glyceraldehyde** is correctly identified as an aldose, this choice would contradict the chemical classification of sugars.
Question 723: Which of the following enzymes participates exclusively in glycolysis?
- A. Phosphofructokinase
- B. Glucose-6-phosphate dehydrogenase
- C. Hexokinase
- D. Pyruvate kinase (Correct Answer)
Explanation: ***Pyruvate kinase*** - This enzyme catalyzes the **final step of glycolysis**, irreversibly converting **phosphoenolpyruvate (PEP)** to pyruvate, producing ATP. - **Exclusively participates in glycolysis** - it has no role in any other metabolic pathway, making it the most definitive answer. - All tissue-specific isoforms (M1, M2, L, R) perform the same glycolysis-exclusive function. *Phosphofructokinase* - **Phosphofructokinase-1 (PFK-1)** catalyzes the committed step of glycolysis (Fructose-6-P → Fructose-1,6-BP) and is technically exclusive to the glycolytic pathway. - However, when the question refers to "phosphofructokinase" generically, it could include **PFK-2**, which produces fructose-2,6-bisphosphate (a regulatory molecule, not a glycolytic intermediate) and is part of the regulatory mechanism rather than the pathway itself. - **Pyruvate kinase is more unambiguously exclusive** to glycolysis as a metabolic enzyme. *Hexokinase* - While essential for the initial step of glycolysis, **hexokinase** phosphorylates multiple hexoses (glucose, mannose, fructose) and its product (G6P) can enter **multiple pathways**: glycolysis, pentose phosphate pathway, or glycogen synthesis. - **Not exclusive to glycolysis** - it serves as a branch point enzyme. *Glucose-6-phosphate dehydrogenase* - This enzyme is the rate-limiting step of the **pentose phosphate pathway (PPP)**, not glycolysis. - It catalyzes the oxidation of G6P to produce **NADPH** and ribose-5-phosphate for nucleotide synthesis, thereby diverting glucose-6-phosphate **away from glycolysis**.
Question 724: Which of the following is not a step in gluconeogenesis?
- A. Conversion of pyruvate to acetyl-CoA (Correct Answer)
- B. Conversion of glucose-6-phosphate to glucose
- C. Conversion of oxaloacetate to phosphoenolpyruvate
- D. Conversion of fructose-1,6-bisphosphate to fructose-6-phosphate
Explanation: ***Conversion of pyruvate to acetyl-CoA*** - This step is a key irreversible reaction catalyzed by the **pyruvate dehydrogenase complex** that commits pyruvate to oxidative metabolism via the **Krebs cycle** or to fatty acid synthesis. - It is **not a part of gluconeogenesis**, as acetyl-CoA cannot be converted back to pyruvate or glucose in mammals. - This reaction is irreversible and represents a point of no return for glucose synthesis. *Conversion of glucose-6-phosphate to glucose* - This is the **final step in gluconeogenesis**, catalyzed by **glucose-6-phosphatase** in the liver and kidney. - This enzyme allows free glucose to be released into the bloodstream. - It is an essential gluconeogenic step that bypasses the irreversible hexokinase/glucokinase reaction of glycolysis. *Conversion of oxaloacetate to phosphoenolpyruvate* - This is a **key bypass step in gluconeogenesis** that overcomes the irreversible pyruvate kinase reaction in glycolysis. - It is catalyzed by **phosphoenolpyruvate carboxykinase (PEPCK)** and requires GTP. - This is crucial for synthesizing glucose from non-carbohydrate precursors like amino acids and lactate. *Conversion of fructose-1,6-bisphosphate to fructose-6-phosphate* - This is an important **bypass step in gluconeogenesis**, catalyzed by **fructose-1,6-bisphosphatase**. - This irreversible reaction bypasses the phosphofructokinase-1 step of glycolysis. - It is one of the three key regulatory steps unique to gluconeogenesis.
Question 725: In the oxidative phase of pentose phosphate pathway, NADPH is produced in?
- A. Cytosol (Correct Answer)
- B. Mitochondria
- C. Ribosome
- D. Peroxisomes
Explanation: ***Cytosol*** - The **oxidative phase** of the **pentose phosphate pathway (PPP)**, which produces **NADPH**, occurs exclusively in the **cytosol**. - Two key enzymes generate NADPH: **glucose-6-phosphate dehydrogenase (G6PD)** and **6-phosphogluconate dehydrogenase**. - **NADPH** is crucial for **reductive biosynthesis** (e.g., fatty acid synthesis, cholesterol synthesis) and for maintaining **redox balance** (e.g., reducing glutathione to protect against oxidative stress). *Mitochondria* - While mitochondria are central to **oxidative phosphorylation** and the **Krebs cycle**, they primarily produce **NADH** and **FADH2** for ATP generation. - The pentose phosphate pathway does not occur in mitochondria. *Ribosome* - **Ribosomes** are responsible for **protein synthesis** (translation) and are not involved in metabolic pathways or NADPH production. - They are cellular machinery for translation, not metabolic compartments. *Peroxisomes* - **Peroxisomes** are involved in **fatty acid β-oxidation** and **detoxification** of hydrogen peroxide. - While peroxisomes have some oxidative enzymes, they are not the site of the pentose phosphate pathway or its NADPH production.
Question 726: Which enzyme converts glucose to sorbitol?
- A. Sorbitol dehydrogenase
- B. Aldose reductase (Correct Answer)
- C. Aldolase B
- D. Glucose-6-phosphate dehydrogenase
Explanation: ***Aldose reductase*** - This enzyme is crucial in the **polyol pathway**, reducing **glucose to sorbitol** by using **NADPH** as a cofactor. - In conditions of high glucose (e.g., uncontrolled diabetes), increased activity of **aldose reductase** leads to sorbitol accumulation, contributing to **osmotic damage** in certain tissues like the lens, nerves, and kidneys. *Sorbitol dehydrogenase* - This enzyme is responsible for the subsequent step in the polyol pathway, **oxidizing sorbitol to fructose** using NAD+ as a cofactor. - While related to sorbitol metabolism, it does not convert glucose to sorbitol; instead, it metabolizes sorbitol further. *Aldolase B* - This enzyme is involved in **fructose metabolism**, specifically cleaving **fructose-1-phosphate** into **dihydroxyacetone phosphate** and **glyceraldehyde**. - It plays no direct role in the conversion of glucose to sorbitol. *Glucose-6-phosphate dehydrogenase* - This is the rate-limiting enzyme of the **pentose phosphate pathway**, catalyzing the oxidation of glucose-6-phosphate to 6-phosphoglucono-δ-lactone. - While it also uses NADPH (producing it rather than consuming it), it is not involved in the polyol pathway or sorbitol synthesis.
Question 727: When the insulin to glucagon ratio decreases, which enzyme is primarily active?
- A. Glucose-6-phosphatase
- B. Pyruvate carboxylase
- C. Fructose 1,6-bisphosphatase
- D. Glycogen phosphorylase (Correct Answer)
Explanation: ***Glycogen phosphorylase*** - A decrease in the **insulin-to-glucagon ratio** indicates a **low blood glucose** state, signaling the need for glucose production. - **Glycogen phosphorylase** is the key enzyme in **glycogenolysis**, which breaks down stored glycogen into glucose-1-phosphate, thereby elevating blood glucose. - This is the **primary and fastest response** to decreased insulin/glucagon ratio. *Fructose-1,6-bisphosphatase* - This enzyme is crucial for **gluconeogenesis**, specifically catalyzing the dephosphorylation of **fructose-1,6-bisphosphate** to **fructose-6-phosphate**. - While active during low insulin/high glucagon states, it is involved in synthesizing glucose, not directly breaking down stored glycogen as quickly as glycogen phosphorylase. *Pyruvate carboxylase* - This enzyme is the first committed step in **gluconeogenesis**, converting **pyruvate to oxaloacetate** in the mitochondria. - Although active in response to a low insulin-to-glucagon ratio, its role is in synthesizing glucose from non-carbohydrate precursors, which is a slower process than immediate glycogen breakdown. *Glucose-6-phosphatase* - This enzyme is found primarily in the **liver and kidneys** and is responsible for dephosphorylating **glucose-6-phosphate** to free glucose, allowing it to exit the cell into the bloodstream. - While essential for the release of glucose from both gluconeogenesis and glycogenolysis, it acts at a later stage to make glucose available rather than initiating the breakdown of glycogen itself.
Question 728: Which of the following dietary fibers is most characteristically insoluble in water?
- A. Pectin
- B. Lignin (Correct Answer)
- C. Hemicellulose
- D. Cellulose
Explanation: ***Lignin*** - **Lignin** is a complex polymer found in plant cell walls, known for its **extreme insolubility** in water. - It provides structural rigidity to plants and is a non-carbohydrate component of **dietary fiber**. *Pectin* - **Pectin** is a type of soluble dietary fiber that forms a **gel-like substance** when mixed with water. - It is often used as a gelling agent in foods and is found in fruits like apples and citrus. *Hemicellulose* - **Hemicellulose** is a diverse group of polysaccharides; some forms are **soluble**, while others are **insoluble**, but it's generally more soluble than lignin. - Its solubility depends on its specific structure and sugar composition. *Cellulose* - **Cellulose** is an insoluble fiber, but it can absorb water and swell, contributing to **bulk in stool**. - While largely insoluble, **lignin** is considered the most characteristically insoluble fiber due to its highly cross-linked and rigid structure, which resists hydration even more effectively than cellulose.
Question 729: An adolescent male patient presents to you with exercise intolerance. He gives a history of developing cramps on exertion. Which of the following enzyme deficiencies could be the cause?
- A. Myophosphorylase (Correct Answer)
- B. Hexokinase
- C. Glucose-6-phosphatase
- D. Hepatic glycogen phosphorylase
Explanation: ***Myophosphorylase*** - A deficiency in **myophosphorylase** (McArdle's disease, Glycogen Storage Disease Type V) impairs muscle glycogen breakdown, leading to **exercise intolerance** and **muscle cramps** due to insufficient ATP production during exertion. - Patients often experience a "second wind" phenomenon where symptoms improve after resting, as free fatty acids become an alternative fuel source. *Hexokinase* - A deficiency in **hexokinase** would affect the first step of glycolysis, impacting glucose phosphorylation in all tissues, not specifically causing exercise-induced muscle cramps. - This deficiency is rare and typically presents with **hemolytic anemia** due to impaired erythrocyte metabolism. *Glucose-6-phosphatase* - A deficiency in **glucose-6-phosphatase** (Von Gierke's disease, Glycogen Storage Disease Type Ia) primarily affects the liver and kidneys, leading to **fasting hypoglycemia**, lactic acidosis, and hepatomegaly, not exercise intolerance. - Muscle glycogen metabolism is unaffected in this condition. *Hepatic glycogen phosphorylase* - A deficiency in **hepatic glycogen phosphorylase** (Hers' disease, Glycogen Storage Disease Type VI) mainly causes **hepatomegaly** and **mild hypoglycemia** because the liver cannot effectively mobilize its glycogen stores. - **Muscle glycogen metabolism** remains normal, so exercise intolerance and cramps are not characteristic symptoms.
Question 730: Classical galactosemia (Type I) is due to deficiency of which enzyme?
- A. Galactose-1-phosphate uridyltransferase (Correct Answer)
- B. Fructose-1,6-bisphosphatase
- C. Adenine phosphoribosyltransferase (APRT)
- D. Hexokinase
Explanation: ***Galactose-1-phosphate uridyltransferase*** - **Galactosemia Type I** (**classical galactosemia**) is caused by a deficiency in **galactose-1-phosphate uridyltransferase (GALT)**. - This enzyme is crucial for converting **galactose-1-phosphate** to **glucose-1-phosphate** in the Leloir pathway of galactose metabolism. *Adenine phosphoribosyltransferase (APRT)* - Deficiency in **adenine phosphoribosyltransferase (APRT)** leads to **APRT deficiency**, characterized by **kidney stones** composed of 2,8-dihydroxyadenine. - This enzyme is involved in **purine salvage pathways**, not carbohydrate metabolism. *Fructose-1,6-bisphosphatase* - A deficiency in **fructose-1,6-bisphosphatase** causes **fructose-1,6-bisphosphatase deficiency**, a disorder of **gluconeogenesis**. - It results in **hypoglycemia** and **lactic acidosis**, especially during fasting. *Hexokinase* - **Hexokinase** phosphorylates glucose to **glucose-6-phosphate**, the first step in glycolysis. - Deficiency is rare but can lead to **nonspherocytic hemolytic anemia**.
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
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Blood Glucose Regulation
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Diabetes Mellitus: Biochemical Aspects
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Glycosylation and Glycoproteins
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Lactose Intolerance and Galactosemia
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