Carbohydrate Metabolism — MCQs
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Which of the following is NOT a key enzyme of gluconeogenesis?
In glycogen synthesis the active form of glucose used is-
Neonatal hypoglycemia that does not respond to treatment with counter-regulatory hormones is diagnostic of which condition?
All of the following are true about lactate utilization in the liver except -
What is the biochemical structure of cellulose?
Gas released from oligosaccharide metabolism by intestinal bacteria is
Which of the following steps is specific for gluconeogenesis?
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?
Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs
Question 711: Which of the following is NOT a key enzyme of gluconeogenesis?
- A. Pyruvate carboxylase
- B. PEP carboxykinase
- C. Pyruvate kinase (Correct Answer)
- D. Glucose-6-phosphatase
Explanation: ***Pyruvate kinase*** - **Pyruvate kinase** is a key regulatory enzyme in **glycolysis**, catalyzing the irreversible conversion of phosphoenolpyruvate (PEP) to pyruvate. - Since gluconeogenesis is essentially the reversal of glycolysis, pyruvate kinase is **NOT involved in gluconeogenesis**. Instead, this glycolytic step must be bypassed by different enzymes. - This is the correct answer as it is NOT a key enzyme of gluconeogenesis. *Pyruvate carboxylase* - This **IS a key enzyme of gluconeogenesis**, converting **pyruvate to oxaloacetate** in the mitochondria, thereby bypassing the pyruvate kinase step of glycolysis. - It uses **biotin** as a coenzyme and requires ATP. - This is one of the four key regulatory enzymes unique to gluconeogenesis. *PEP carboxykinase* - This **IS a key enzyme of gluconeogenesis**, converting **oxaloacetate to phosphoenolpyruvate (PEP)**, bypassing the irreversible pyruvate kinase step of glycolysis. - This enzyme is located in both the cytoplasm and mitochondria, depending on the species (cytoplasmic in humans). - This is one of the four key regulatory enzymes unique to gluconeogenesis. *Glucose-6-phosphatase* - This **IS a key enzyme of gluconeogenesis**, catalyzing the final step by dephosphorylating **glucose-6-phosphate to free glucose**, enabling its release from the liver into the bloodstream. - This enzyme bypasses the irreversible hexokinase/glucokinase step of glycolysis. - Located in the endoplasmic reticulum, it is exclusively found in gluconeogenic tissues like the liver and kidney.
Question 712: In glycogen synthesis the active form of glucose used is-
- A. Glucose 6 phosphate
- B. UDP glucose (Correct Answer)
- C. GTP glucose
- D. Glucose 1 phosphate
Explanation: **UDP glucose** - **UDP-glucose** (uridine diphosphate glucose) is the activated form of glucose that donates glucose units for the elongation of the **glycogen chain** during glycogen synthesis. - The formation of UDP-glucose from **glucose-1-phosphate** and **UTP** (uridine triphosphate) is catalyzed by UDP-glucose pyrophosphorylase, making glucose-1-phosphate a precursor, not the active form. *Glucose 6 phosphate* - **Glucose 6-phosphate** is an important intermediate in glycolysis and gluconeogenesis, and it can be isomerized to glucose 1-phosphate, but it is not the direct substrate for glycogen synthase. - Its formation is the first committed step in glucose metabolism within the cell, trapping glucose inside. *Glucose I phosphate* - **Glucose 1-phosphate** is a precursor to UDP-glucose, formed from glucose 6-phosphate by **phosphoglucomutase**. - While essential for glycogen synthesis, it is not the directly active form that donates glucose to the glycogen chain itself. *GTP glucose* - **GTP glucose** is not a known active form of glucose involved in glycogen synthesis. - **GTP** (guanosine triphosphate) is primarily involved in other metabolic processes, such as protein synthesis and signal transduction.
Question 713: Neonatal hypoglycemia that does not respond to treatment with counter-regulatory hormones is diagnostic of which condition?
- A. Von Gierke's disease (Correct Answer)
- B. Hereditary fructose intolerance
- C. Cori's disease (Glycogen storage disease type III)
- D. Anderson's disease (Glycogen storage disease type IV)
Explanation: ***Von Gierke's disease*** - This condition (Glycogen Storage Disease Type I) results from a **deficiency of glucose-6-phosphatase**, essential for releasing glucose from the liver. - The inability to produce free glucose from glycogen or gluconeogenesis leads to severe hypoglycemia that **does not respond to counter-regulatory hormones** like glucagon, as the enzyme needed for glucose release is non-functional. *Hereditary fructose intolerance* - This condition involves a deficiency in **aldolase B**, leading to the accumulation of fructose-1-phosphate after fructose ingestion. - While it can cause hypoglycemia, it generally occurs after **fructose exposure** and is not characterized by hypoglycemia refractory to counter-regulatory hormones in the neonatal period without such exposure. *Cori's disease (Glycogen storage disease type III)* - Caused by a deficiency in the **glycogen debranching enzyme**, leading to the accumulation of abnormal glycogen. - Patients can present with hypoglycemia, but often respond to glucagon administration, as the remaining glycogen structure can still be partially broken down, unlike in Von Gierke's. *Anderson's disease (Glycogen storage disease type IV)* - Result of a deficiency in the **glycogen branching enzyme**, leading to the formation of abnormally structured glycogen with long, unbranched chains. - This disease primarily affects the liver and muscles, causing **cirrhosis** and muscle weakness, and typically does not present with severe, refractory neonatal hypoglycemia as the primary or most characteristic symptom.
Question 714: All of the following are true about lactate utilization in the liver except -
- A. Cori's cycle shifts the metabolic burden from muscle to liver
- B. Cori's cycle can not be sustained indefinitely because it is energetically unfavourable
- C. Cori's cycle is linked to glycogen synthesis in muscle
- D. Total net number of ATP formed because of Cori's cycle is 4 (Correct Answer)
Explanation: ***Total net number of ATP formed because of cori's cycle is 4*** - This statement is incorrect. The **Cori cycle (lactic acid cycle)** is an energy-consuming process overall, as **6 ATP** molecules are consumed in the liver for gluconeogenesis to resynthesize glucose from lactate, while only a total of **2 ATP** are gained from glycolysis in the muscle. - The primary purpose of the Cori cycle is not net ATP production, but rather to shift the metabolic burden and regenerate glucose for tissues that rely on glycolysis (e.g., muscle, red blood cells). *Cori's cycle shifts the metabolic burden from muscle to liver* - This is true because **lactate produced in muscle** (during anaerobic conditions) is transported to the liver, where it is converted back to glucose. - The liver then bears the metabolic cost of **gluconeogenesis**, allowing the muscle to continue glycolysis and ATP production. *Cori's cycle can not be sustained indefinitely because it is energetically unfavourable* - This is true because the cycle involves a net consumption of ATP. **Six ATP equivalents** are used in gluconeogenesis in the liver to convert two molecules of lactate to one molecule of glucose. - In contrast, the glycolysis that produces the two lactate molecules in muscle yields only **two net ATP**. This energy deficit makes prolonged reliance on the Cori cycle unsustainable. *Cori's cycle is linked to glycogen synthesis in muscle* - This is true because the **glucose produced by the liver** via gluconeogenesis (from lactate) is released into the bloodstream. - This glucose can then be taken up by muscles and other tissues to **replenish glycogen stores** or be used for energy.
Question 715: What is the biochemical structure of cellulose?
- A. β (1,4) glucose (Correct Answer)
- B. β (1,6) glucose
- C. α (1,6) glucose
- D. α (1,4) glucose
Explanation: ***β (1,4) glucose*** - Cellulose is a linear polysaccharide made of repeating **glucose units** joined by **β-1,4 glycosidic bonds**. - This specific linkage allows for strong hydrogen bonding between adjacent cellulose chains, contributing to its structural rigidity in plant cell walls. *α (1,4) glucose* - This linkage is characteristic of starch (amylose) and glycogen, forming helical structures that are readily digestible by humans. - Unlike cellulose, these **α-1,4 linkages** result in a coiled, rather than linear, polysaccharide structure. *β (1,6) glucose* - While beta linkages are present in some polysaccharides, the **β-1,6 linkage** is not the primary linkage for the main chain of cellulose. - This linkage is primarily found at branch points in certain complex carbohydrates. *α (1,6) glucose* - This linkage forms branch points in branched polysaccharides like amylopectin (a component of starch) and glycogen. - It allows for a more compact and easily accessible energy storage molecule, very different from the structural role of cellulose.
Question 716: Gas released from oligosaccharide metabolism by intestinal bacteria is
- A. Carbon dioxide (Correct Answer)
- B. Sulfur dioxide
- C. Nitric oxide
- D. Methane
Explanation: ***Carbon dioxide*** - **Carbon dioxide (CO₂)** is the **most universally produced gas** from oligosaccharide and carbohydrate fermentation by intestinal bacteria in the colon. - Nearly all colonic bacteria produce CO₂ during the fermentation of undigested carbohydrates and oligosaccharides. - Along with **hydrogen (H₂)**, CO₂ forms the bulk of intestinal gas from bacterial metabolism. - This is part of normal gut flora activity contributing to **flatulence**. *Methane* - While **methane (CH₄)** is produced during oligosaccharide fermentation, it is only generated by individuals harboring **methanogenic archaea** (approximately 30-50% of the population). - Methane production is not universal, unlike CO₂, making it less representative as "THE gas" from oligosaccharide metabolism. - Methanogens use H₂ and CO₂ to produce methane as a secondary process. *Sulfur dioxide* - **Sulfur dioxide (SO₂)** is primarily associated with industrial pollution and is not a product of normal intestinal bacterial metabolism. - Hydrogen sulfide (H₂S) may be produced from sulfur-containing compounds, but not sulfur dioxide. *Nitric oxide* - **Nitric oxide (NO)** is a signaling molecule involved in vasodilation and immune responses. - It is not a major gas produced from bacterial fermentation of oligosaccharides in the intestines.
Question 717: 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 718: 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 719: 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 720: 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.
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
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Pentose Phosphate Pathway
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Metabolism of Fructose and Galactose
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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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