Pentose Phosphate Pathway — MCQs
Which is not a feature of G6PD deficiency?
Which of the following statements is true regarding the pentose phosphate pathway?
Which coenzyme is required by 6-phosphogluconate dehydrogenase?
Which enzyme is primarily associated with the reduction of NADP+ to NADPH in the pentose phosphate pathway?
Which of the following substances does not inhibit glycolysis?
Which compound serves as a central metabolic intermediate that connects glycolysis with glycogenesis and glycogenolysis?
What cofactor is required for the proper functioning of glucose-6-phosphate dehydrogenase?
A patient with hemolytic anemia has a defect in the enzyme glucose-6-phosphate dehydrogenase. Which of the following pathways is directly affected by this defect?
Riboflavin deficiency is assessed by?
Which of the following is not the source of cytosolic NADPH:
Pentose Phosphate Pathway Indian Medical PG Practice Questions and MCQs
Question 1: Which is not a feature of G6PD deficiency?
- A. Presence of Heinz bodies
- B. Males and females are equally affected (Correct Answer)
- C. Absence of NADPH
- D. Oxidative stress
Explanation: ***Males and females are equally affected*** - G6PD deficiency is an **X-linked recessive disorder**, meaning males are predominantly and more severely affected because they have only one X chromosome [2]. - Females are typically carriers and are less commonly affected, or may experience milder symptoms, due to **X-chromosome inactivation** (Lyonization). *Presence of Heinz bodies* - **Heinz bodies** are formed from denatured hemoglobin precipitates within red blood cells, a characteristic feature of **oxidative stress** in G6PD deficiency [2]. - These bodies are removed by the spleen, contributing to **hemolytic anemia**. *Absence of NADPH* - G6PD is the rate-limiting enzyme in the **pentose phosphate pathway**, which generates **NADPH** [1], [2]. - Without sufficient G6PD, the production of **NADPH** is severely impaired, leading to a deficiency in this critical reducing agent. *Oxidative stress* - **NADPH** is crucial for reducing **glutathione**, which in turn detoxifies reactive oxygen species [2]. - The lack of NADPH makes red blood cells vulnerable to **oxidative damage**, manifesting as hemolytic anemia upon exposure to oxidative agents [3].
Question 2: Which of the following statements is true regarding the pentose phosphate pathway?
- A. Glucose is the only substrate that can enter this pathway
- B. The pentose phosphate pathway is a direct oxidative pathway of glucose metabolism (Correct Answer)
- C. The pathway has only monophosphates as intermediates
- D. None of the options
Explanation: ***The pentose phosphate pathway is a direct oxidative pathway of glucose metabolism*** - The pathway **oxidizes glucose-6-phosphate** to generate **NADPH** and **pentose phosphates**, notably ribose-5-phosphate. - It is considered "direct oxidative" because it bypasses the initial steps of glycolysis to produce these crucial products. - The oxidative phase directly converts glucose-6-phosphate through two irreversible NADPH-generating steps. *Glucose is the only substrate that can enter this pathway* - While **glucose-6-phosphate** is the primary entry point, other intermediates from glycolysis can be channeled into the non-oxidative phase. - For example, **fructose-6-phosphate** and **glyceraldehyde-3-phosphate** can be interconverted with pentose phosphates, allowing their entry. *The pathway has only monophosphates as intermediates* - The pathway involves various intermediates, including **sugar phosphates** like ribulose-5-phosphate, xylulose-5-phosphate, and sedoheptulose-7-phosphate. - Many of these are **phosphates of different sugar lengths**, not exclusively monophosphates in the strict sense. *None of the options* - This statement is incorrect because the option regarding the pentose phosphate pathway being a **direct oxidative pathway of glucose metabolism** is accurate. - The pathway's central role involves both the oxidative production of **NADPH** and the non-oxidative interconversion of sugars.
Question 3: Which coenzyme is required by 6-phosphogluconate dehydrogenase?
- A. FAD
- B. NADP (Correct Answer)
- C. FMN
- D. NAD
Explanation: ***Correct NADP*** - **6-phosphogluconate dehydrogenase** is a key enzyme in the **oxidative phase of the pentose phosphate pathway**, which generates **NADPH**. - **NADP+** acts as an electron acceptor, getting reduced to **NADPH**, which is crucial for reductive biosynthesis and **antioxidant defense**. *Incorrect FAD* - **Flavin adenine dinucleotide (FAD)** is a derivative of **riboflavin (vitamin B2)** and is typically involved in reactions transferring two electrons, often found in **dehydrogenases** that form carbon-carbon double bonds. - Enzymes like **succinate dehydrogenase** (in the TCA cycle) utilize **FAD**, which is not the case for 6-phosphogluconate dehydrogenase. *Incorrect FMN* - **Flavin mononucleotide (FMN)**, also derived from **riboflavin**, is primarily found in **flavoproteins** and acts as a prosthetic group in various **redox reactions**. - It is a component of **Complex I (NADH dehydrogenase)** in the electron transport chain, but not involved in the pentose phosphate pathway's 6-phosphogluconate dehydrogenase reaction. *Incorrect NAD* - **Nicotinamide adenine dinucleotide (NAD+)** is a coenzyme primarily involved in catabolic reactions where it accepts electrons and is reduced to **NADH**, typically in glycolysis, the TCA cycle, and fatty acid oxidation. - While structurally similar to NADP+, **NAD+** is involved in ATP production, whereas **NADP+** is used in anabolic processes.
Question 4: Which enzyme is primarily associated with the reduction of NADP+ to NADPH in the pentose phosphate pathway?
- A. G6PD (Correct Answer)
- B. APDH
- C. α-keto glutarate dehydrogenases
- D. None of the options
Explanation: ***G6PD*** - **Glucose-6-phosphate dehydrogenase (G6PD)** catalyzes the first committed step in the pentose phosphate pathway, converting **glucose-6-phosphate** to **6-phosphogluconolactone**. - This reaction involves the reduction of **NADP+ to NADPH**, making G6PD the primary enzyme for NADPH production in this pathway. *APDH* - **APDH (adenosine phosphosulfate reductase)** is involved in sulfur metabolism and has no direct role in the pentose phosphate pathway or NADPH production. - This enzyme primarily functions in prokaryotes for the **reduction of APS (adenosine 5'-phosphosulfate)**. *α-keto glutarate dehydrogenases* - **Alpha-ketoglutarate dehydrogenase** is a mitochondrial enzyme part of the **Krebs cycle**, converting **alpha-ketoglutarate to succinyl-CoA**. - This enzyme is crucial for ATP production and generates **NADH**, not NADPH, in its reaction. *None of the options* - This option is incorrect because **G6PD** is indeed the primary enzyme responsible for NADPH generation in the pentose phosphate pathway.
Question 5: Which of the following substances does not inhibit glycolysis?
- A. Fluoride
- B. Arsenite
- C. Iodoacetate
- D. Fluoroacetate (Correct Answer)
Explanation: ***Fluoroacetate*** - **Fluoroacetate** is not a direct inhibitor of glycolysis. Instead, it is metabolized to **fluorocitrate**, which then acts as an inhibitor of **aconitase** in the **Krebs cycle (TCA cycle)**, thereby affecting cellular respiration at a later stage. - Its primary role in metabolic inhibition is within the **mitochondria**, impacting energy production via the TCA cycle rather than the glycolytic pathway. *Fluoride* - **Fluoride** is a known inhibitor of **enolase**, an enzyme in the penultimate step of glycolysis. - It forms a complex with **magnesium** and **phosphate** to block the active site of enolase, preventing the conversion of 2-phosphoglycerate to phosphoenolpyruvate. *Arsenite* - **Arsenite** inhibits glycolysis by targeting enzymes containing **sulfhydryl (–SH) groups**, particularly **glyceraldehyde-3-phosphate dehydrogenase (GAPDH)**, a critical enzyme in the glycolytic pathway. - It also inhibits the **pyruvate dehydrogenase complex** (linking glycolysis to the TCA cycle) and TCA cycle enzymes like **α-ketoglutarate dehydrogenase**, thereby affecting multiple stages of cellular respiration. *Iodoacetate* - **Iodoacetate** is a potent inhibitor of the enzyme **glyceraldehyde-3-phosphate dehydrogenase (GAPDH)**. - It specifically alkylates the **cysteine residue** at the active site of GAPDH, preventing the conversion of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate, thereby blocking glycolysis.
Question 6: Which compound serves as a central metabolic intermediate that connects glycolysis with glycogenesis and glycogenolysis?
- A. Glucose 1,6-bisphosphate
- B. Glucose 1-phosphate
- C. Glucose 6-phosphate (Correct Answer)
- D. Fructose 1,6-bisphosphate
Explanation: ***Glucose 6-phosphate*** - **Glucose 6-phosphate** is the central metabolic hub connecting glycolysis, glycogenesis, and glycogenolysis - Can be **isomerized to fructose 6-phosphate** to enter glycolysis for energy production - Can be **converted to glucose 1-phosphate** via phosphoglucomutase for glycogen synthesis (glycogenesis) - During **glycogenolysis**, it is formed from glucose 1-phosphate and can either enter glycolysis or be dephosphorylated to free glucose (in liver) for release into bloodstream - This unique position makes it the **key branch point** connecting all three pathways *Glucose 1,6-bisphosphate* - Acts as a **cofactor for phosphoglucomutase enzyme**, facilitating the interconversion between glucose 1-phosphate and glucose 6-phosphate - Not a direct metabolic intermediate in the main pathways - Present in trace amounts and functions catalytically rather than as a pathway substrate *Glucose 1-phosphate* - Direct product of **glycogen breakdown** (glycogenolysis) via glycogen phosphorylase - Must be **converted to glucose 6-phosphate** by phosphoglucomutase before entering glycolysis - Converted to **UDP-glucose** for glycogen synthesis (glycogenesis) - Does not directly connect all three pathways as it requires conversion to G6P first *Fructose 1,6-bisphosphate* - Committed intermediate **exclusively in glycolysis**, formed by phosphofructokinase-1 (PFK-1) - Represents the **committed step** in glycolysis (irreversible under physiological conditions) - Does not participate in glycogenesis or glycogenolysis - Located downstream of the branch point, after pathway commitment
Question 7: What cofactor is required for the proper functioning of glucose-6-phosphate dehydrogenase?
- A. NAD
- B. NADP (Correct Answer)
- C. FAD
- D. FMN
Explanation: ***NADP*** - **NADP+** (nicotinamide adenine dinucleotide phosphate) acts as the **electron acceptor** in the **glucose-6-phosphate dehydrogenase (G6PD)** reaction, becoming **NADPH**. - **NADPH** is crucial for maintaining the **redox balance** in cells, particularly in red blood cells, by reducing **oxidative stress**. *NAD* - **NAD+** (nicotinamide adenine dinucleotide) is a primary cofactor for many **dehydrogenase reactions** in catabolic pathways like **glycolysis** and the **Krebs cycle**. - It primarily functions as an electron acceptor in pathways that generate **ATP**, distinct from the role of **NADPH** in reductive biosynthesis and antioxidant defense. *FAD* - **FAD** (flavin adenine dinucleotide) is a coenzyme derived from **riboflavin (vitamin B2)** that is involved in various redox reactions, often in the form of **flavoproteins**. - Enzymes like **succinate dehydrogenase** in the electron transport chain utilize **FAD** as an electron acceptor, which is not the case for G6PD. *FMN* - **FMN** (flavin mononucleotide) is another coenzyme derived from **riboflavin**, structurally similar to FAD but lacking the additional adenosine monophosphate. - It participates in electron transfer reactions, particularly within **complex I** of the **electron transport chain**, but is not a cofactor for G6PD.
Question 8: A patient with hemolytic anemia has a defect in the enzyme glucose-6-phosphate dehydrogenase. Which of the following pathways is directly affected by this defect?
- A. Glycolysis
- B. Pentose phosphate pathway (Correct Answer)
- C. TCA cycle
- D. Urea cycle
Explanation: ***Pentose phosphate pathway*** - **Glucose-6-phosphate dehydrogenase (G6PD)** is the **rate-limiting enzyme** in the **pentose phosphate pathway (PPP)**, initiating the oxidative phase. - Deficiency in G6PD impairs the production of **NADPH**, which is crucial for reducing **oxidative stress** in red blood cells. *Glycolysis* - This pathway metabolizes glucose to pyruvate for **ATP production** and does not directly involve G6PD. - While G6P is an intermediate in both pathways, its conversion in glycolysis is catalyzed by phosphoglucose isomerase, not G6PD. *TCA cycle* - The **tricarboxylic acid (TCA) cycle** is a central metabolic pathway for energy production occurring in the **mitochondria**. - It involves the oxidation of acetyl-CoA and does not directly utilize G6PD. *Urea cycle* - The **urea cycle** is responsible for detoxifying ammonia by converting it into urea, primarily occurring in the **liver**. - This pathway is unrelated to glucose metabolism or G6PD activity.
Question 9: Riboflavin deficiency is assessed by?
- A. None of the options
- B. Glutathione reductase activity (Correct Answer)
- C. Pyruvate dehydrogenase activity
- D. Transketolase activity
Explanation: ***Glutathione reductase activity*** - Riboflavin is a precursor to **flavin adenine dinucleotide (FAD)**, a coenzyme for **glutathione reductase**. - Reduced glutathione reductase activity and its activation coefficient in erythrocytes are reliable indicators of **riboflavin deficiency**. *Pyruvate dehydrogenase activity* - **Pyruvate dehydrogenase** complex requires **thiamine pyrophosphate** (from thiamine, vitamin B1), not riboflavin, as a coenzyme. - Its activity is used to assess **thiamine status**, not riboflavin. *None of the options* - This option is incorrect as **glutathione reductase activity** is a valid method for assessing riboflavin deficiency. - The other options provided target different vitamin deficiencies. *Transketolase activity* - **Transketolase** activity is specifically used to assess **thiamine (vitamin B1) status**. - It requires **thiamine pyrophosphate** as a coenzyme, which is derived from thiamine.
Question 10: Which of the following is not the source of cytosolic NADPH:
- A. ATP citrate lyase (Correct Answer)
- B. G6PD
- C. Malic enzyme
- D. Isocitrate dehydrogenase
Explanation: ***ATP citrate lyase*** - **ATP citrate lyase** is an enzyme that converts **citrate** and **CoA** into **acetyl-CoA** and **oxaloacetate** in the cytoplasm. - This enzyme is crucial for fatty acid synthesis but does not directly produce **NADPH**. *G6PD* - **Glucose-6-phosphate dehydrogenase (G6PD)** is the rate-limiting enzyme of the **pentose phosphate pathway**. - It catalyzes the first step, producing **NADPH** during the conversion of glucose-6-phosphate to 6-phosphogluconolactone. *Malic enzyme* - **Malic enzyme** converts **malate** to **pyruvate**, generating **NADPH** in the cytosol. - This reaction helps to replenish cytosolic **NADPH** needed for lipid biosynthesis and other reductive processes. *Isocitrate dehydrogenase* - In the cytoplasm, the **cytosolic isocitrate dehydrogenase** isoform oxidizes **isocitrate** to **α-ketoglutarate**. - This reaction generates **NADPH**, which is vital for reductive biosynthesis and antioxidant defense.
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