Oxidative Phosphorylation — MCQs
In the electron transport chain (ETC), which enzyme does cyanide inhibit?
The electron transport chain is a series of redox reactions that result in ATP synthesis. Which of the following is a cytochrome complex IV inhibitor?
What is the net number of ATP molecules and NADH formed in glycolysis per glucose molecule?
ATP is generated in the Electron Transport Chain (ETC) specifically by which enzyme?
Increased H+ ions in the intermembrane space of mitochondria are due to?
Chemiosmotic coupling of oxidative phosphorylation is related to which of the following?
What is the function of Complex I in the electron transport chain?
Phosphofructokinase-1 occupies a key position in regulating glycolysis and is also subjected to feedback control. Which among the following are the allosteric activators of phosphofructokinase-1?
Final common pathway of metabolism of carbohydrates, lipids and proteins is?
All are cofactors for Dehydrogenase except:
Oxidative Phosphorylation Indian Medical PG Practice Questions and MCQs
Question 1: In the electron transport chain (ETC), which enzyme does cyanide inhibit?
- A. Complex II (Succinate dehydrogenase)
- B. Cytochrome c oxidase (Complex IV) (Correct Answer)
- C. Complex I (NADH dehydrogenase)
- D. Complex III (Cytochrome bc1 complex)
Explanation: ***Cytochrome c oxidase (Complex IV)*** - Cyanide binds to the **ferric iron (Fe3+)** in the heme a3 component of cytochrome c oxidase, blocking the final transfer of electrons to oxygen. - This inhibition effectively halts the entire **electron transport chain** and **oxidative phosphorylation**, leading to rapid cellular energy depletion. *Complex I (NADH dehydrogenase)* - While other toxins can inhibit Complex I (e.g., rotenone, amytal), **cyanide specifically targets Complex IV**. - Inhibition here prevents the entry of electrons from **NADH** into the ETC, but it's not cyanide's primary site of action. *Complex III (Cytochrome bc1 complex)* - Complex III is involved in transferring electrons from **ubiquinol** to cytochrome c, but it is not directly inhibited by cyanide. - Antimycin A is a well-known inhibitor of Complex III. *Complex II (Succinate dehydrogenase)* - Complex II directly receives electrons from **succinate** in the citric acid cycle and passes them to ubiquinone, bypassing Complex I. - Cyanide does not inhibit Complex II; inhibitors of this complex include malonate.
Question 2: The electron transport chain is a series of redox reactions that result in ATP synthesis. Which of the following is a cytochrome complex IV inhibitor?
- A. Cyanide (Correct Answer)
- B. Carbon dioxide
- C. Oligomycin
- D. Ouabain
Explanation: ***Cyanide*** - **Cyanide** is a potent inhibitor of **cytochrome c oxidase (Complex IV)** in the electron transport chain, binding to the ferric iron (Fe3+) in the heme group of the enzyme. - This binding prevents the transfer of electrons to **oxygen**, thereby halting cellular respiration and ATP production. *Carbon dioxide* - **Carbon dioxide** is a metabolic waste product and a component of the **bicarbonate buffer system**, but it does not directly inhibit cytochrome complex IV. - While high levels can affect physiological pH and enzyme function, its primary role is not as an electron transport chain inhibitor. *Oligomycin* - **Oligomycin** inhibits **ATP synthase (Complex V)** by binding to its Fo subunit, which blocks the flow of protons through the ATP synthase channel. - This prevents the synthesis of ATP but does not directly affect the electron transfer steps of cytochrome complex IV. *Ouabain* - **Ouabain** is a cardiac glycoside that inhibits the **Na+/K+-ATPase pump** in the cell membrane. - It does not have any direct inhibitory effect on the components of the electron transport chain, including cytochrome complex IV.
Question 3: What is the net number of ATP molecules and NADH formed in glycolysis per glucose molecule?
- A. 4 ATP, 2 NADH
- B. 4 ATP, 4 NADH
- C. 2 ATP, 4 NADH
- D. 2 ATP, 2 NADH (Correct Answer)
Explanation: **2 ATP, 2 NADH** - Glycolysis has a net yield of **2 molecules of ATP** because 4 ATP molecules are produced, but 2 ATP molecules are consumed during the initial energy investment phase. - **2 molecules of NADH** are also produced during the energy generation phase when glyceraldehyde-3-phosphate is oxidized. *4 ATP, 2 NADH* - While 4 ATP molecules are indeed produced during glycolysis, this option does not account for the **2 ATP molecules consumed** in the initial steps, leading to an incorrect net value. - The production of **2 NADH** is correct, but the ATP count is the gross rather than the net. *4 ATP, 4 NADH* - This option overstates the production of both ATP and NADH. While **4 ATP are produced (gross)**, the net is 2 ATP. - Only **2 NADH** molecules are formed per glucose molecule in glycolysis, not 4. *2 ATP, 4 NADH* - This option accurately reflects the **net ATP yield of 2 molecules**. - However, it exaggerates the production of NADH, as only **2 molecules of NADH** are formed during glycolysis, not 4.
Question 4: ATP is generated in the Electron Transport Chain (ETC) specifically by which enzyme?
- A. Cl- ATPase
- B. ADP Kinase
- C. FoF1 ATPase (Correct Answer)
- D. Na+/K+ ATPase
Explanation: ***FoF1 ATPase*** - The **FoF1 ATPase**, also known as **ATP synthase**, is the complex enzyme responsible for synthesizing ATP using the **proton gradient** generated by the electron transport chain. - The **Fo subunit** forms a channel that allows protons to flow back into the mitochondrial matrix, driving the rotation of the **F1 subunit** which catalyzes ATP synthesis from ADP and inorganic phosphate. *Na+/K+ ATPase* - This enzyme is a **pump** that actively transports **three sodium ions out** of the cell and **two potassium ions into** the cell, maintaining membrane potential. - It uses **ATP hydrolysis** as its energy source, meaning it **consumes ATP** rather than producing it directly in the ETC. *Cl- ATPase* - **Cl- ATPase** refers to a family of pumps that transport **chloride ions**, typically using ATP hydrolysis as an energy source. - These enzymes are involved in ion homeostasis and fluid balance, but they do **not generate ATP** in the electron transport chain. *ADP Kinase* - **ADP Kinase** is a general term for enzymes that catalyze the phosphorylation of ADP to ATP, often by transferring a phosphate group from another high-energy molecule. - While it produces ATP, it is not the specific enzyme that directly harnesses the **proton gradient** in the electron transport chain for oxidative phosphorylation.
Question 5: Increased H+ ions in the intermembrane space of mitochondria are due to?
- A. Decreased ATP synthase activity
- B. Reduced proton pumping
- C. Impaired inner mitochondrial membrane integrity
- D. Increased electron transport chain activity (Correct Answer)
Explanation: ***Increased electron transport chain activity*** - The **electron transport chain (ETC)** complexes (I, III, and IV) actively pump **protons (H+)** from the mitochondrial matrix into the intermembrane space during electron transfer. - **Increased ETC activity** directly causes more protons to be pumped, creating a higher H+ concentration in the intermembrane space. - This is the **primary mechanism** for establishing the proton-motive force used in ATP synthesis. *Decreased ATP synthase activity* - While decreased ATP synthase activity would cause **passive accumulation** of protons in the intermembrane space (since fewer H+ flow back through ATP synthase), it does **not actively increase** proton pumping. - The question asks what causes the **increase** in H+ ions, which requires active transport by the ETC, not passive accumulation. - This option confuses the consequence (accumulation) with the cause (active pumping). *Reduced proton pumping* - **Reduced proton pumping** by the ETC would lead to a **decrease** in H+ concentration in the intermembrane space, as fewer protons are being actively transported. - This produces the opposite effect of what the question describes. *Impaired inner mitochondrial membrane integrity* - **Impaired membrane integrity** would cause protons to **leak back** into the mitochondrial matrix, dissipating the proton gradient. - This would **decrease**, not increase, the H+ concentration in the intermembrane space. - This is seen in uncoupling conditions where the membrane becomes permeable to protons.
Question 6: Chemiosmotic coupling of oxidative phosphorylation is related to which of the following?
- A. ATP generation by pumping of neutrons
- B. Formation of ATP at substrate level
- C. ATP generation by pumping of protons (Correct Answer)
- D. ATP formation by transport of electrons
Explanation: ***ATP generation by pumping of protons*** - **Chemiosmotic coupling** links the electron transport chain's activity to ATP synthesis through the generation of a **proton gradient** across the inner mitochondrial membrane. - The energy released from the flow of electrons through complexes I, III, and IV is used to pump protons from the mitochondrial matrix to the intermembrane space, creating a **proton motive force** that drives ATP synthase. *Formation of ATP at substrate level* - **Substrate-level phosphorylation** involves the direct transfer of a phosphate group from a high-energy substrate to ADP to form ATP, independently of a proton gradient. - This process occurs in reactions like those in **glycolysis** and the **Krebs cycle**, not in oxidative phosphorylation via chemiosmosis. *ATP generation by pumping of neutrons* - **Neutrons** are subatomic particles with no electric charge and are not involved in biological processes like ATP generation or membrane transport. - Pumping of neutrons has no physiological relevance in cellular energy metabolism. *ATP formation by transport of electrons* - While **electron transport** is an integral part of oxidative phosphorylation, it does not directly form ATP. - The energy released during electron transport is used to create the **proton gradient** (chemiosmotic coupling), which then drives ATP synthesis, rather than ATP being formed directly by electron movement.
Question 7: What is the function of Complex I in the electron transport chain?
- A. NADH - CoQ reductase (Correct Answer)
- B. CoQ - cytochrome C reductase
- C. Cytochrome-C oxidase
- D. None of the options
Explanation: ***NADH - CoQ reductase*** - Complex I, also known as **NADH dehydrogenase**, transfers electrons from **NADH** to **Coenzyme Q (CoQ)**. - This process oxidizes NADH to NAD+ and pumps protons from the **mitochondrial matrix** to the **intermembrane space**, contributing to the proton gradient. - Complex I is the entry point for electrons from NADH into the electron transport chain and generates approximately **4 H+ ions** pumped per 2 electrons. *CoQ - cytochrome C reductase* - This describes the function of **Complex III**, not Complex I. - Complex III (cytochrome bc1 complex) transfers electrons from **reduced CoQ (ubiquinol)** to **cytochrome c**. - It also contributes to proton pumping via the Q-cycle mechanism. *Cytochrome-C oxidase* - This describes the function of **Complex IV**, not Complex I. - Complex IV transfers electrons from **cytochrome c** to **molecular oxygen (O2)**, forming water (H2O). - It is the terminal enzyme of the electron transport chain and pumps protons across the membrane. *None of the options* - This option is incorrect because Complex I clearly functions as **NADH - CoQ reductase**. - Each complex (I, II, III, and IV) has distinct enzymatic functions in the electron transport chain, and Complex I's role is well-established.
Question 8: Phosphofructokinase-1 occupies a key position in regulating glycolysis and is also subjected to feedback control. Which among the following are the allosteric activators of phosphofructokinase-1?
- A. 2,3-Bisphosphoglycerate (2,3-BPG)
- B. Fructose 2,6-bisphosphate (Correct Answer)
- C. Glucokinase
- D. Phosphoenolpyruvate (PEP)
Explanation: ***Fructose 2,6-bisphosphate*** - **Fructose 2,6-bisphosphate** is a potent **allosteric activator** of **phosphofructokinase-1 (PFK-1)**, increasing its affinity for fructose 6-phosphate and overcoming ATP inhibition. - Its synthesis is regulated by **insulin** (stimulating) and **glucagon** (inhibiting), linking glucose availability to glycolytic flux. *2,3-Bisphosphoglycerate (2,3-BPG)* - **2,3-BPG** is an important regulator of **hemoglobin oxygen affinity** in red blood cells. - It is not an allosteric activator of **PFK-1**; its primary role is in oxygen delivery. *Glucokinase* - **Glucokinase** is an **enzyme** in glycolysis, specifically catalyzing the phosphorylation of glucose to glucose 6-phosphate in the liver and pancreatic beta cells. - It is not an allosteric activator of **PFK-1** but rather an upstream enzyme in the pathway. *Phosphoenolpyruvate (PEP)* - **PEP** is an intermediate in glycolysis, formed from 2-phosphoglycerate and converted to pyruvate by pyruvate kinase. - It acts as an **allosteric inhibitor** of phosphofructokinase-1, signaling high energy status and slowing down glycolysis.
Question 9: Final common pathway of metabolism of carbohydrates, lipids and proteins is?
- A. Gluconeogenesis
- B. Glycogenesis
- C. TCA cycle (Correct Answer)
- D. None of the options
Explanation: ***TCA cycle*** - The **TCA cycle** (also known as the **Krebs cycle** or **citric acid cycle**) is the central metabolic pathway through which acetyl-CoA, derived from the breakdown of carbohydrates, fats, and proteins, is oxidized to produce energy. - Intermediates of the breakdown of **glucose (pyruvate)**, **fatty acids (acetyl-CoA)**, and **certain amino acids (keto acids)** feed into the TCA cycle, making it the final common pathway. *Gluconeogenesis* - **Gluconeogenesis** is the process of synthesizing glucose from non-carbohydrate precursors, primarily occurring in the liver and kidneys. - It is an anabolic pathway that creates glucose, rather than a catabolic pathway for energy generation from diverse macromolecules. *Glycogenesis* - **Glycogenesis** is the process of synthesizing glycogen from glucose, primarily in the liver and muscles, for storage. - It is a specific anabolic pathway for glucose storage and not a common pathway for the metabolism of all three major macronutrients. *None of the options* - The TCA cycle is indeed the final common pathway for the complete oxidation of carbohydrates, lipids, and proteins, making this option incorrect. - All major macronutrients are ultimately broken down to molecules that enter the TCA cycle.
Question 10: All are cofactors for Dehydrogenase except:
- A. SAM (Correct Answer)
- B. NADP
- C. NAD
- D. FAD
Explanation: ***SAM*** - **S-adenosylmethionine (SAM)** is a cofactor involved in **methyl group transfer reactions**, carried out by enzymes known as methyltransferases. - Dehydrogenase enzymes catalyze **redox reactions**, typically involving the transfer of hydride ions, and thus do not utilize SAM as a cofactor. *NADP* - **Nicotinamide adenine dinucleotide phosphate (NADP)** is a crucial coenzyme for many **dehydrogenase reactions**, particularly in **anabolic pathways** like fatty acid synthesis and the pentose phosphate pathway. - It acts as an **electron carrier**, accepting or donating hydride ions. *NAD* - **Nicotinamide adenine dinucleotide (NAD)** is a highly common coenzyme for numerous **dehydrogenase enzymes**, especially in **catabolic pathways** such as glycolysis, the Krebs cycle, and oxidative phosphorylation. - It functions as an **electron acceptor** or donor in redox reactions. *FAD* - **Flavin adenine dinucleotide (FAD)** is a coenzyme derived from **riboflavin (Vitamin B2)** and is associated with various dehydrogenase enzymes, particularly those involved in **electron transport** and fatty acid oxidation. - FAD can accept two hydrogen atoms (one hydride and one proton) to become FADH₂.
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