Energy Production and Metabolism — MCQs

Energy Production and Metabolism Indian Medical PG Practice Questions and MCQs

Question 281: 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 282: Which of the following statements best describes the role of inorganic phosphate in the Electron Transport Chain (ETC)?

A. Generates ATP
B. Occurs in mitochondria
C. Inorganic phosphate is essential for ATP synthesis in the ETC. (Correct Answer)
D. No role of inorganic phosphate

Explanation: ***Inorganic phosphate is essential for ATP synthesis in the ETC*** - **Inorganic phosphate (Pi)** serves as a crucial **substrate** in oxidative phosphorylation, combining with ADP to form ATP. - The reaction catalyzed by **ATP synthase** is: ADP + Pi → ATP, powered by the proton motive force generated by the ETC. - Without Pi, the ETC cannot fulfill its primary function of ATP production through **oxidative phosphorylation**. - This represents the **direct and essential role** of inorganic phosphate in the context of the Electron Transport Chain. *Generates ATP* - While Pi is involved in ATP synthesis, it does not itself "generate" ATP. - Pi is a **substrate** (reactant), not an energy source; the energy comes from the **proton gradient** created by the ETC. - This option incorrectly attributes ATP generation to Pi alone rather than recognizing it as one component of the synthesis process. *No role of inorganic phosphate* - This is factually incorrect as inorganic phosphate plays a **direct and essential role** in ATP synthesis. - Without Pi, ADP cannot be phosphorylated to form ATP during oxidative phosphorylation. - Pi is an indispensable substrate for the ATP synthase enzyme. *Occurs in mitochondria* - This statement describes the **location of the ETC**, not the role of inorganic phosphate. - While the ETC does occur in the inner mitochondrial membrane, this does not answer what role Pi plays in the process. - The question specifically asks about the role of inorganic phosphate, not where the ETC is located.

Question 283: NADH CoQ reductase is inhibited by ?

A. Antimycin (inhibits cytochrome bc1 complex)
B. Atractyloside (inhibits ATP/ADP translocase)
C. Rotenone (Correct Answer)
D. Carbon monoxide (inhibits cytochrome c oxidase)

Explanation: ***Rotenone*** - **Rotenone** is a potent inhibitor of **NADH CoQ reductase**, also known as **Complex I** of the electron transport chain. - It blocks the transfer of electrons from **NADH** to **ubiquinone (CoQ)**, thereby halting oxidative phosphorylation. *Antimycin (inhibits cytochrome bc1 complex)* - **Antimycin A** specifically inhibits **Complex III (cytochrome bc1 complex)**, not **NADH CoQ reductase**. - Its action blocks electron transfer from **ubiquinol** to **cytochrome c**. *Atractyloside (inhibits ATP/ADP translocase)* - **Atractyloside** inhibits the **adenine nucleotide translocase (ATP/ADP translocase)**, which is responsible for exchanging ATP for ADP across the inner mitochondrial membrane. - It does not directly affect the electron transport chain components like **NADH CoQ reductase**. *Carbon monoxide (inhibits cytochrome c oxidase)* - **Carbon monoxide (CO)** is a classic inhibitor of **Complex IV (cytochrome c oxidase)**. - It binds to the **heme iron** of **cytochrome a3** with high affinity, preventing oxygen from acting as the final electron acceptor.

Question 284: In starvation, earliest to become depleted -

A. Carbohydrates (Correct Answer)
B. Proteins
C. Fats
D. None of the options

Explanation: ***Carbohydrates*** - **Glycogen stores** (primarily liver and muscle glycogen) are the body's most readily accessible energy source and are depleted within hours of starvation. - The liver initially maintains blood glucose levels by breaking down **glycogen** before resorting to gluconeogenesis. *Proteins* - **Proteins** are conserved as much as possible during early starvation to preserve vital body functions. - Significant **protein breakdown** for energy (gluconeogenesis) typically occurs in later stages of prolonged starvation, after carbohydrate and most fat reserves are diminished. *Fats* - **Fats** (in the form of triglycerides stored in adipose tissue) become the primary energy source after glycogen stores are depleted. - While they provide a large energy reserve, their mobilization and utilization as fuel take longer than glycogen, and they are not the **earliest to be depleted**. *None of the options* - This option is incorrect because **carbohydrates** are indeed the earliest to be depleted during starvation.

Question 285: What is the primary role of molecular oxygen in the electron transport chain (ETC)?

A. Transfer of electrons to CoQ
B. Transfer of electrons from cytosol to mitochondria
C. Acting as the final electron acceptor (Correct Answer)
D. Facilitating ATP synthesis

Explanation: ***Acting as the final electron acceptor*** - **Molecular oxygen** is the terminal electron acceptor in the **electron transport chain**, combining with electrons and protons (H+) to form **water**. - Without oxygen, electron flow would cease, leading to a build-up of reduced electron carriers and halting ATP production via **oxidative phosphorylation**. *Transfer of electrons to CoQ* - **Coenzyme Q (CoQ)** accepts electrons from Complexes I and II but is an intermediate carrier, not the final destination. - The primary role of molecular oxygen occurs much later in the chain. *Transfer of electrons from cytosol to mitochondria* - This process involves specific shuttle systems (e.g., malate-aspartate, glycerol phosphate shuttle) but is distinct from oxygen's role within the ETC. - Oxygen's function is internal to the electron transport process in the mitochondrial matrix. *Facilitating ATP synthesis* - While oxygen's role as the final electron acceptor indirectly enables **ATP synthesis** by maintaining electron flow and the proton gradient, it does not directly synthesize ATP. - **ATP synthase** uses the proton gradient to produce ATP, a separate but dependent step.

Question 286: Hyperammonaemia inhibits the TCA cycle by depleting which of the following?

A. succinate
B. α-ketoglutarate (Correct Answer)
C. malate
D. fumarate

Explanation: ***a keto glutarate*** - **Hyperammonemia** leads to the depletion of **α-ketoglutarate** through its amination to form **glutamate** by glutamate dehydrogenase and subsequently glutamine by glutamine synthetase. - The removal of **α-ketoglutarate** from the TCA cycle impairs its ability to produce energy and essential intermediates, contributing to neurological dysfunction in hyperammonemia. *succinate* - **Succinate** is an intermediate in the TCA cycle, but its depletion is not the primary mechanism by which hyperammonemia inhibits the cycle. - The direct consumption of **α-ketoglutarate** for ammonia detoxification is the more direct and significant impact. *malate* - **Malate** is another intermediate in the TCA cycle but is downstream from **α-ketoglutarate**. - Its depletion is a consequence of overall TCA cycle inhibition, not the initial cause mediated by hyperammonemia. *fumarate* - **Fumarate** is also a TCA cycle intermediate and is produced after succinate. - Its levels would be affected by the overall inhibition of the cycle, but it is not the direct target or substrate for ammonia detoxification that depletes the cycle.

Question 287: In the malate shuttle, how many ATPs are produced from one NADH?

A. 1 ATP
B. 3 ATP
C. 2 ATP
D. 2.5 ATP (Correct Answer)

Explanation: ***2.5 ATP*** - In the **malate-aspartate shuttle**, mitochondrial **NADH** is regenerated from cytosolic NADH, and then enters the electron transport chain at **Complex I**. - **Complex I** entry means that NADH contributes to the pumping of enough protons to generate approximately **2.5 ATP** through oxidative phosphorylation. *1 ATP* - **1 ATP** is not the direct equivalent produced from the reoxidation of one NADH via the malate shuttle into the electron transport chain. - This value is typically associated with the direct hydrolysis of **ATP** or the energy equivalent of **GTP** produced in the citric acid cycle. *3 ATP* - Historically, **3 ATP** was the accepted stoichiometry for one NADH, but more accurate measurements of proton pumping and ATP synthase activity have revised this. - The value of 3 ATP per NADH does not reflect the most current understanding of mitochondrial bioenergetics. *2 ATP* - **2 ATP** is the approximate yield for **FADH2** entering the electron transport chain at **Complex II**, bypassing Complex I, and thus pumping fewer protons. - This value is not applicable to NADH transferred via the malate-aspartate shuttle, as NADH enters at Complex I.

Question 288: Which of the following is the most reactive free radical?

A. Alkyl radical
B. Superoxide radical
C. Peroxide radical
D. Hydroxyl radical (Correct Answer)

Explanation: ***Hydroxyl radical*** - The **hydroxyl radical (•OH)** is the most reactive free radical in biological systems due to its extremely high oxidation potential and short half-life. - It readily reacts with virtually all cellular macromolecules, including **DNA, proteins, and lipids**, causing widespread damage. *Peroxide radical* - The **peroxide radical (ROO•)**, or more specifically the peroxyl radical, is less reactive than the hydroxyl radical, but still significant in lipid peroxidation. - It plays a role in propagating chain reactions of **lipid damage** in cell membranes. *Alkyl radical* - **Alkyl radicals (R•)** are generally formed as intermediates during the abstraction of hydrogen atoms from saturated compounds. - While reactive, they are typically less reactive and less frequently encountered in biological systems compared to oxygen-centered radicals like the hydroxyl radical. *Superoxide radical* - The **superoxide radical (O₂•−)** is a relatively less reactive free radical compared to the hydroxyl radical, but it is the precursor to many other reactive oxygen species (ROS). - It is primarily involved in **initiation of oxidative stress** and can lead to the formation of more damaging species through reactions like the Haber-Weiss reaction.

Question 289: Pyruvate dehydrogenase requires all cofactors except:

A. Thiamin
B. Pyridoxin (Vitamin B6) (Correct Answer)
C. Riboflavin
D. Niacin

Explanation: ***Pyridoxin (Vitamin B6)*** - **Pyridoxin** (vitamin B6) is a coenzyme for many enzymes involved in **amino acid metabolism**, but it is **not directly required** by the pyruvate dehydrogenase complex. - The pyruvate dehydrogenase complex uses **thiamine pyrophosphate**, **lipoic acid**, **FAD**, **NAD+**, and **Coenzyme A** as cofactors. *Thiamin* - **Thiamin pyrophosphate** (TPP), derived from thiamin (vitamin B1), is a crucial coenzyme for the **E1 subunit** of the pyruvate dehydrogenase complex. - It participates in the **decarboxylation** of pyruvate, releasing CO2. *Riboflavin* - **FAD** (flavin adenine dinucleotide), derived from riboflavin (vitamin B2), is a coenzyme for the **E3 subunit** (dihydrolipoyl dehydrogenase) of the pyruvate dehydrogenase complex. - It is involved in the **regeneration of oxidized lipoamide**. *Niacin* - **NAD+** (nicotinamide adenine dinucleotide), derived from niacin (vitamin B3), is a coenzyme for the **E3 subunit** of the pyruvate dehydrogenase complex. - It acts as an **electron acceptor** during the reoxidation of FADH2.

Question 290: The mechanism of action of uncouplers of oxidative phosphorylation involves:

A. Inhibition of ATP synthase
B. Stimulation of ATP synthase
C. Disruption of proton gradient across the inner membrane (Correct Answer)
D. Blocking electron transport chain complexes

Explanation: ***Disruption of proton gradient across the inner membrane*** - Uncouplers such as **2,4-dinitrophenol** increase the permeability of the **inner mitochondrial membrane** to protons. - This dissipates the **proton motive force** that is normally used by ATP synthase to produce ATP, leading to the uncoupling of electron transport from ATP synthesis. *Inhibition of ATP synthase* - Inhibitors of ATP synthase directly block the enzyme's activity, preventing the synthesis of ATP while the **proton gradient** remains intact. - This mechanism is distinct from uncouplers, which allow electron transport to continue while dissipating the proton gradient. *Stimulation of ATP synthase* - Uncouplers do not stimulate ATP synthase; rather, their action prevents ATP synthase from effectively utilizing the **proton gradient** for ATP production. - Stimulation of ATP synthase would lead to increased ATP synthesis, which is contrary to the effect of uncouplers. *Blocking electron transport chain complexes* - Inhibitors of the **electron transport chain** (e.g., cyanide, rotenone) directly prevent the flow of electrons, thereby preventing the pumping of protons and the formation of a **proton gradient**. - Uncouplers, in contrast, allow electron transport to proceed but dissipate the proton gradient after it has been established.

Practice by Chapter

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ATP as Energy Currency

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Tricarboxylic Acid Cycle

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Electron Transport Chain

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Oxidative Phosphorylation

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Mitochondrial Diseases

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Uncouplers and Inhibitors of Oxidative Phosphorylation

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Shuttle Systems: Malate-Aspartate and Glycerol-Phosphate

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Energy Yield from Nutrients

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Metabolic Rate and Basal Metabolism

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Brown Adipose Tissue and Thermogenesis

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Oxygen Toxicity and Free Radicals

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