Energy Production and Metabolism — MCQs

Energy Production and Metabolism Indian Medical PG Practice Questions and MCQs

Question 311: Mechanism of cyanide poisoning is by inhibiting: NEET 2013

A. DNA synthesis
B. Cytochrome oxidase (Correct Answer)
C. Protein breakdown
D. Protein synthesis

Explanation: ***Cytochrome oxidase*** - **Cyanide** is a potent poison because it binds to the **ferric iron (Fe3+)** in the active site of **cytochrome c oxidase**. - This binding completely inhibits the enzyme, halting **cellular respiration** and **ATP production**, leading to rapid cell death. *DNA synthesis* - **Cyanide** does not directly inhibit **DNA polymerase** or other enzymes involved in DNA replication. - While overall cellular processes are disrupted, its primary toxic effect is not on DNA synthesis. *Protein breakdown* - **Cyanide** does not directly interfere with proteasomes or lysosomal enzymes responsible for **protein degradation**. - Its mechanism of action is upstream, affecting energy production necessary for all cellular processes, including protein turnover. *Protein synthesis* - **Cyanide** does not directly inhibit **ribosomes** or the enzymatic machinery for **protein synthesis**. - The lack of **ATP** caused by cyanide poisoning would eventually shut down protein synthesis, but this is a secondary effect, not the primary mechanism of action.

Question 312: Which of the following pairs of compounds has the highest standard reduction potential?

A. NADH/NAD+
B. Succinate/Fumarate
C. Ubiquinone/Ubiquinol
D. Fe³⁺/Fe²⁺ (Correct Answer)

Explanation: ***Fe³⁺/Fe²⁺*** - The **Fe³⁺/Fe²⁺ couple** has a **standard reduction potential (E'0)** of **+0.77 V**, making it the highest among the given options. - A higher positive E'0 indicates a stronger tendency for the oxidized form to accept electrons and be reduced. *NADH/NAD+* - The **NADH/NAD+ couple** has a **standard reduction potential** of **-0.32 V**, indicating it is a strong reducing agent. - Its negative reduction potential means it readily donates electrons during metabolic processes. *Succinate/Fumarate* - The **succinate/fumarate couple** has a **standard reduction potential** of **+0.03 V**. - This pair is involved in the **TCA cycle**, where succinate is oxidized to fumarate, releasing electrons. *Ubiquinone/Ubiquinol* - The **ubiquinone/ubiquinol couple** has a **standard reduction potential** varying around **+0.05 to +0.10 V**, depending on the specific state. - It acts as a mobile electron carrier in the **electron transport chain**, accepting electrons from NADH and FADH2.

Question 313: 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 314: NADH via glycerophosphate shunt makes how many ATP?

A. 1
B. 4
C. 2 (Correct Answer)
D. 3

Explanation: ***2*** - The **glycerol phosphate shuttle** transfers electrons from **cytosolic NADH** to **FAD** in the mitochondrial electron transport chain. - Each **FADH2** molecule produced then enters the electron transport chain at **Complex II**, ultimately leading to the generation of approximately **2 ATP** molecules. *1* - This option would be correct if the electrons were transferred to a molecule that yields only **one ATP** equivalent, which is not the case for **FADH2**. - No direct mechanism in a shunt generates exactly one ATP per NADH equivalent. *3* - This value represents the ATP yield from **NADH** when it directly enters the electron transport chain via the **malate-aspartate shuttle**, not the **glycerophosphate shuttle**. - The **glycerophosphate shuttle** is less efficient than the **malate-aspartate shuttle**. *4* - This number is not a standard ATP yield for either **NADH** or **FADH2** in the electron transport chain. - The maximum yield for NADH is typically considered to be 2.5 or 3 ATP, and for FADH2 is 1.5 or 2 ATP, depending on the shuttle and precise calculations.

Question 315: What metabolic changes occur during overnight fasting?

A. Blood glucose decreases slightly
B. Fat breakdown increases
C. Glucose production increases (Correct Answer)
D. Ketone levels rise slightly

Explanation: ***Glucose production increases*** - During overnight fasting (typically 8-12 hours), the body's **primary metabolic priority** is to maintain **blood glucose homeostasis** to fuel the brain and other glucose-dependent tissues. - As **hepatic glycogen stores** become depleted, the liver significantly increases **gluconeogenesis** (glucose production from non-carbohydrate sources like amino acids, lactate, and glycerol) to supply glucose. - This represents the **most critical metabolic adaptation** during overnight fasting, as the brain requires a constant glucose supply (~120g/day) and cannot initially use alternative fuels. *Blood glucose decreases slightly* - During a normal overnight fast, blood glucose levels remain **relatively stable** (70-100 mg/dL) due to compensatory mechanisms. - The body's homeostatic mechanisms (increased glucose production, decreased glucose utilization by muscles) prevent any significant drop in blood glucose. - A significant decrease would indicate **hypoglycemia**, which is prevented by the metabolic changes described above. *Fat breakdown increases* - **Lipolysis** (fat breakdown) does indeed increase significantly during overnight fasting to provide **fatty acids** as an alternative fuel source for skeletal muscle, cardiac muscle, and liver. - This is an important metabolic change, but is **secondary to glucose production** in terms of priority, as it serves to spare glucose for the brain rather than directly maintaining glucose levels. - Increased fatty acid oxidation provides acetyl-CoA for **ketone body synthesis** and reduces glucose consumption by peripheral tissues (glucose-sparing effect). *Ketone levels rise slightly* - **Ketone body production** (acetoacetate, β-hydroxybutyrate) does begin to increase as fasting progresses beyond 8-12 hours. - However, during an *overnight* fast, ketone levels rise only **modestly** (typically <1 mM); clinically significant ketosis develops during **prolonged fasting** (24-72 hours), when ketone bodies become a major fuel source for the brain. - The overnight period represents the **transition phase** where glucose production remains the dominant metabolic response.

Question 316: Which enzyme primarily initiates the electron transport process in oxidative phosphorylation?

A. Pyruvate kinase
B. Succinyl CoA thiokinase
C. NADH dehydrogenase (Correct Answer)
D. ATP synthase

Explanation: ***Correct NADH dehydrogenase*** - **NADH dehydrogenase**, also known as Complex I, is the enzyme that accepts electrons from **NADH** during oxidative phosphorylation, initiating the electron transport chain. - This enzyme **oxidizes NADH** to NAD+ and pumps protons from the mitochondrial matrix to the intermembrane space, contributing to the **proton gradient**. *Incorrect Pyruvate kinase* - **Pyruvate kinase** is an enzyme involved in **glycolysis**, catalyzing the final step of converting phosphoenolpyruvate to pyruvate. - It functions in the **cytoplasm** and is not directly involved in the electron transport chain or oxidative phosphorylation. *Incorrect Succinyl CoA thiokinase* - **Succinyl CoA thiokinase** (also known as succinate thiokinase or succinyl-CoA synthetase) is an enzyme in the **Krebs cycle** (citric acid cycle). - It catalyzes the reversible reaction of converting succinyl-CoA to succinate and is not directly part of the electron transport chain. *Incorrect ATP synthase* - **ATP synthase** (Complex V) is the enzyme responsible for synthesizing ATP using the **proton gradient** established by the electron transport chain. - While crucial for oxidative phosphorylation, it acts at the end of the process, utilizing the energy generated, rather than initiating electron transport.

Question 317: Which of the following is not the source of cytosolic NADPH ?

A. Malic enzyme
B. G6PD
C. Isocitrate dehydrogenase
D. ATP citrate lyase (Correct Answer)

Explanation: ***ATP citrate lyase*** - **ATP citrate lyase** is an enzyme involved in the synthesis of **acetyl-CoA** from citrate in the cytosol, which is then used for **fatty acid synthesis**. It does not generate NADPH. - While the **acetyl-CoA** produced is used in pathways that require NADPH, ATP citrate lyase itself does not directly produce NADPH. *Isocitrate dehydrogenase* - Cytosolic **isocitrate dehydrogenase** catalyzes the oxidative decarboxylation of **isocitrate** to alpha-ketoglutarate, producing **NADPH**. - This reaction is an important source of **cytosolic NADPH**, especially in non-photosynthetic tissues. *Malic enzyme* - **Malic enzyme** catalyzes the oxidative decarboxylation of **malate** to pyruvate, simultaneously reducing **NADP+ to NADPH**. - This enzyme is a significant source of **cytosolic NADPH** in various tissues, contributing to fatty acid synthesis and other reductive processes. *G6PD* - **Glucose-6-phosphate dehydrogenase (G6PD)** is the rate-limiting enzyme in the **pentose phosphate pathway** (PPP). - It catalyzes the first step of the PPP, converting **glucose-6-phosphate** to 6-phosphogluconolactone and producing **NADPH** as a crucial coenzyme.

Question 318: 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 319: Major source of energy for brain in fasting/starvation?

A. Glucose
B. Glycogen
C. Fatty acids
D. Ketone bodies (Correct Answer)

Explanation: ***Ketone bodies*** - During **prolonged fasting or starvation**, the body depletes its **glycogen stores** and begins to break down fatty acids. The liver converts these fatty acids into **ketone bodies**, such as **acetoacetate and beta-hydroxybutyrate**. - These **ketone bodies** can cross the **blood-brain barrier** and be used by the brain as an alternative energy source when glucose becomes scarce, preventing protein breakdown for gluconeogenesis. *Glucose* - While **glucose** is the primary and preferred energy source for the brain under normal physiological conditions, its availability significantly decreases during **prolonged fasting or starvation**. - The brain requires a continuous supply of glucose, but in states of severe caloric restriction, the body must conserve glucose for other critical functions and adapt by using alternative fuels. *Glycogen* - **Glycogen** is a stored form of glucose found predominantly in the **liver and muscles**. - The brain itself has minimal **glycogen stores**, which are rapidly depleted during fasting, and thus cannot be a major long-term energy source. *Fatty acids* - **Fatty acids** are a major energy source for many tissues in the body, especially during fasting, but they **cannot directly cross the blood-brain barrier** in significant amounts to fuel the brain. - Instead, **fatty acids** are metabolized into **ketone bodies** in the liver, which then serve as the brain's alternative fuel.

Question 320: Which of the following represents the most significant regulatory control point among these TCA cycle reactions?

A. Succinyl-CoA to Succinate (Succinyl-CoA synthetase)
B. Isocitrate to Alpha-ketoglutarate (Isocitrate dehydrogenase) (Correct Answer)
C. Acetyl-CoA + Oxaloacetate to Citrate (Citrate synthase)
D. Alpha-ketoglutarate to Succinyl-CoA (Alpha-ketoglutarate dehydrogenase complex)

Explanation: ***Isocitrate to Alpha-ketoglutarate (Isocitrate dehydrogenase)*** - **Isocitrate dehydrogenase** is the **rate-limiting enzyme** and the **most significant regulatory control point** of the TCA cycle - It catalyzes the first **irreversible NADH-generating step** after citrate formation, making it the key determinant of cycle flux - Strongly **activated by ADP** (indicating low energy status) and **Ca²⁺** (in mitochondria) - Strongly **inhibited by NADH and ATP** (indicating high energy status), providing sensitive energy-status regulation - This is the primary control point recognized in standard biochemistry references *Alpha-ketoglutarate to Succinyl-CoA (Alpha-ketoglutarate dehydrogenase complex)* - The **alpha-ketoglutarate dehydrogenase complex** is an important regulatory enzyme with irreversible catalysis - Inhibited by its products **NADH** and **succinyl-CoA**, as well as by **ATP** - While it is one of the three main control points, it is considered a **secondary regulatory site** compared to isocitrate dehydrogenase *Acetyl-CoA + Oxaloacetate to Citrate (Citrate synthase)* - **Citrate synthase** catalyzes the first committed step of the TCA cycle and is the entry point for acetyl-CoA - Subject to **product inhibition by citrate** and allosteric inhibition by **ATP, NADH, and succinyl-CoA** - Although highly regulated and crucial for initiating the cycle, it is not the rate-limiting step *Succinyl-CoA to Succinate (Succinyl-CoA synthetase)* - This reaction involves **substrate-level phosphorylation** to produce **GTP (or ATP)** - It is a **reversible reaction** and generally not a primary regulatory step - Regulation depends mainly on substrate availability rather than complex allosteric control mechanisms

Practice by Chapter

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Shuttle Systems: Malate-Aspartate and Glycerol-Phosphate

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