Energy Production and Metabolism — MCQs
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Mechanism of cyanide poisoning is by inhibiting: NEET 2013
Chemiosmotic coupling of oxidative phosphorylation is related to which of the following?
Fluoroacetate inhibits?
Which of the following processes does not occur in mitochondria?
Which of the following is not the source of cytosolic NADPH ?
Organ that can utilize glucose, fatty acids and ketone bodies is:
Which enzyme primarily initiates the electron transport process in oxidative phosphorylation?
Anaplerotic reaction is catalyzed by?
Which of the following represents the most significant regulatory control point among these TCA cycle reactions?
What metabolic changes occur during overnight fasting?
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: 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 313: Fluoroacetate inhibits?
- A. Citrate synthase
- B. Succinate dehydrogenase
- C. Alpha-ketoglutarate dehydrogenase
- D. Aconitase (Correct Answer)
Explanation: ***Aconitase*** - **Fluoroacetate** is metabolically converted to **fluorocitrate**, which is a potent competitive inhibitor of **aconitase**. - **Aconitase** is the enzyme responsible for converting **citrate to isocitrate** in the **Krebs cycle**, and its inhibition blocks the cycle. *Citrate synthase* - This enzyme is responsible for the formation of **citrate** from **acetyl-CoA** and **oxaloacetate**. - While fluoroacetate indirectly affects the cycle, it does not directly inhibit **citrate synthase**. *Succinate dehydrogenase* - This enzyme is part of the **Krebs cycle** and the **electron transport chain**, converting **succinate to fumarate**. - **Malonate** is a competitive inhibitor of succinate dehydrogenase, not **fluoroacetate**. *Alpha-ketoglutarate dehydrogenase* - This enzyme catalyzes the conversion of **alpha-ketoglutarate to succinyl-CoA** in the **Krebs cycle**. - Specific inhibitors of this enzyme include **arsenite** and **mercury compounds**, but not fluoroacetate.
Question 314: Which of the following processes does not occur in mitochondria?
- A. Fatty acid oxidation
- B. Electron transport chain
- C. Glycogenolysis (Correct Answer)
- D. Citric acid cycle (Kreb's cycle)
Explanation: ***Glycogenolysis*** - **Glycogenolysis** is the breakdown of **glycogen** into glucose, which primarily occurs in the **cytosol** of cells, mainly in the liver and muscles. - This process is crucial for maintaining blood glucose levels and providing energy during periods of fasting or increased demand, and it does not take place within the mitochondria. *Fatty acid oxidation* - **Fatty acid oxidation**, also known as beta-oxidation, is a mitochondrial process that breaks down fatty acids into **acetyl-CoA** for energy production. - This occurs extensively within the mitochondrial matrix, producing ATP. *Electron transport chain* - The **electron transport chain** is located in the **inner mitochondrial membrane** and is the final stage of aerobic respiration, producing the majority of ATP. - It involves a series of protein complexes that transfer electrons to oxygen, creating a proton gradient for ATP synthesis. *Citric acid cycle (Kreb's cycle)* - The **citric acid cycle**, or **Krebs cycle**, is a central metabolic pathway that occurs in the **mitochondrial matrix**. - It oxidizes acetyl-CoA, derived from carbohydrates, fats, and proteins, to produce ATP, NADH, and FADH2.
Question 315: 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 316: Organ that can utilize glucose, fatty acids and ketone bodies is:
- A. Liver
- B. Brain
- C. Skeletal muscle (Correct Answer)
- D. RBC
Explanation: ***Skeletal muscle*** - Skeletal muscle is highly adaptable and can utilize **glucose**, **fatty acids (FAs)**, and **ketone bodies** as fuel sources, especially during prolonged exercise or starvation. - Its metabolic flexibility allows it to switch between these substrates depending on their availability and the body's energy demands. *Liver* - The liver is central to metabolism but primarily **produces ketone bodies** from fatty acids rather than utilizing them as a major fuel source for its own energy needs. - While it uses glucose and FAs, its role in ketone body metabolism is largely synthetic. *Brain* - The brain preferentially uses **glucose** as its primary fuel. - During prolonged starvation, it can adapt to utilize **ketone bodies** as an alternative fuel source, but it does not significantly use fatty acids directly. *RBC* - Red blood cells (RBCs) lack mitochondria and therefore rely exclusively on **anaerobic glycolysis** for energy, metabolizing only **glucose**. - They cannot utilize fatty acids or ketone bodies.
Question 317: 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 318: Anaplerotic reaction is catalyzed by?
- A. Enolase
- B. Pyruvate kinase
- C. G6PD
- D. Pyruvate carboxylase (Correct Answer)
Explanation: ***Pyruvate carboxylase*** - **Pyruvate carboxylase** catalyzes the ATP-dependent carboxylation of **pyruvate** to **oxaloacetate**. - This reaction is crucial for replenishing intermediates of the **citric acid cycle**, making it an anaplerotic reaction. *Enolase* - **Enolase** catalyzes the conversion of **2-phosphoglycerate** to **phosphoenolpyruvate** in **glycolysis**. - This reaction is part of catabolism and does not replenish citric acid cycle intermediates. *Pyruvate kinase* - **Pyruvate kinase** catalyzes the final step of **glycolysis**, converting **phosphoenolpyruvate** to **pyruvate**. - This enzyme is involved in ATP production and the overall catabolic pathway of glucose. *G6PD* - **Glucose-6-phosphate dehydrogenase (G6PD)** is the rate-limiting enzyme in the **pentose phosphate pathway**. - It produces **NADPH** and precursors for nucleotide synthesis, but not directly involved in anaplerotic reactions for the citric acid cycle.
Question 319: 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
Question 320: 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.
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