Energy Production and Metabolism — MCQs

Energy Production and Metabolism Indian Medical PG Practice Questions and MCQs

Question 321: Why is the citric acid cycle called an amphibolic pathway?

A. Both exergonic and endergonic reactions take place
B. Metabolites are utilized in other pathways. (Correct Answer)
C. It can proceed in both forward and backward directions.
D. The same enzymes can be used in reverse directions.

Explanation: ***Metabolites are utilized in other pathways.*** - The citric acid cycle is termed **amphibolic** because it serves both catabolic (breakdown) and anabolic (synthetic) functions. - Its intermediates are constantly drawn off for biosynthesis of molecules like **amino acids**, **heme**, and **glucose**, meaning it's not solely degradative. *Both exergonic and endergonic reactions take place* - While both types of reactions do occur in many metabolic pathways, this is a general characteristic of metabolism and not specific to the definition of an **amphibolic pathway**. - The amphibolic nature specifically refers to the dual role in both **catabolism** and **anabolism**. *It can proceed in both forward and backward directions.* - This statement typically describes a **reversible pathway** or individual reversible reactions, not necessarily an amphibolic pathway. - The citric acid cycle is primarily an oxidative cycle that proceeds in a forward, cyclic direction under aerobic conditions. *The same enzymes can be used in reverse directions.* - While some individual enzymes within metabolic pathways can catalyze reversible reactions, this is not the defining characteristic of an **amphibolic pathway**. - The amphibolic designation refers to the overall pathway's contribution to both breakdown and synthesis of molecules.

Question 322: 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 323: 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 324: 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 325: 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 326: Which enzyme in the TCA cycle catalyzes the step where substrate-level phosphorylation occurs?

A. Isocitrate dehydrogenase
B. Malate dehydrogenase
C. Aconitase
D. Succinate thiokinase (Correct Answer)

Explanation: ***Succinate thiokinase*** - This enzyme (also known as **succinyl-CoA synthetase**) catalyzes the conversion of **succinyl-CoA** to **succinate**. - During this reaction, the energy released from breaking the **thioester bond** in succinyl-CoA is directly used to synthesize **GTP** (or ATP in some organisms) from GDP (or ADP) and inorganic phosphate, which is a classic example of **substrate-level phosphorylation**. *Isocitrate dehydrogenase* - This enzyme catalyzes the **oxidative decarboxylation** of isocitrate to $\alpha$-ketoglutarate. - This step produces **NADH** and **CO2** but does not involve substrate-level phosphorylation. *Malate dehydrogenase* - This enzyme catalyzes the oxidation of **L-malate** to **oxaloacetate** in the final step of the TCA cycle. - It produces **NADH** but does not involve the direct synthesis of ATP or GTP. *Aconitase* - This enzyme catalyzes the **isomerization** of **citrate** to **isocitrate** via an aconitate intermediate. - No energy is generated or consumed in the form of ATP/GTP during this rearrangement.

Question 327: In ETC NADH generates -

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

Explanation: ***3 ATPs*** - Each molecule of **NADH** donates electrons to **Complex I** of the electron transport chain (ETC), resulting in the pumping of enough protons to generate approximately **3 ATP molecules** via **oxidative phosphorylation**. - This high yield is due to NADH's ability to activate multiple proton pumps along the ETC, maximizing the **proton gradient** for ATP synthesis. *1 ATPs* - This is an incorrect yield for NADH; **FADH2** typically generates fewer ATPs (around 2) because it enters the ETC at a later stage, bypassing the initial proton pump. - Generating only 1 ATP from NADH would be very inefficient and is not physiologically accurate for oxidative phosphorylation. *2 ATPs* - While closer, 2 ATPs is the approximate yield for **FADH2**, which enters the ETC at **Complex II**, bypassing Complex I and thus pumping fewer protons. - NADH enters at Complex I, which provides enough energy for a higher ATP yield. *4 ATPs* - 4 ATPs is an overestimation of the ATP yield from NADH in the electron transport chain. - The maximum theoretical yield from NADH via oxidative phosphorylation is typically considered to be 3 ATPs.

Question 328: Which enzyme is involved in substrate level phosphorylation?

A. Enolase
B. Aldolase
C. Creatine kinase (Correct Answer)
D. Lactate dehydrogenase

Explanation: ***Creatine kinase*** - **Creatine kinase** catalyzes the direct transfer of a high-energy phosphate group from **phosphocreatine** to **ADP** to form **ATP**. - This is a classic example of **substrate-level phosphorylation** - ATP formation by direct phosphate transfer from a high-energy donor molecule. - This reaction is crucial in muscle cells for rapid ATP regeneration during high-energy demand. - Other substrate-level phosphorylation enzymes include **phosphoglycerate kinase** and **pyruvate kinase** in glycolysis, and **succinyl-CoA synthetase** in the citric acid cycle. *Enolase* - **Enolase** converts **2-phosphoglycerate** to **phosphoenolpyruvate (PEP)** in glycolysis. - While this creates a high-energy phosphate compound, enolase itself does **not** catalyze substrate-level phosphorylation. - The actual ATP formation from PEP is catalyzed by **pyruvate kinase**, not enolase. *Aldolase* - **Aldolase** cleaves **fructose-1,6-bisphosphate** into **dihydroxyacetone phosphate** and **glyceraldehyde-3-phosphate**. - This is a cleavage reaction in glycolysis that does not involve ATP synthesis. *Lactate dehydrogenase* - **Lactate dehydrogenase** catalyzes the conversion of **pyruvate** to **lactate** with oxidation of **NADH to NAD+**. - This reaction regenerates NAD+ for glycolysis to continue but does not produce ATP.

Question 329: Which enzyme catalyzes the rate limiting step in the TCA cycle?

A. Fumarase
B. Aconitase
C. Thiokinase
D. α-ketoglutarate dehydrogenase (Correct Answer)

Explanation: **α-ketoglutarate dehydrogenase** - The **α-ketoglutarate dehydrogenase complex** catalyzes the oxidative decarboxylation of α-ketoglutarate to succinyl-CoA, producing NADH and CO2. - This step is a **major control point** in the TCA cycle and is highly regulated by: - **Product inhibition**: Succinyl-CoA and NADH - **Calcium ions**: Activate the enzyme - Along with isocitrate dehydrogenase and citrate synthase, it represents one of the three key regulatory enzymes of the TCA cycle. *Fumarase* - **Fumarase** catalyzes the reversible hydration of fumarate to L-malate. - This enzyme is **not a regulatory step** in the TCA cycle; its activity is typically high and not a control point for the overall flux of the cycle. *Aconitase* - **Aconitase** catalyzes the reversible isomerization of citrate to isocitrate, via the intermediate cis-aconitate. - While important for the cycle's progression, aconitase activity is **not considered a rate-limiting step** for the overall regulation of the TCA cycle. *Thiokinase* - The term **thiokinase** (or succinyl-CoA synthetase) catalyzes the reversible conversion of succinyl-CoA to succinate, coupled with GTP/ATP production. - This enzyme is responsible for **substrate-level phosphorylation** in the TCA cycle but does not represent a primary regulatory or rate-limiting step.

Question 330: Which of the following vitamins forms a coenzyme that acts as the primary electron acceptor in cellular oxidation-reduction reactions?

A. Vitamin B3 (Niacin) (Correct Answer)
B. Vitamin B1 (Thiamine)
C. Vitamin B6 (Pyridoxine)
D. Vitamin B2 (Riboflavin)

Explanation: ***Vitamin B3 (Niacin)*** - **Niacin (Vitamin B3)** is a precursor to **NAD+** (nicotinamide adenine dinucleotide) and **NADP+**, which function as primary electron acceptors in cellular metabolism. - **NAD+** accepts electrons from various metabolic intermediates during glycolysis, beta-oxidation, and the TCA cycle, becoming **NADH**. - **NADH** then transfers these electrons to Complex I of the electron transport chain to generate ATP. - NAD+/NADH is the most abundant and widely used electron carrier in cellular metabolism. *Vitamin B2 (Riboflavin)* - **Riboflavin (Vitamin B2)** is a precursor to **FAD** (flavin adenine dinucleotide) and **FMN** (flavin mononucleotide), which are also electron carriers. - While FAD and FMN are important electron acceptors (e.g., in succinate dehydrogenase and fatty acid oxidation), **NAD+** is quantitatively more significant and accepts electrons from a greater number of reactions. *Vitamin B1 (Thiamine)* - **Thiamine** acts as a coenzyme, **thiamine pyrophosphate (TPP)**, primarily involved in carbohydrate metabolism (e.g., pyruvate dehydrogenase complex, alpha-ketoglutarate dehydrogenase). - It facilitates decarboxylation reactions but does not function as an electron acceptor. *Vitamin B6 (Pyridoxine)* - **Pyridoxine (Vitamin B6)** is converted to **pyridoxal phosphate (PLP)**, a coenzyme primarily involved in amino acid metabolism, including transamination, decarboxylation, and racemization. - It has no role as an electron acceptor in oxidation-reduction reactions.

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