Energy Production and Metabolism Practice Questions

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

Fe³⁺/Fe²⁺. ***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.

Q: In ETC NADH generates -

3 ATPs. ***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.

Q: Anaplerotic reaction is catalyzed by?

Pyruvate carboxylase. ***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.

Q: The citric acid cycle is a hub of energy transformation. Which citric acid cycle enzyme produces ATP by substrate-level phosphorylation?

Succinate thiokinase. ***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 the thioester bond of succinyl-CoA is used to directly phosphorylate ADP to **ATP** (or GDP to GTP), a classic example of **substrate-level phosphorylation**. *Succinate dehydrogenase* - This enzyme is part of both the **citric acid cycle** and the **electron transport chain** (Complex II). - It catalyzes the oxidation of **succinate to fumarate** and reduces FAD to **FADH2**, which then donates electrons to the electron transport chain, but it does not produce ATP directly. *Isocitrate dehydrogenase* - Catalyzes the oxidative decarboxylation of **isocitrate to α-ketoglutarate**, producing **NADH** and **CO2**. - The NADH generated contributes to ATP production through oxidative phosphorylation, not substrate-level phosphorylation. *Aconitase* - Catalyzes the reversible isomerization of **citrate to isocitrate**, with **cis-aconitate** as an intermediate. - This enzyme rearranges the molecule for subsequent oxidative steps but does not directly produce ATP or reduce coenzymes.

Q: The mechanism by which cytosolic pyruvate is transported to mitochondria is

Proton symport. ***Proton symport*** - Pyruvate is transported into the mitochondrial matrix by the **mitochondrial pyruvate carrier (MPC)**, which functions as a **proton symporter**. - This transport mechanism couples the movement of pyruvate with the movement of a proton down its electrochemical gradient. *Chloride antiport (specific for chloride ions)* - **Chloride antiport** involves the movement of chloride ions in the opposite direction to another molecule, which is not the mechanism for pyruvate transport. - This mechanism is typically involved in maintaining **ionic balance** or specific solute transport across membranes, distinct from pyruvate uptake. *Facilitated uniport (passive transport)* - While pyruvate transport is a form of facilitated diffusion, it is specifically a **symport** mechanism, not a simple uniport. - **Uniport** involves the transport of a single solute down its concentration gradient without coupling to another molecule or ion. *ATP-dependent transport* - Pyruvate transport into the mitochondrion is **not directly ATP-dependent**; it utilizes the proton gradient rather than ATP hydrolysis. - **ATP-dependent transport** involves energy derived directly from ATP hydrolysis to move molecules against their concentration gradient.

Q: Which of the following statements about the electron transport chain is CORRECT?

FADH2 gives 1.5 ATP. ***FADH2 gives 1.5 ATP*** - Each **FADH2** molecule that enters the electron transport chain generates approximately **1.5 ATP molecules** via oxidative phosphorylation based on modern P/O ratio calculations. - FADH2 bypasses Complex I and enters at **Complex II (succinate dehydrogenase)**, thus contributing to fewer proton pumping sites (only Complexes III and IV) compared to NADH. - This is the modern, accurate value based on current understanding of mitochondrial bioenergetics. *NADH enters at Complex I* - While this statement is **factually true**, NADH entering at Complex I is well-established biochemistry. - However, when combined with the context of other options, **option C provides the most clinically relevant quantitative information** about ATP yield. - NADH oxidation at Complex I pumps protons at three sites (Complexes I, III, and IV). *FADH2 enters at Complex II* - This statement is also **factually correct** - FADH2 does enter the electron transport chain at Complex II. - However, without mentioning the ATP yield, this statement is less complete than option C which provides quantitative information. *NADH gives 3 ATP* - This is **INCORRECT** based on modern biochemistry. - The current accepted value is approximately **2.5 ATP** per NADH molecule. - The older estimate of 3 ATP was based on integer P/O ratios that didn't account for the energy cost of ATP/ADP translocase and phosphate transporter.

Q: Final common pathway of metabolism of carbohydrate, lipids, and protein metabolism is?

TCA. ***TCA (Tricarboxylic Acid Cycle)*** - The **TCA cycle** (also called Krebs cycle or citric acid cycle) is the **final common oxidative pathway** where all three macronutrients converge - **Carbohydrates** → Pyruvate → **Acetyl-CoA** (via pyruvate dehydrogenase) - **Lipids** → Fatty acids → **Acetyl-CoA** (via beta-oxidation) - **Proteins** → Amino acids → **Acetyl-CoA or TCA intermediates** (via deamination/transamination) - Complete oxidation of acetyl-CoA occurs in the TCA cycle, producing **NADH, FADH2, and GTP** for energy production *Gluconeogenesis* - This is a **biosynthetic pathway** that synthesizes glucose from non-carbohydrate precursors (lactate, glycerol, amino acids) - It is an **anabolic process**, not the catabolic final common pathway for energy production from all macronutrients *Glycolysis* - **Carbohydrate-specific pathway** that converts glucose to pyruvate - It is only the initial breakdown pathway for carbohydrates, not the common pathway where lipids and proteins also converge - Pyruvate from glycolysis must enter TCA cycle for complete oxidation *HMP pathway (Pentose Phosphate Pathway)* - Parallel pathway to glycolysis that generates **NADPH** (for biosynthesis and antioxidant defense) and **ribose-5-phosphate** (for nucleotide synthesis) - Processes only **glucose-6-phosphate** from carbohydrate metabolism - Not involved in lipid or protein metabolism integration

Q: Hyperammonemia impairs the citric acid cycle by depleting which of the following?

α-ketoglutarate. ***α-ketoglutarate*** - Excess ammonia in the brain reacts with **α-ketoglutarate** via **glutamate dehydrogenase** to form glutamate, and glutamate then converts to glutamine. - This depletion of α-ketoglutarate reduces the availability of a crucial citric acid cycle intermediate, thereby impairing the cycle's function and **ATP production**. *Pyruvate* - **Pyruvate** is primarily involved in the entry into the citric acid cycle via its conversion to acetyl-CoA, but hyperammonemia does not directly deplete it. - Ammonia metabolism primarily affects the glutamate-glutamine pathway, which consumes α-ketoglutarate, not pyruvate. *Oxaloacetate* - **Oxaloacetate** is a citric acid cycle intermediate, but its direct depletion is not the primary mechanism by which hyperammonemia impairs the cycle. - While cycle impairment may indirectly affect oxaloacetate levels, the direct consumption of **α-ketoglutarate** is the more immediate impact of hyperammonemia. *Succinate* - **Succinate** is an intermediate of the citric acid cycle and its levels would be affected by a general impairment, but it is not directly consumed by ammonia detoxification. - The primary target for ammonia in this context is **α-ketoglutarate** due to its role in glutamate synthesis.

Q: Which molecule serves as the primary acceptor for acetyl-CoA to initiate the TCA cycle?

Oxaloacetate. ***Oxaloacetate*** - **Oxaloacetate** is the **4-carbon acceptor molecule** that condenses with acetyl-CoA to form citrate, initiating each turn of the TCA cycle. - It acts as a **catalytic molecule** that is regenerated at the end of the cycle, allowing continuous operation. - Without oxaloacetate, acetyl-CoA cannot enter the cycle, making it the **rate-limiting requirement** for cycle initiation. *Acetyl-CoA* - Acetyl-CoA is the **2-carbon substrate** that enters the TCA cycle by combining with oxaloacetate. - While essential for providing carbons to be oxidized, it is not the acceptor molecule; rather, it is the molecule being accepted. *Citrate* - Citrate is the **6-carbon product** formed when acetyl-CoA condenses with oxaloacetate in the first reaction of the TCA cycle. - It is an intermediate product, not the acceptor molecule needed to initiate the cycle. *ATP* - ATP is an **energy product** of cellular respiration and acts as an **allosteric inhibitor** of the TCA cycle. - It does not serve as a substrate or acceptor molecule in the cycle's reactions.

Question 1

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

Accepted Answer

Fe³⁺/Fe²⁺

Answer

NADH/NAD+

Answer

Succinate/Fumarate

Answer

Ubiquinone/Ubiquinol

Question 2

In ETC NADH generates -

Accepted Answer

3 ATPs

Answer

1 ATPs

Answer

4 ATPs

Answer

2 ATPs

Question 3

Anaplerotic reaction is catalyzed by?

Accepted Answer

Pyruvate carboxylase

Answer

Enolase

Answer

Pyruvate kinase

Answer

G6PD

Question 4

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

Accepted Answer

Isocitrate to Alpha-ketoglutarate (Isocitrate dehydrogenase)

Answer

Succinyl-CoA to Succinate (Succinyl-CoA synthetase)

Answer

Acetyl-CoA + Oxaloacetate to Citrate (Citrate synthase)

Answer

Alpha-ketoglutarate to Succinyl-CoA (Alpha-ketoglutarate dehydrogenase complex)

Question 5

The citric acid cycle is a hub of energy transformation. Which citric acid cycle enzyme produces ATP by substrate-level phosphorylation?

Accepted Answer

Succinate thiokinase

Answer

Isocitrate dehydrogenase

Answer

Succinate dehydrogenase

Answer

Aconitase

Question 6

The mechanism by which cytosolic pyruvate is transported to mitochondria is

Accepted Answer

Proton symport

Answer

Chloride antiport

Answer

ATP-dependent transport

Answer

Facilitated uniport

Question 7

Which of the following statements about the electron transport chain is CORRECT?

Accepted Answer

FADH2 gives 1.5 ATP

Answer

NADH enters at Complex I

Answer

FADH2 enters at Complex II

Answer

NADH gives 3 ATP

Question 8

Final common pathway of metabolism of carbohydrate, lipids, and protein metabolism is?

Accepted Answer

TCA

Answer

Gluconeogenesis

Answer

HMP pathway

Answer

Glycolysis

Question 9

Hyperammonemia impairs the citric acid cycle by depleting which of the following?

Accepted Answer

α-ketoglutarate

Answer

Pyruvate

Answer

Oxaloacetate

Answer

Succinate

Question 10

Which molecule serves as the primary acceptor for acetyl-CoA to initiate the TCA cycle?

Accepted Answer

Oxaloacetate

Answer

Acetyl CoA

Answer

Citrate

Answer

ATP

Energy Production and Metabolism — MCQs

Energy Production and Metabolism — MCQs

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