Energy Production and Metabolism Practice Questions

Q: Which coenzyme serves as the primary electron acceptor in multiple dehydrogenase reactions of the Kreb's cycle?

NAD. ***NAD⁺ (NAD)*** - **NAD⁺ (Nicotinamide Adenine Dinucleotide)** serves as the primary **electron acceptor** in the Kreb's cycle, being reduced to **NADH** in three key dehydrogenase reactions. - These reactions occur at: **isocitrate dehydrogenase**, **α-ketoglutarate dehydrogenase**, and **malate dehydrogenase** steps. - The **NAD⁺/NADH** coenzyme system is essential for extracting energy from acetyl-CoA, with NADH subsequently donating electrons to the **electron transport chain** for ATP synthesis. *NADP* - **NADP⁺ (Nicotinamide Adenine Dinucleotide Phosphate)** is primarily involved in **anabolic reactions**, such as **fatty acid synthesis** and the **pentose phosphate pathway**. - While structurally similar to NAD⁺ (differing only by a phosphate group), it functions in different metabolic pathways and is not utilized in the **Kreb's cycle**. *NADPH* - **NADPH** is the reduced form of **NADP⁺** and functions as a reducing agent in various **biosynthetic pathways**, including synthesis of **fatty acids**, **cholesterol**, and **nucleotides**. - It also plays a crucial role in **antioxidant defense** (glutathione reduction) and the **respiratory burst** in phagocytes. - NADPH is not involved in the **Kreb's cycle**, which uses the NAD⁺/NADH system instead. *NADH* - **NADH** is the **reduced form** of NAD⁺ produced during the Kreb's cycle reactions. - While NADH and NAD⁺ are two forms of the same coenzyme, the question asks for the **electron acceptor** form, which is **NAD⁺** (oxidized form). - **NADH** carries the extracted electrons to **Complex I** of the electron transport chain, where it is reoxidized back to NAD⁺, generating approximately **2.5 ATP** per NADH molecule.

Q: Which molecule acts as the primary electron carrier in the Krebs cycle?

NADH. ***NADH*** - **NADH** (reduced nicotinamide adenine dinucleotide) is the **primary electron carrier** produced during the Krebs cycle. - **Three molecules of NADH** are generated per cycle (at isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, and malate dehydrogenase steps). - These high-energy electrons are transferred to the **electron transport chain** to generate approximately **7.5 ATP per NADH**. *NAD+* - **NAD+** (oxidized nicotinamide adenine dinucleotide) is the coenzyme that *accepts* electrons during the Krebs cycle. - It is the **oxidized form** that gets reduced to NADH, not the carrier itself. *FADH₂* - **FADH₂** (reduced flavin adenine dinucleotide) is also produced in the Krebs cycle at the **succinate dehydrogenase** step. - However, only **one molecule of FADH₂** is produced per cycle compared to three NADH molecules. - FADH₂ generates approximately **5 ATP** in the electron transport chain, making NADH the quantitatively dominant electron carrier. *NADPH* - **NADPH** (reduced nicotinamide adenine dinucleotide phosphate) is NOT involved in the Krebs cycle. - It is primarily used in **anabolic pathways** such as fatty acid synthesis, cholesterol synthesis, and the **pentose phosphate pathway**. - NADPH serves as a **reducing agent** in biosynthetic reactions and protects against oxidative stress.

Q: Among the following enzymes, which one produces NADH in the citric acid cycle?

Isocitrate dehydrogenase. ***Isocitrate dehydrogenase*** - This enzyme catalyzes the conversion of **isocitrate to $\alpha$-ketoglutarate**, a reaction that releases **carbon dioxide** and reduces NAD+ to **NADH**. - This is one of the three **irreversible** (rate-limiting) reactions of the citric acid cycle. *Succinate thiokinase* - This enzyme, also known as **succinyl-CoA synthetase**, catalyzes the conversion of succinyl-CoA to succinate. - This reaction produces **GTP** (which can be readily interconverted to ATP), not NADH. *Succinate dehydrogenase* - This enzyme catalyzes the conversion of **succinate to fumarate**. - This reaction reduces **FAD to FADH2**, not NAD+ to NADH. *Fumarase* - This enzyme catalyzes the **hydration of fumarate to malate**. - This reaction does not involve the production of either NADH or FADH2; it simply adds a water molecule.

Q: What is the theoretical yield of ATP generated in one TCA cycle?

10. ***Correct: 10*** - One turn of the **TCA cycle** produces 3 NADH, 1 FADH₂, and 1 GTP (which is equivalent to ATP) - Using modern **P/O ratios**: 3 NADH yield 7.5 ATP (3 × 2.5 ATP/NADH) and 1 FADH₂ yields 1.5 ATP (1 × 1.5 ATP/FADH₂) - Adding the 1 GTP/ATP from substrate-level phosphorylation gives a **total of 10 ATP** *Incorrect: 2* - This only accounts for **substrate-level phosphorylation** (1 GTP converted to ATP) and ignores the substantial ATP generated from NADH and FADH₂ through **oxidative phosphorylation** - The total theoretical yield including electron transport chain is much higher *Incorrect: 8* - This is based on **outdated calculations** using older P/O ratios (3 ATP/NADH and 2 ATP/FADH₂ = 3×3 + 1×2 - 1 = 10, or miscalculation) - Modern biochemistry uses 2.5 ATP per NADH and 1.5 ATP per FADH₂, yielding 10 ATP total *Incorrect: 11* - This **overestimates** the ATP yield, possibly by using incorrect P/O ratios or miscounting the number of reduced cofactors produced - The standard calculation with 3 NADH, 1 FADH₂, and 1 GTP yields exactly 10 ATP

Q: Which of the following primarily occurs in the mitochondria?

ETC. ***Correct Option: ETC*** - The **electron transport chain (ETC)** is a series of protein complexes located **exclusively in the inner mitochondrial membrane**. - It occurs **solely in mitochondria** with no cytosolic component, making it the process that most "primarily" occurs in this organelle. - Its primary role is to generate a **proton gradient** through electron transfer, ultimately producing ATP via **oxidative phosphorylation**. - This is the definitive mitochondrial process among the options. *Ketogenesis* - **Ketogenesis** does occur entirely in the **mitochondrial matrix** of liver cells during fasting or low carbohydrate intake. - While mitochondrial, it is tissue-specific (primarily liver) and metabolically conditional (occurs during fasting states). - It involves synthesis of **ketone bodies** (acetoacetate, β-hydroxybutyrate) from acetyl-CoA. *Urea cycle* - The **urea cycle** is compartmentalized between the **mitochondrial matrix** and **cytosol** of liver cells. - First two steps (carbamoyl phosphate synthetase I and ornithine transcarbamylase) occur in mitochondria. - Remaining steps occur in cytosol, so it is NOT primarily mitochondrial. - Functions to detoxify **ammonia** by converting it to urea. *Steroid synthesis* - **Steroid synthesis** primarily occurs in the **smooth endoplasmic reticulum**. - Only specific steps (e.g., cholesterol side-chain cleavage by CYP11A1) occur in mitochondria. - Most of the steroidogenic pathway is extra-mitochondrial.

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

α-ketoglutarate. ***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.

Q: Which of the following is the most reactive free radical?

Hydroxyl radical. ***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.

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

2.5 ATP. ***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.

Q: The mechanism of action of uncouplers of oxidative phosphorylation involves:

Disruption of proton gradient across the inner membrane. ***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.

Q: Which metabolic pathway is least active during 12 days of fasting?

Glycogenolysis. ***Correct: Glycogenolysis*** - **Glycogenolysis**, the breakdown of glycogen stores, is very active during the **initial hours of fasting** (first 24-48 hours) to maintain blood glucose levels. - However, after **12 days of fasting**, liver and muscle **glycogen stores are completely depleted**, making this pathway **essentially inactive** or the least active of all the metabolic pathways. - Once glycogen is exhausted, this pathway cannot contribute further to energy metabolism. *Incorrect: Gluconeogenesis* - This pathway becomes **increasingly active** during prolonged fasting to **synthesize new glucose** from non-carbohydrate precursors (amino acids, lactate, glycerol). - Essential for maintaining blood glucose for **glucose-dependent tissues** like red blood cells and parts of the brain that haven't fully adapted to ketones. - Remains a **crucial and active pathway** throughout prolonged fasting. *Incorrect: Ketogenesis* - **Ketogenesis** is **highly active** during prolonged fasting, producing **ketone bodies** (acetoacetate, β-hydroxybutyrate) from fatty acids in the liver. - Provides the **primary alternative fuel** for the brain (up to 70% of brain energy needs) and other tissues. - This is a **key metabolic adaptation** to preserve protein and glucose during starvation. *Incorrect: Lipolysis* - **Lipolysis** (breakdown of triglycerides into fatty acids and glycerol) is **highly active** during fasting to mobilize stored energy. - Provides **fatty acids** for direct oxidation by most tissues and **glycerol** as a gluconeogenic substrate. - A **fundamental process** for energy supply during nutrient deprivation.

Question 1

Which coenzyme serves as the primary electron acceptor in multiple dehydrogenase reactions of the Kreb's cycle?

Accepted Answer

NAD

Answer

NADP

Answer

NADPH

Answer

NADH

Question 2

Which molecule acts as the primary electron carrier in the Krebs cycle?

Accepted Answer

NADH

Answer

NAD

Answer

FADH₂

Answer

NADPH

Question 3

Among the following enzymes, which one produces NADH in the citric acid cycle?

Accepted Answer

Isocitrate dehydrogenase

Answer

Succinate thiokinase

Answer

Succinate dehydrogenase

Answer

Fumarase

Question 4

What is the theoretical yield of ATP generated in one TCA cycle?

Accepted Answer

10

Answer

2

Answer

8

Answer

11

Question 5

Which of the following primarily occurs in the mitochondria?

Accepted Answer

ETC

Answer

Ketogenesis

Answer

Urea cycle

Answer

Steroid synthesis

Question 6

Hyperammonaemia inhibits the TCA cycle by depleting which of the following?

Accepted Answer

α-ketoglutarate

Answer

succinate

Answer

malate

Answer

fumarate

Question 7

Which of the following is the most reactive free radical?

Accepted Answer

Hydroxyl radical

Answer

Alkyl radical

Answer

Superoxide radical

Answer

Peroxide radical

Question 8

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

Accepted Answer

2.5 ATP

Answer

1 ATP

Answer

3 ATP

Answer

2 ATP

Question 9

The mechanism of action of uncouplers of oxidative phosphorylation involves:

Accepted Answer

Disruption of proton gradient across the inner membrane

Answer

Inhibition of ATP synthase

Answer

Stimulation of ATP synthase

Answer

Blocking electron transport chain complexes

Question 10

Which metabolic pathway is least active during 12 days of fasting?

Accepted Answer

Glycogenolysis

Answer

Gluconeogenesis

Answer

Ketogenesis

Answer

Lipolysis

Energy Production and Metabolism — MCQs

Energy Production and Metabolism — MCQs

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