Energy Production and Metabolism Practice Questions

Q: Major source of energy for brain in fasting/starvation?

Ketone bodies. ***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.

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

α-ketoglutarate dehydrogenase. **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.

Q: Which enzyme in the TCA cycle catalyzes the step where substrate-level phosphorylation occurs?

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 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.

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

Metabolites are utilized in other pathways.. ***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.

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: Which of the following processes does not occur in mitochondria?

Glycogenolysis. ***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.

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

Vitamin B3 (Niacin). ***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.

Question 1

Major source of energy for brain in fasting/starvation?

Accepted Answer

Ketone bodies

Answer

Glucose

Answer

Glycogen

Answer

Fatty acids

Question 2

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 3

Which enzyme catalyzes the rate limiting step in the TCA cycle?

Accepted Answer

α-ketoglutarate dehydrogenase

Answer

Fumarase

Answer

Aconitase

Answer

Thiokinase

Question 4

Which enzyme in the TCA cycle catalyzes the step where substrate-level phosphorylation occurs?

Accepted Answer

Succinate thiokinase

Answer

Isocitrate dehydrogenase

Answer

Malate dehydrogenase

Answer

Aconitase

Question 5

Why is the citric acid cycle called an amphibolic pathway?

Accepted Answer

Metabolites are utilized in other pathways.

Answer

Both exergonic and endergonic reactions take place

Answer

It can proceed in both forward and backward directions.

Answer

The same enzymes can be used in reverse directions.

Question 6

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 7

Which of the following processes does not occur in mitochondria?

Accepted Answer

Glycogenolysis

Answer

Fatty acid oxidation

Answer

Electron transport chain

Answer

Citric acid cycle (Kreb's cycle)

Question 8

Anaplerotic reaction is catalyzed by?

Accepted Answer

Pyruvate carboxylase

Answer

Enolase

Answer

Pyruvate kinase

Answer

G6PD

Question 9

In ETC NADH generates -

Accepted Answer

3 ATPs

Answer

1 ATPs

Answer

4 ATPs

Answer

2 ATPs

Question 10

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

Accepted Answer

Vitamin B3 (Niacin)

Answer

Vitamin B1 (Thiamine)

Answer

Vitamin B6 (Pyridoxine)

Answer

Vitamin B2 (Riboflavin)

Energy Production and Metabolism — MCQs

Energy Production and Metabolism — MCQs

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