Energy Production and Metabolism Practice Questions

Q: Which is the primary energy molecule that gives approximately 7.3 kcal/mol?

ATP. ***ATP*** - **Adenosine triphosphate (ATP)** is the primary energy currency of the cell, providing approximately **7.3 kcal/mol** upon hydrolysis of its terminal phosphate group. - This energy is released when ATP is converted to **ADP (adenosine diphosphate)** and an inorganic phosphate (Pi), driving various cellular processes. *GTP* - **Guanosine triphosphate (GTP)** is another nucleotide triphosphate that carries energy, but it is primarily involved in specific processes like **protein synthesis** and **signal transduction**, not as the ubiquitous primary energy molecule like ATP. - While it also releases energy upon hydrolysis, its standard free energy change is similar to ATP but it's not the main universal energy carrier. *Glucose-6-phosphate* - **Glucose-6-phosphate** is an important intermediate in **glycolysis** and **gluconeogenesis**, but it is not an energy-storing molecule in the same way as ATP. - Its high-energy phosphate bond is used in metabolic pathways, but it doesn't directly release 7.3 kcal/mol as a direct energy source for cellular work. *Creatine phosphate* - **Creatine phosphate** serves as an energy reserve in muscle and nerve cells, rapidly generating ATP from ADP during periods of intense activity. - While it is a high-energy phosphate compound, it functions to **replenish ATP** rather than being the direct energy molecule that performs cellular work.

Q: Mechanism of cyanide poisoning is by inhibiting: NEET 2013

Cytochrome oxidase. ***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.

Q: In the electron transport chain (ETC), which enzyme does cyanide inhibit?

Cytochrome c oxidase (Complex IV). ***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.

Q: Which enzyme is involved in substrate level phosphorylation?

Creatine kinase. ***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.

Q: NADH via glycerophosphate shunt makes how many ATP?

2. ***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.

Q: Which enzyme primarily initiates the electron transport process in oxidative phosphorylation?

NADH dehydrogenase. ***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.

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: 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: 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.

Question 1

Which is the primary energy molecule that gives approximately 7.3 kcal/mol?

Accepted Answer

ATP

Answer

GTP

Answer

Glucose-6-phosphate

Answer

Creatine phosphate

Question 2

Mechanism of cyanide poisoning is by inhibiting: NEET 2013

Accepted Answer

Cytochrome oxidase

Answer

DNA synthesis

Answer

Protein breakdown

Answer

Protein synthesis

Question 3

In the electron transport chain (ETC), which enzyme does cyanide inhibit?

Accepted Answer

Cytochrome c oxidase (Complex IV)

Answer

Complex II (Succinate dehydrogenase)

Answer

Complex I (NADH dehydrogenase)

Answer

Complex III (Cytochrome bc1 complex)

Question 4

Which enzyme is involved in substrate level phosphorylation?

Accepted Answer

Creatine kinase

Answer

Enolase

Answer

Aldolase

Answer

Lactate dehydrogenase

Question 5

NADH via glycerophosphate shunt makes how many ATP?

Accepted Answer

2

Answer

1

Answer

4

Answer

3

Question 6

Which enzyme primarily initiates the electron transport process in oxidative phosphorylation?

Accepted Answer

NADH dehydrogenase

Answer

Pyruvate kinase

Answer

Succinyl CoA thiokinase

Answer

ATP synthase

Question 7

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

Accepted Answer

α-ketoglutarate dehydrogenase

Answer

Fumarase

Answer

Aconitase

Answer

Thiokinase

Question 8

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 9

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 10

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.

Energy Production and Metabolism — MCQs

Energy Production and Metabolism — MCQs

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