Energy Production and Metabolism Practice Questions

Q: Ketone body formation without glycosuria is seen in ?

Starvation. ***Starvation*** - During **starvation**, the body depletes its **glycogen stores** and begins to break down **fat for energy**. This process leads to the production of **ketone bodies** (acetoacetate, beta-hydroxybutyrate, and acetone) as an alternative fuel source for the brain and other tissues. - Since there is no underlying problem with **insulin production** or action, blood glucose levels are typically low or normal, and therefore, **glycosuria** (glucose in the urine) is absent. *Diabetes mellitus* - In **uncontrolled diabetes mellitus**, especially Type 1, the body cannot effectively use **glucose** due to lack of insulin, leading to high blood glucose levels (**hyperglycemia**) and subsequently **glycosuria**. - The body then compensates by breaking down **fats**, leading to the formation of **ketone bodies** (**diabetic ketoacidosis**), which results in both **ketonuria** and **glycosuria**. *Diabetes insipidus* - **Diabetes insipidus** is a condition characterized by the inability to conserve water due to insufficient **antidiuretic hormone (ADH)** production or action, leading to excessive urination and thirst. - It does not involve abnormalities in **glucose metabolism** or **ketone body production** and therefore does not typically present with ketonuria or glycosuria. *Obesity* - While **obesity** can lead to **insulin resistance** and is a risk factor for Type 2 Diabetes, it does not directly cause **ketone body formation** in the absence of metabolic derangements such as those seen in uncontrolled diabetes or prolonged starvation. - In most cases of obesity without diabetes, **glucose metabolism** is still adequate enough to prevent significant reliance on **fat breakdown** for energy, meaning there is usually no ketonuria or glycosuria.

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: 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: 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 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: 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 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: 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

Ketone body formation without glycosuria is seen in ?

Accepted Answer

Starvation

Answer

Diabetes mellitus

Answer

Diabetes insipidus

Answer

Obesity

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

NADH via glycerophosphate shunt makes how many ATP?

Accepted Answer

2

Answer

1

Answer

4

Answer

3

Question 4

Anaplerotic reaction is catalyzed by?

Accepted Answer

Pyruvate carboxylase

Answer

Enolase

Answer

Pyruvate kinase

Answer

G6PD

Question 5

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 6

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 7

In ETC NADH generates -

Accepted Answer

3 ATPs

Answer

1 ATPs

Answer

4 ATPs

Answer

2 ATPs

Question 8

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 9

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

Accepted Answer

α-ketoglutarate dehydrogenase

Answer

Fumarase

Answer

Aconitase

Answer

Thiokinase

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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