Energy Yield from Nutrients — MCQs
Which amino acid can be utilized in both gluconeogenesis and ketogenesis?
Which two enzymes are required for the beta oxidation of polyunsaturated fatty acids (PUFA)?
Which of these is an example of anaplerotic reaction?
Which is the primary energy molecule that gives approximately 7.3 kcal/mol?
Which of the following is not a substrate for gluconeogenesis?
Which of the following is not a substrate for glucose formation?
What is the effect of insulin administration on the respiratory quotient (RQ) in diabetes?
In beta-oxidation of fatty acids, carnitine is required for:
Which of the following statements BEST describes the net ATP production in glycolysis?
During a 100 m sprint which of the following is used by the muscle for meeting energy demands?
Energy Yield from Nutrients Indian Medical PG Practice Questions and MCQs
Question 1: Which amino acid can be utilized in both gluconeogenesis and ketogenesis?
- A. Leucine
- B. Valine
- C. Arginine
- D. Tyrosine (Correct Answer)
Explanation: ***Tyrosine (Correct Answer)*** - Tyrosine is **both glucogenic and ketogenic**, making it the correct answer. - It is **glucogenic** because its metabolism yields **fumarate**, which can enter the TCA cycle and contribute to **gluconeogenesis**. - It is also **ketogenic** because its degradation produces **acetoacetate**, a **ketone body**. *Leucine* - Leucine is a purely **ketogenic** amino acid, meaning its catabolism only produces **acetyl-CoA** and **acetoacetate**. - It cannot be converted into glucose precursors and therefore does not contribute to gluconeogenesis. *Valine* - Valine is a purely **glucogenic** amino acid, meaning its metabolism produces **succinyl-CoA**. - Succinyl-CoA can be converted into **oxaloacetate** and then to glucose via gluconeogenesis, but it does not produce ketone bodies. *Arginine* - Arginine is a purely **glucogenic** amino acid, serving as a precursor for **α-ketoglutarate** in the TCA cycle. - This pathway allows its carbon skeleton to be diverted into glucose production, but it does not yield ketone bodies.
Question 2: Which two enzymes are required for the beta oxidation of polyunsaturated fatty acids (PUFA)?
- A. Dienoyl CoA isomerase and Enoyl CoA isomerase
- B. Dienoyl CoA isomerase and 2,4 Dienoyl CoA reductase
- C. Enoyl CoA isomerase and Enoyl CoA reductase
- D. Enoyl CoA isomerase and 2,4 Dienoyl CoA reductase (Correct Answer)
Explanation: **Enoyl CoA isomerase and 2,4 Dienoyl CoA reductase** - **Enoyl CoA isomerase** is necessary to convert *cis* double bonds to *trans* double bonds at the 3,4 position, which allows the beta-oxidation enzymes to continue processing the fatty acid. - **2,4 Dienoyl CoA reductase** is required to reduce *cis-2, cis-4* or *trans-2, cis-4* dienoyl intermediates into a single *trans-3* enoyl CoA, which can then be isomerized by enoyl CoA isomerase. *Dienoyl CoA isomerase and Enoyl CoA isomerase* - This option is incorrect because **Dienoyl CoA isomerase** is not a commonly recognized single enzyme directly involved in PUFA beta-oxidation in the way described. The key is to reduce a diene, which reductase does. - While **Enoyl CoA isomerase** is crucial, pairing it with another isomerase in this context does not fully address the reduction step needed for certain PUFAs. *Dienoyl CoA isomerase and 2,4 Dienoyl CoA reductase* - This option incorrectly names **Dienoyl CoA isomerase** as one of the two main required enzymes. A 2,4 Dienoyl CoA reductase does exist. - While **2,4 Dienoyl CoA reductase** is essential, the other enzyme should be Enoyl CoA isomerase to handle the initial *cis* to *trans* isomerizations. *Enoyl CoA isomerase and Enoyl CoA reductase* - This option is incorrect because **Enoyl CoA reductase** without the "2,4" prefix generally refers to the enzyme involved in fatty acid synthesis, not beta-oxidation of PUFAs. - **Enoyl CoA isomerase** is correctly identified, but the other enzyme specifically for PUFA oxidation is the **2,4 Dienoyl CoA reductase**.
Question 3: Which of these is an example of anaplerotic reaction?
- A. Pyruvate to acetaldehyde
- B. Pyruvate to lactic acid
- C. Pyruvate to acetyl-CoA
- D. Pyruvate to oxaloacetate (Correct Answer)
Explanation: ***Pyruvate to oxaloacetate*** - This reaction, catalyzed by **pyruvate carboxylase**, replenishes intermediates of the **TCA cycle (Krebs cycle)**. - **Oxaloacetate** is a key intermediate that combines with acetyl-CoA to initiate the TCA cycle, thus anaplerotic reactions ensure the cycle can continue. *Pyruvate to acetaldehyde* - This conversion occurs in alcoholic fermentation, primarily in yeast, and is not an anaplerotic reaction in human metabolism. - It involves the enzyme **pyruvate decarboxylase** and produces **carbon dioxide** as a byproduct. *Pyruvate to lactic acid* - This is an anaerobic pathway for pyruvate metabolism, catalyzed by **lactate dehydrogenase**, which regenerates NAD+ for glycolysis. - It does not directly replenish intermediates of the **TCA cycle**. *Pyruvate to acetyl-CoA* - This reaction, catalyzed by the **pyruvate dehydrogenase complex**, links glycolysis to the TCA cycle by producing acetyl-CoA. - However, it consumes pyruvate and forms an entry point for the cycle, rather than replenishing existing intermediates.
Question 4: Which is the primary energy molecule that gives approximately 7.3 kcal/mol?
- A. ATP (Correct Answer)
- B. GTP
- C. Glucose-6-phosphate
- D. Creatine phosphate
Explanation: ***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.
Question 5: Which of the following is not a substrate for gluconeogenesis?
- A. Leucine (Correct Answer)
- B. Lactate
- C. Propionate
- D. Glycerol
Explanation: ***Leucine*** - **Leucine** is an exclusively **ketogenic amino acid**, meaning its breakdown products can only be converted into **ketone bodies** or fatty acids, not glucose. - It does not have a carbon skeleton that can be directly converted into **pyruvate** or **oxaloacetate**, which are key intermediates in gluconeogenesis. *Lactate* - **Lactate** is a major substrate for gluconeogenesis, particularly during exercise or fasting. - It is converted to **pyruvate** by **lactate dehydrogenase**, and pyruvate can then enter the gluconeogenic pathway. *Propionate* - **Propionate** is a fatty acid with an odd number of carbon atoms, primarily derived from the catabolism of odd-chain fatty acids or from bacterial fermentation in the colon. - It can be converted into **succinyl CoA**, an intermediate of the citric acid cycle, which can then be used for gluconeogenesis. *Glycerol* - **Glycerol**, released during the breakdown of triglycerides, is an important substrate for gluconeogenesis. - It is phosphorylated to **glycerol-3-phosphate**, which is then oxidized to **dihydroxyacetone phosphate (DHAP)**, an intermediate in glycolysis and gluconeogenesis.
Question 6: Which of the following is not a substrate for glucose formation?
- A. Lactate
- B. Glycerol
- C. Alanine
- D. Acetyl coenzyme A (Correct Answer)
Explanation: ***Acetyl coenzyme A*** - **Acetyl CoA** cannot be converted to glucose because the two carbons from the acetyl group are lost as carbon dioxide in the **Krebs cycle**, making a net synthesis of glucose impossible. - The irreversible nature of the **pyruvate dehydrogenase complex** prevents the conversion of Acetyl CoA back to **pyruvate**, which is a crucial step for gluconeogenesis. *Lactate* - **Lactate** is a major substrate for gluconeogenesis, particularly during exercise and fasting, via the **Cori cycle**. - **Lactate dehydrogenase** converts lactate to **pyruvate**, which can then enter the gluconeogenic pathway. *Glycerol* - **Glycerol**, derived from triglyceride breakdown, enters gluconeogenesis by being converted to **glycerol-3-phosphate** and then to **dihydroxyacetone phosphate (DHAP)**. - DHAP is an intermediate in glycolysis and gluconeogenesis, allowing for its conversion to glucose. *Alanine* - **Alanine** is a **glucogenic amino acid** that can be transaminated to **pyruvate**. - **Pyruvate** can then proceed through the gluconeogenic pathway to synthesize glucose, especially during prolonged fasting.
Question 7: What is the effect of insulin administration on the respiratory quotient (RQ) in diabetes?
- A. RQ remains unchanged in Diabetes.
- B. RQ decreases in Diabetes and increases with insulin administration. (Correct Answer)
- C. In diabetes, RQ increases with insulin administration.
- D. RQ increases in Diabetes and decreases with insulin administration
Explanation: ***RQ decreases in Diabetes and increases with insulin administration.*** - In uncontrolled diabetes, the body primarily metabolizes **fats for energy** due to insufficient insulin, leading to a lower **Respiratory Quotient (RQ)** (closer to 0.7). - Insulin administration shifts metabolism towards **carbohydrate utilization**, which has an RQ of 1.0, thereby **increasing the overall RQ**. *RQ remains unchanged in Diabetes.* - This is incorrect because the metabolic state in diabetes, particularly the shift to **fat metabolism**, directly impacts the RQ. - The RQ in uncontrolled diabetes is typically **lower than normal**, not unchanged. *In diabetes, RQ increases with insulin administration.* - While this statement is partially correct (RQ does increase with insulin), it is **incomplete** as it fails to mention that RQ is **decreased in the untreated diabetic state**. - The complete picture requires understanding that **RQ decreases in diabetes** due to fat metabolism, and then **increases with insulin** as glucose utilization is restored. *RQ increases in Diabetes and decreases with insulin administration* - This is completely incorrect as **RQ decreases** (not increases) in untreated diabetes due to predominant fat metabolism. - Insulin administration leads to an **increase** (not decrease) in RQ as carbohydrate utilization is favored.
Question 8: In beta-oxidation of fatty acids, carnitine is required for:
- A. Transport of long chain fatty acid to mitochondrial inner layer (Correct Answer)
- B. Conversion of long chain fatty acids to short chain fatty acids
- C. Transport of long chain fatty acid across the outer mitochondrial membrane
- D. Conversion of short chain fatty acids to medium chain fatty acids
Explanation: ***Transport of long chain fatty acid to mitochondrial inner layer*** - **Carnitine** acts as a shuttle, transporting **long-chain fatty acids** from the cytosol across the **inner mitochondrial membrane** for beta-oxidation. - This process involves the enzyme **carnitine palmitoyltransferase I (CPT-I)** on the outer membrane and **CPT-II** on the inner membrane. - The carnitine shuttle system is essential because the inner mitochondrial membrane is impermeable to long-chain fatty acyl-CoA molecules. *Conversion of long chain fatty acids to short chain fatty acids* - **Carnitine** is not involved in the conversion or shortening of fatty acid chains. - The breakdown of long-chain fatty acids into shorter chains occurs *during* beta-oxidation, not as a function of carnitine transport. *Transport of long chain fatty acid across the outer mitochondrial membrane* - Long-chain fatty acids do not require carnitine to cross the **outer mitochondrial membrane**, which is freely permeable to fatty acyl-CoA. - Carnitine is specifically required for transport across the **inner mitochondrial membrane**, which is impermeable to fatty acyl-CoA. - CPT-I on the outer membrane surface and CPT-II on the inner membrane work together to facilitate this transport. *Conversion of short chain fatty acids to medium chain fatty acids* - **Carnitine** does not facilitate the elongation or interconversion of fatty acid chains. - These processes relate to fatty acid synthesis pathways, not transport for beta-oxidation.
Question 9: Which of the following statements BEST describes the net ATP production in glycolysis?
- A. Glycolysis produces 2 molecules of pyruvate
- B. Glycolysis produces a net gain of 2 ATP per glucose molecule (Correct Answer)
- C. Hexokinase consumes ATP during glycolysis
- D. Aldolase catalyzes the conversion of fructose-1,6-bisphosphate into two three-carbon molecules
Explanation: ***Glycolysis produces a net gain of 2 ATP per glucose molecule*** - In the initial "investment" phase of glycolysis, **2 ATP molecules are consumed** to phosphorylate glucose. - In the subsequent "payoff" phase, **4 ATP molecules are produced** through substrate-level phosphorylation, resulting in a net gain of 2 ATP. *Glycolysis produces 2 molecules of pyruvate* - While glycolysis does produce **2 molecules of pyruvate** from one glucose molecule, this statement describes the end product of the pathway, not the net ATP production. - Pyruvate is a crucial product that can be further metabolized in aerobic or anaerobic conditions, but it does not directly represent the energy yield in terms of ATP. *Hexokinase consumes ATP during glycolysis* - **Hexokinase** is indeed the enzyme that catalyzes the first ATP-consuming step in glycolysis, phosphorylating glucose to glucose-6-phosphate. - However, this statement describes only one aspect of ATP utilization within the pathway and does not account for the total ATP produced or the overall net gain. *Aldolase catalyzes the conversion of fructose-1,6-bisphosphate into two three-carbon molecules* - **Aldolase** is a key enzyme in glycolysis responsible for cleaving **fructose-1,6-bisphosphate** into dihydroxyacetone phosphate and glyceraldehyde-3-phosphate. - This step is part of the preparatory phase of glycolysis but does not directly describe the net ATP production.
Question 10: During a 100 m sprint which of the following is used by the muscle for meeting energy demands?
- A. Phosphofructokinase
- B. Phosphocreatine (Correct Answer)
- C. Glucose 1 - phosphate
- D. Creatine phosphokinase
Explanation: ***Phosphocreatine*** - **Phosphocreatine (PCr)** is the primary energy source for a **100m sprint** (lasting 10-20 seconds). - The **ATP-PC (phosphagen) system** provides **immediate energy** by rapidly regenerating **ATP** from ADP through the transfer of a high-energy phosphate group. - This system is crucial for **short bursts of maximal intensity exercise** where energy demand exceeds the capacity of glycolysis and oxidative phosphorylation to respond quickly enough. - Phosphocreatine stores can fuel maximum effort for approximately **10-15 seconds**, making it ideal for sprint activities. *Phosphofructokinase* - **Phosphofructokinase (PFK)** is a key regulatory enzyme in **glycolysis**, not an energy substrate. - While PFK-catalyzed glycolysis contributes ATP during intense exercise, it cannot provide energy as rapidly as the phosphocreatine system. - Glycolysis becomes more prominent after the first 10-15 seconds of maximal effort. *Glucose 1-phosphate* - **Glucose 1-phosphate** is an intermediate in **glycogenolysis** (breakdown of glycogen to glucose-6-phosphate). - It is part of the pathway leading to glucose availability for glycolysis, but is not a **direct, immediate energy source** for muscle contraction. - Unlike phosphocreatine, it cannot directly regenerate ATP. *Creatine phosphokinase* - **Creatine phosphokinase (CPK)**, also known as **creatine kinase (CK)**, is the **enzyme** that catalyzes the reversible transfer of phosphate from phosphocreatine to ADP. - It facilitates the energy transfer reaction but is **not an energy substrate** itself. - The enzyme enables the phosphocreatine system to function, but the actual energy comes from phosphocreatine.
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