Metabolic Regulation: Substrate Availability — MCQs
Which of the following is NOT required for gluconeogenesis from lactate?
Which of the following statements about gluconeogenesis is correct?
Which metabolic pathway provides instant energy to muscles?
Which of the following is not a substrate for gluconeogenesis?
What is the diagnosis in a patient who presents with nausea and vomiting, initially responds to intravenous glucose, but later develops increased blood glutamine and orotic acid levels?
Which of the following is the primary tissue dependent on insulin for glucose uptake?
Which of the following is not utilized in the process of gluconeogenesis?
Which hormone is known to repress the biosynthesis of the enzyme pyruvate carboxylase?
Which of these is an example of anaplerotic reaction?
Which enzyme in the Krebs cycle is indirectly affected by hyperammonemia due to its impact on metabolic pathways?
Metabolic Regulation: Substrate Availability Indian Medical PG Practice Questions and MCQs
Question 1: Which of the following is NOT required for gluconeogenesis from lactate?
- A. Transamination of pyruvate to alanine (Correct Answer)
- B. Transport of lactate from muscle to liver
- C. Conversion of lactate to pyruvate
- D. None of the above
Explanation: ***Transamination of pyruvate to alanine*** - While **alanine** can be a substrate for gluconeogenesis, **lactate** is directly converted to pyruvate, which then enters the gluconeogenesis pathway. **Transamination to alanine** is not a required intermediate step for lactate-derived glucose production. - The direct conversion of **lactate to pyruvate** by **lactate dehydrogenase** is the key initial step, not its conversion to alanine. *Transport of lactate from muscle to liver* - **Lactate** produced in muscles (e.g., during intense exercise) must be transported to the **liver** via the bloodstream to be used for **gluconeogenesis** in the **Cori cycle**. - This transport is essential for clearing lactate from the periphery and supplying the liver with a gluconeogenic precursor. *Conversion of lactate to pyruvate* - **Lactate dehydrogenase** catalyzes the reversible conversion of **lactate to pyruvate**, which is the critical first step in converting lactate into a gluconeogenic substrate. - This reaction regenerates **NAD+** (not NADH), which is necessary for glycolysis to continue in muscle tissue. *None of the above* - This option is incorrect because there IS a step listed above that is not required: **transamination of pyruvate to alanine** is indeed not necessary for gluconeogenesis from lactate, making Option A the correct answer to this "NOT required" question.
Question 2: Which of the following statements about gluconeogenesis is correct?
- A. Occurs mainly in the liver (Correct Answer)
- B. It uses exactly the same enzymes as glycolysis in reverse
- C. It only occurs during fed state when insulin levels are high
- D. Fatty acids are the primary substrate for gluconeogenesis
Explanation: ***Occurs mainly in the liver*** - The **liver** is the primary site for **gluconeogenesis**, responsible for maintaining blood glucose levels during fasting. - The kidneys also contribute, especially during prolonged fasting, but to a lesser extent. *It uses exactly the same enzymes as glycolysis in reverse* - While gluconeogenesis shares some enzymes with glycolysis, there are **three irreversible steps in glycolysis** that require different enzymes in gluconeogenesis to bypass them. - Key bypass enzymes include **pyruvate carboxylase**, **phosphoenolpyruvate carboxykinase (PEPCK)**, **fructose-1,6-bisphosphatase**, and **glucose-6-phosphatase**. *It only occurs during fed state when insulin levels are high* - **Gluconeogenesis is activated during fasting or starvation** when blood glucose levels are low, and it is largely **inhibited by high insulin levels**. - Its purpose is to produce new glucose to prevent hypoglycemia, not to store excess glucose. *Fatty acids are the primary substrate for gluconeogenesis* - **Fatty acids cannot be directly converted to glucose** in significant amounts in humans because they are broken down into acetyl-CoA, which cannot be used for net glucose synthesis. - Primary substrates include **lactate**, **amino acids** (from protein breakdown), and **glycerol** (from triglyceride breakdown).
Question 3: Which metabolic pathway provides instant energy to muscles?
- A. Embden-Meyerhof pathway (Correct Answer)
- B. HMP shunt
- C. Cori cycle
- D. TCA cycle
Explanation: ***Embden-Meyerhof pathway*** - This pathway, also known as **glycolysis**, rapidly breaks down glucose into pyruvate to produce **ATP without oxygen**, providing instant energy to muscles during high-intensity activity. - Generates a net of **two ATP molecules** per glucose molecule, which is crucial for quick bursts of energy. *HMP shunt* - The **hexose monophosphate shunt** primarily produces **NADPH** for reductive biosynthesis and **ribose-5-phosphate** for nucleotide synthesis, not immediate large-scale ATP for muscle contraction. - Plays a role in protecting cells from **oxidative stress** and synthesizing precursors for DNA and RNA. *Cori cycle* - The **Cori cycle** involves the recycling of **lactate** produced in muscles back to glucose in the liver, which is a slower process for maintaining glucose homeostasis rather than providing instant muscle energy. - It helps prevent **lactic acidosis** during strenuous activity but is not a primary pathway for rapid ATP generation. *TCA cycle* - The **TCA cycle (Krebs cycle)** is part of **aerobic respiration** and produces a significant amount of ATP, but it is a slower, more sustained energy production pathway that requires oxygen. - Primarily active during **lower-intensity**, longer-duration activities when oxygen supply is adequate.
Question 4: 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 5: What is the diagnosis in a patient who presents with nausea and vomiting, initially responds to intravenous glucose, but later develops increased blood glutamine and orotic acid levels?
- A. CPS-I deficiency
- B. Arginino succinate synthetase deficiency
- C. CPS-II deficiency
- D. Ornithine transcarbamoylase deficiency (Correct Answer)
Explanation: ***Ornithine transcarbamoylase deficiency*** - **Ornithine transcarbamoylase (OTC) deficiency** is an X-linked urea cycle disorder that leads to the accumulation of **carbamoyl phosphate**. - This excess carbamoyl phosphate is shunted into pyrimidine synthesis, resulting in increased **orotic acid** and **glutamine** levels, and symptoms like nausea and vomiting due to hyperammonemia. *CPS-I deficiency* - **Carbamoyl phosphate synthetase I (CPS-I) deficiency** also causes hyperammonemia but does not involve elevated **orotic acid**, as the pathway leading to pyrimidine synthesis is not overstimulated. - This deficiency would present with high ammonia and glutamine levels, but **normal or low orotic acid**. *Arginino succinate synthetase deficiency* - **Argininosuccinate synthetase deficiency** (citrullinemia) is characterized by very high plasma **citrulline** levels, which are not mentioned in this patient's presentation. - While it is a urea cycle disorder causing hyperammonemia, the diagnostic marker of elevated citrulline differentiates it from OTC deficiency. *CPS-II deficiency* - **Carbamoyl phosphate synthetase II (CPS-II)** is involved in *de novo* pyrimidine synthesis and is not part of the urea cycle. - A deficiency in CPS-II would typically lead to **pyrimidine starvation** rather than hyperammonemia or elevated orotic acid.
Question 6: Which of the following is the primary tissue dependent on insulin for glucose uptake?
- A. Adipose tissue
- B. Brain
- C. Muscle (Correct Answer)
- D. Pancreas
Explanation: ***Muscle (Correct Answer)*** - **Skeletal muscle** is the **primary insulin-dependent tissue** for glucose uptake, accounting for approximately **80-90% of insulin-stimulated glucose disposal** in the postprandial state. - Insulin promotes the translocation of **GLUT4 transporters** to the cell membrane in muscle cells, enabling rapid and substantial glucose uptake. - This makes muscle the most quantitatively significant site for insulin-mediated glucose clearance from the bloodstream. *Adipose tissue (Incorrect)* - While **adipose tissue** does exhibit insulin-dependent glucose uptake via **GLUT4 transporters**, its contribution to overall glucose disposal is **much smaller** compared to skeletal muscle. - Glucose uptake in adipocytes is important for **lipogenesis** and triglyceride storage, but represents only a minor fraction of total body insulin-stimulated glucose uptake. *Brain (Incorrect)* - The **brain** has a constant, high glucose requirement but utilizes **insulin-independent** glucose uptake mechanisms, primarily through **GLUT1 and GLUT3 transporters**. - Glucose uptake in the brain is regulated by **blood glucose concentration gradients**, not by insulin signaling. - This ensures continuous glucose supply to the brain regardless of insulin levels. *Pancreas (Incorrect)* - The **pancreas**, particularly beta cells, employs **insulin-independent** glucose uptake via **GLUT1 and GLUT2 transporters**. - These transporters function as **glucose sensors**, allowing beta cells to detect blood glucose levels and regulate insulin secretion accordingly. - The pancreas produces insulin but does not depend on insulin for its own glucose uptake.
Question 7: Which of the following is not utilized in the process of gluconeogenesis?
- A. Succinate
- B. Oleate (Correct Answer)
- C. Glutamate
- D. Aspartate
Explanation: ***Oleate*** - **Oleate is a fatty acid** and cannot be used for gluconeogenesis in humans because its breakdown product, **acetyl-CoA**, cannot be converted back to pyruvate. - The conversion of **acetyl-CoA** to pyruvate or oxaloacetate is not possible in mammals, as this would require the **glyoxylate cycle**, which is absent in humans. *Succinate* - **Succinate is an intermediate of the citric acid cycle** and can be converted to oxaloacetate, a direct precursor for gluconeogenesis. - As a **glucogenic substrate**, succinate can contribute to glucose synthesis. *Glutamate* - **Glutamate is an amino acid** that can be deaminated to **α-ketoglutarate**, an intermediate of the citric acid cycle. - **α-ketoglutarate** can then be converted to oxaloacetate and subsequently to glucose via gluconeogenesis. *Aspartate* - **Aspartate is an amino acid** that can be converted to **oxaloacetate**, a key intermediate in gluconeogenesis. - Its carbon skeleton can directly enter the gluconeogenic pathway.
Question 8: Which hormone is known to repress the biosynthesis of the enzyme pyruvate carboxylase?
- A. Cortisol
- B. Glucagon
- C. Insulin (Correct Answer)
- D. Growth hormone
Explanation: ***Insulin*** - **Insulin** is an anabolic hormone that promotes glucose utilization and opposes **gluconeogenesis**. - While insulin does inhibit hepatic glucose production, it primarily acts by **repressing PEPCK (phosphoenolpyruvate carboxykinase)**, the rate-limiting enzyme of gluconeogenesis, rather than directly repressing pyruvate carboxylase biosynthesis. - **Note:** Modern biochemistry emphasizes that insulin's main transcriptional target in gluconeogenesis is **PEPCK**, not pyruvate carboxylase. However, this was the expected answer for **NEET-2012**, reflecting the understanding at that time. - Insulin also promotes dephosphorylation and inactivation of gluconeogenic enzymes and enhances glucose uptake and glycolysis. *Glucagon* - **Glucagon** is a catabolic hormone that **activates** enzymes involved in **gluconeogenesis** and glycogenolysis to raise blood glucose levels. - It would **increase**, not repress, the biosynthesis and activity of gluconeogenic enzymes including **pyruvate carboxylase**. *Cortisol* - **Cortisol** is a glucocorticoid hormone that **stimulates gluconeogenesis** in the liver as part of the stress response. - It typically **upregulates** the synthesis and activity of gluconeogenic enzymes like **pyruvate carboxylase** and **PEPCK**. *Growth hormone* - **Growth hormone** generally **increases insulin resistance** and can have a **diabetogenic effect**, promoting glucose production rather than repressing gluconeogenic enzymes. - It does not directly repress gluconeogenic enzyme biosynthesis; its metabolic effects favor lipolysis and protein synthesis.
Question 9: 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 10: Which enzyme in the Krebs cycle is indirectly affected by hyperammonemia due to its impact on metabolic pathways?
- A. Alpha-Ketoglutarate dehydrogenase (Correct Answer)
- B. Isocitrate dehydrogenase
- C. Succinate dehydrogenase
- D. Malate dehydrogenase
Explanation: ***Alpha-Ketoglutarate dehydrogenase*** - Hyperammonemia leads to the conversion of **alpha-ketoglutarate** into **glutamate** by glutamate dehydrogenase, which then uses ammonia to form **glutamine**. - This depletion of **alpha-ketoglutarate**, a substrate for alpha-ketoglutarate dehydrogenase, indirectly inhibits the enzyme's activity and thus the Krebs cycle. *Isocitrate dehydrogenase* - This enzyme is regulated by factors like **ATP**, **NADH**, and **ADP**, but not directly by ammonia or a substrate depletion caused by hyperammonemia. - Its activity is crucial for the cycle but not the primary or most direct target of ammonia's metabolic effects. *Succinate dehydrogenase* - This enzyme is part of both the **Krebs cycle** and the **electron transport chain**, but its activity is not directly or indirectly affected by ammonia detoxification pathways. - Its regulation is primarily linked to **FADH2** production and the electron transport chain. *Malate dehydrogenase* - This enzyme converts **malate** to **oxaloacetate** and is not directly impacted by the metabolic shunting of **alpha-ketoglutarate** due to hyperammonemia. - Its activity is critical for regenerating **oxaloacetate** to continue the cycle.
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