Gluconeogenesis: Reactions and Regulation Indian Medical PG Practice Questions and MCQs
Practice Indian Medical PG questions for Gluconeogenesis: Reactions and Regulation. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Gluconeogenesis: Reactions and Regulation Indian Medical PG Question 1: Which pathway is primarily affected in glucose-6-phosphate dehydrogenase deficiency?
- A. Gluconeogenesis
- B. Glycolysis
- C. Pentose phosphate pathway (Correct Answer)
- D. Beta-oxidation
Gluconeogenesis: Reactions and Regulation Explanation: ***Correct: Pentose phosphate pathway***
- Glucose-6-phosphate dehydrogenase (G6PD) is the **rate-limiting enzyme** of the pentose phosphate pathway (PPP)
- G6PD deficiency leads to impaired **NADPH production**, which is critical for maintaining reduced glutathione
- Reduced glutathione protects red blood cells from **oxidative damage**
- Deficiency results in **hemolytic anemia** when exposed to oxidative stressors (infections, certain drugs, fava beans)
*Incorrect: Gluconeogenesis*
- This pathway synthesizes **glucose from non-carbohydrate precursors** (primarily in liver and kidney)
- G6PD deficiency does not affect the enzymes or substrates involved in glucose synthesis
- Gluconeogenesis uses different enzymes (glucose-6-phosphatase, fructose-1,6-bisphosphatase, etc.)
*Incorrect: Glycolysis*
- Glycolysis is the **metabolic pathway that breaks down glucose** into pyruvate to generate ATP
- While glucose-6-phosphate is a substrate for both glycolysis and PPP, G6PD is **not involved in glycolysis**
- G6PD deficiency specifically impacts the PPP branch, not the glycolytic enzymes
*Incorrect: Beta-oxidation*
- This process involves the **breakdown of fatty acids** into acetyl-CoA for energy production
- Beta-oxidation is a **mitochondrial process** unrelated to G6PD function
- The pentose phosphate pathway occurs in the cytoplasm and involves carbohydrate metabolism
Gluconeogenesis: Reactions and Regulation Indian Medical PG Question 2: Which of the following statements correctly describes the effect of insulin and glucagon on gluconeogenesis?
- A. Glucagon decreases fructose 2,6-bisphosphate levels, stimulating gluconeogenesis. (Correct Answer)
- B. Insulin increases the levels of fructose 2,6-bisphosphate, which inhibits gluconeogenesis.
- C. Insulin acts through a kinase to promote glycolysis.
- D. Fructose 2,6-bisphosphate is an activator of glycolysis.
Gluconeogenesis: Reactions and Regulation Explanation: ***Glucagon decreases fructose 2,6-bisphosphate levels, stimulating gluconeogenesis.***
- **Glucagon** activates **cAMP-dependent protein kinase (PKA)**, which phosphorylates the bifunctional enzyme **PFK-2/FBPase-2**.
- Phosphorylation activates the **fructose-2,6-bisphosphatase (FBPase-2)** activity, which breaks down **fructose 2,6-bisphosphate (F-2,6-BP)**.
- Decreased **F-2,6-BP** removes the inhibition of **fructose-1,6-bisphosphatase**, a key regulatory enzyme in gluconeogenesis, thereby **stimulating gluconeogenesis**.
- This is the primary mechanism by which glucagon promotes glucose production during fasting states.
*Insulin increases the levels of fructose 2,6-bisphosphate, which inhibits gluconeogenesis.*
- While this statement is biochemically accurate, **insulin's primary role is to inhibit gluconeogenesis**, not stimulate it.
- Insulin activates the **kinase activity (PFK-2)** of the bifunctional enzyme, increasing **F-2,6-BP** levels.
- Elevated **F-2,6-BP** inhibits **fructose-1,6-bisphosphatase**, thereby inhibiting gluconeogenesis.
- However, the question asks about effects on gluconeogenesis, and **glucagon's stimulatory effect is more directly relevant** to understanding gluconeogenesis regulation.
*Fructose 2,6-bisphosphate is an activator of glycolysis.*
- This statement is true but incomplete in the context of the question.
- **F-2,6-BP** is a potent allosteric activator of **phosphofructokinase-1 (PFK-1)**, the rate-limiting enzyme of glycolysis.
- However, this option doesn't directly address the hormonal regulation of **gluconeogenesis** as requested in the question stem.
*Insulin acts through a kinase to promote glycolysis.*
- While insulin does activate various kinases (e.g., **Akt/PKB**) that promote glycolysis, this statement is too vague.
- The question specifically asks about effects on **gluconeogenesis**, not glycolysis.
- Insulin's effect on gluconeogenesis is through inhibition (via increased F-2,6-BP levels), which is not clearly stated in this option.
Gluconeogenesis: Reactions and Regulation Indian Medical PG Question 3: Which of the following is not a substrate for gluconeogenesis?
- A. Leucine (Correct Answer)
- B. Lactate
- C. Propionate
- D. Glycerol
Gluconeogenesis: Reactions and Regulation 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.
Gluconeogenesis: Reactions and Regulation Indian Medical PG Question 4: Enzyme activated by decrease in Insulin: glucagon ratio:
- A. PFK
- B. Glucose 6 phosphatase (Correct Answer)
- C. Glucokinase
- D. Hexokinase
Gluconeogenesis: Reactions and Regulation Explanation: ***Glucose 6 phosphatase***
- A decreased **insulin:glucagon ratio** signifies a catabolic state, promoting glucose release into the blood.
- **Glucose-6-phosphatase** is the key enzyme in **gluconeogenesis** and **glycogenolysis** in the liver, dephosphorylating **glucose-6-phosphate** to **free glucose**, which can then be exported from the liver.
*PFK*
- **Phosphofructokinase (PFK)** is a key regulatory enzyme in **glycolysis**, which is inhibited in a state of low insulin:glucagon ratio.
- Its activity would decrease, not increase, to reduce glucose utilization.
*Glucokinase*
- **Glucokinase** phosphorylates glucose to **glucose-6-phosphate** in the liver, trapping it for metabolism; its activity is increased by high insulin levels.
- In a low insulin:glucagon ratio, its activity would be reduced to conserve glucose.
*Hexokinase*
- **Hexokinase** phosphorylates glucose in most peripheral tissues but has a lower Km for glucose than glucokinase, becoming saturated even at low glucose concentrations.
- Its activity is not primarily regulated by the insulin:glucagon ratio; it is generally involved in glucose uptake for cellular energy needs.
Gluconeogenesis: Reactions and Regulation Indian Medical PG Question 5: Which of the following is not a substrate for glucose formation?
- A. Lactate
- B. Glycerol
- C. Alanine
- D. Acetyl coenzyme A (Correct Answer)
Gluconeogenesis: Reactions and Regulation 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.
Gluconeogenesis: Reactions and Regulation Indian Medical PG Question 6: Which of the following is true about the synthesis of glucose from pyruvate by gluconeogenesis?
- A. Occurs exclusively in the cytosol.
- B. Is inhibited by an elevated level of glucagon
- C. Involves lactate as an intermediate
- D. Requires the participation of biotin (Correct Answer)
Gluconeogenesis: Reactions and Regulation Explanation: ***Requires the participation of biotin***
- **Biotin** is a required cofactor for **pyruvate carboxylase**, an enzyme that converts **pyruvate to oxaloacetate**, a crucial step in gluconeogenesis that bypasses the irreversible pyruvate kinase step.
- This carboxylation reaction is the first committed step in overcoming the irreversible steps of glycolysis in gluconeogenesis.
*Occurs exclusively in the cytosol.*
- Gluconeogenesis is a complex process that occurs in **multiple cellular compartments**.
- While many steps occur in the cytosol, the initial conversion of **pyruvate to oxaloacetate** by pyruvate carboxylase occurs in the **mitochondria**.
*Is inhibited by an elevated level of glucagon*
- **Glucagon** is a hormone that **stimulates gluconeogenesis**, not inhibits it.
- High glucagon levels signal a need for increased glucose production, especially during fasting or hypoglycemia.
*Involves lactate as an intermediate*
- While **lactate can be a precursor for gluconeogenesis**, it is not an intermediate in the direct synthesis of glucose from pyruvate.
- Lactate is converted to pyruvate, which then enters the gluconeogenic pathway.
Gluconeogenesis: Reactions and Regulation Indian Medical PG Question 7: Which amino acid can be utilized in both gluconeogenesis and ketogenesis?
- A. Leucine
- B. Valine
- C. Arginine
- D. Tyrosine (Correct Answer)
Gluconeogenesis: Reactions and Regulation 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.
Gluconeogenesis: Reactions and Regulation Indian Medical PG Question 8: Gluconeogenesis is inhibited by?
- A. Cholecystokinin
- B. 5-alpha reductase
- C. Insulin (Correct Answer)
- D. Glucagon
Gluconeogenesis: Reactions and Regulation Explanation: ***Insulin***
- **Insulin** is a key hormone released in response to high blood glucose, promoting glucose uptake and storage, and **inhibiting hepatic glucose production** through gluconeogenesis and glycogenolysis.
- It achieves this by decreasing the transcription and activity of key gluconeogenic enzymes like **phosphoenolpyruvate carboxykinase (PEPCK)** and **glucose-6-phosphatase**.
*Cholecystokinin*
- **Cholecystokinin (CCK)** is a gastrointestinal hormone primarily involved in digestion, stimulating bile release and pancreatic enzyme secretion.
- It does not directly regulate gluconeogenesis; its main role is related to **fat and protein digestion**.
*5-alpha reductase*
- **5-alpha reductase** is an enzyme involved in steroid metabolism, converting testosterone to the more potent androgen, dihydrotestosterone (DHT).
- This enzyme has no direct role in the regulation of **gluconeogenesis**.
*Glucagon*
- **Glucagon** is a hormone that has the opposite effect of insulin, stimulating gluconeogenesis and glycogenolysis to increase blood glucose levels during fasting or hypoglycemia.
- Its primary action is to **promote** hepatic glucose output, not inhibit it.
Gluconeogenesis: Reactions and Regulation Indian Medical PG Question 9: Which one of the following statements concerning gluconeogenesis is correct?
- A. It occurs primarily in the liver.
- B. It is stimulated by elevated levels of acetyl CoA.
- C. It is important in maintaining blood glucose during the normal overnight fast. (Correct Answer)
- D. It is primarily inhibited by insulin.
Gluconeogenesis: Reactions and Regulation Explanation: ***It is important in maintaining blood glucose during the normal overnight fast.***
- **This is the BEST answer** as it emphasizes the **primary physiological role** of gluconeogenesis in human metabolism.
- During the **overnight fast** (8-12 hours), hepatic glycogen stores become depleted, making gluconeogenesis the **critical mechanism** to maintain blood glucose for glucose-dependent tissues like the **brain** (requires ~120g glucose/day) and **red blood cells**.
- Without gluconeogenesis, blood glucose would drop dangerously during fasting, leading to hypoglycemia and neurological dysfunction.
*It occurs primarily in the liver.*
- This statement is **technically correct** - the liver accounts for approximately **90%** of total gluconeogenesis under normal conditions.
- However, the **kidney cortex** also contributes significantly (10% normally, up to 40% during prolonged fasting), and the **intestine** plays a minor role.
- While true, this is more of a **anatomical fact** rather than highlighting the critical physiological importance of the pathway, making it a less comprehensive answer than Option 1.
*It is stimulated by elevated levels of acetyl CoA.*
- This statement is **biochemically correct** - **Acetyl-CoA** is an important **allosteric activator** of **pyruvate carboxylase**, the first committed enzyme of gluconeogenesis.
- However, this represents just **one regulatory mechanism** at the enzymatic level, not the overall physiological significance.
- Primary regulation occurs through **hormones** (glucagon, cortisol, epinephrine) that coordinate the entire pathway, making this a narrower answer than Option 1.
*It is primarily inhibited by insulin.*
- This statement is also **correct** - **Insulin** is the primary hormonal **inhibitor** of gluconeogenesis.
- Insulin suppresses gluconeogenesis by inhibiting key enzymes (PEPCK, glucose-6-phosphatase) and decreasing transcription of gluconeogenic genes.
- However, this describes **inhibition** rather than the positive physiological role, making it less representative of gluconeogenesis's essential function than Option 1.
**Note:** All four statements are technically correct, but Option 1 best captures the **essential physiological importance** of gluconeogenesis in human metabolism, which is why it is the preferred answer for this question.
Gluconeogenesis: Reactions and Regulation Indian Medical PG Question 10: When the insulin:glucagon ratio is decreased, which enzyme is active?
- A. Glucokinase
- B. Phosphofructokinase
- C. Hexokinase
- D. Glucose-6-phosphatase (Correct Answer)
Gluconeogenesis: Reactions and Regulation Explanation: ***Glucose-6-phosphatase***
- A decreased insulin:glucagon ratio indicates a **fasting state** or **catabolic state**, promoting glucose production and release rather than storage.
- **Glucose-6-phosphatase** is the key enzyme that enables glucose release from the liver by removing the phosphate group from glucose-6-phosphate, producing free glucose that can exit hepatocytes.
- This enzyme is active during both **gluconeogenesis** and **glycogenolysis** and is only present in liver, kidney, and intestinal cells.
*Glucokinase*
- **Glucokinase** is active in the **fed state** when insulin levels are high and the insulin:glucagon ratio is increased.
- It phosphorylates glucose to trap it in hepatocytes for glycogen synthesis and metabolism, which is the opposite of what occurs during fasting.
*Phosphofructokinase*
- **Phosphofructokinase (PFK-1)** is the rate-limiting enzyme of **glycolysis**, active when glucose needs to be broken down for energy.
- It is stimulated by high insulin:glucagon ratios and inhibited during fasting when gluconeogenesis (the reverse pathway) is active.
*Hexokinase*
- **Hexokinase** phosphorylates glucose in peripheral tissues for intracellular utilization.
- During a low insulin:glucagon ratio, the priority is glucose **release** from the liver, not glucose **uptake** and phosphorylation in tissues.
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