Gluconeogenesis: Reactions and Regulation — MCQs
Which pathway is primarily affected in glucose-6-phosphate dehydrogenase deficiency?
Which of the following statements correctly describes the effect of insulin and glucagon on gluconeogenesis?
Which of the following is not a substrate for gluconeogenesis?
Enzyme activated by decrease in Insulin: glucagon ratio:
Which of the following is not a substrate for glucose formation?
Which of the following is true about the synthesis of glucose from pyruvate by gluconeogenesis?
Which of the following is not utilized in the process of gluconeogenesis?
Which amino acid can be utilized in both gluconeogenesis and ketogenesis?
Gluconeogenesis is inhibited by?
Which one of the following statements concerning gluconeogenesis is correct?
Gluconeogenesis: Reactions and Regulation Indian Medical PG Practice Questions and MCQs
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
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
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.
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.
Question 3: 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 4: Enzyme activated by decrease in Insulin: glucagon ratio:
- A. PFK
- B. Glucose 6 phosphatase (Correct Answer)
- C. Glucokinase
- D. Hexokinase
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.
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)
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 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)
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.
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 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 9: Gluconeogenesis is inhibited by?
- A. Cholecystokinin
- B. 5-alpha reductase
- C. Insulin (Correct Answer)
- D. Glucagon
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.
Question 10: 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.
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.
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