Metabolic Integration — MCQs

Metabolic Integration Indian Medical PG Practice Questions and MCQs

Question 61: 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 62: Urea & Kreb's cycle are linked at?

A. Arginine
B. Ornithine
C. Oxaloacetate
D. Fumarate (Correct Answer)

Explanation: ***Fumarate*** - **Fumarate** is a key intermediate produced during the **urea cycle** when argininosuccinate is cleaved into arginine and fumarate. - This fumarate then enters the **Krebs cycle** (citric acid cycle) as an intermediate to be converted into malate and then oxaloacetate, thus linking the two cycles. *Arginine* - **Arginine** is an amino acid that participates in the urea cycle, serving as a precursor for the formation of urea. - While arginine is a part of the urea cycle, it does not directly enter the Krebs cycle or serve as its linking metabolite. *Ornithine* - **Ornithine** is another amino acid central to the urea cycle, being regenerated at the end of the cycle to combine with carbamoyl phosphate. - It is a carrier molecule for the nitrogen atoms, but it does not directly link to the Krebs cycle. *Oxaloacetate* - **Oxaloacetate** is a central intermediate in the Krebs cycle, and it can be a precursor for intermediates in the urea cycle (e.g., through aspartate). - However, it is not the direct molecule that links the two cycles in the direction of the urea cycle feeding into the Krebs cycle.

Question 63: Which molecule serves as a key link between carbohydrate metabolism and fatty acid synthesis?

A. Glucose-6-phosphate
B. Acetyl-CoA
C. Citrate (Correct Answer)
D. Succinyl-CoA

Explanation: ***Citrate*** - **Citrate** is the crucial molecule that links carbohydrate metabolism to fatty acid synthesis via the **citrate-malate shuttle** - In the fed state, excess **acetyl-CoA** (derived from glucose metabolism via glycolysis and pyruvate dehydrogenase) condenses with oxaloacetate to form citrate in the mitochondria - **Citrate** is then transported from mitochondria to the cytosol, where **ATP-citrate lyase** cleaves it to regenerate **acetyl-CoA** and **oxaloacetate** for fatty acid synthesis - This is the primary mechanism for transporting acetyl-CoA equivalents from mitochondria (where glucose is oxidized) to the cytosol (where fatty acids are synthesized) - Citrate also acts as an **allosteric activator** of **acetyl-CoA carboxylase**, the rate-limiting enzyme of fatty acid synthesis *Glucose-6-phosphate* - While **glucose-6-phosphate** is a key intermediate in glycolysis and gluconeogenesis, it is not the molecule that directly links carbohydrate breakdown to fatty acid synthesis - It is several steps removed from the generation of cytosolic acetyl-CoA needed for fatty acid synthesis *Acetyl-CoA* - **Acetyl-CoA** is the direct precursor for fatty acid synthesis - However, acetyl-CoA generated in mitochondria from glucose oxidation **cannot directly cross the mitochondrial membrane** - It must be transported as citrate, making citrate the actual linking molecule between the two compartments *Succinyl-CoA* - **Succinyl-CoA** is a Krebs cycle intermediate involved in heme synthesis and propionate metabolism - It is not involved in transporting acetyl units from mitochondria to cytosol for fatty acid synthesis

Question 64: All are activated by insulin except?

A. Lipoprotein lipase
B. Pyruvate kinase
C. Acetyl-CoA carboxylase
D. Hormone sensitive lipase (Correct Answer)

Explanation: ***Hormone sensitive lipase*** - **Insulin** is an **anabolic hormone** that promotes energy storage; it **inhibits** hormone-sensitive lipase (HSL) activity which is responsible for **fat breakdown (lipolysis)**. - When insulin levels are high, the body stores fat rather than breaks it down, thus **decreasing** HSL activity. *Lipoprotein lipase* - **Insulin activates lipoprotein lipase (LPL)**, an enzyme that breaks down triglycerides in **chylomicrons** and **VLDL** into fatty acids for storage in adipose tissue. - This activation promotes the uptake of fatty acids into fat cells, aligning with insulin's role in **energy storage**. *Pyruvate kinase* - **Insulin activates pyruvate kinase** in glycolysis, promoting the conversion of **phosphoenolpyruvate to pyruvate**. - This enzyme's activation enhances glucose utilization and energy production following a meal when insulin levels are high. *Acetyl-CoA carboxylase* - **Insulin activates acetyl-CoA carboxylase (ACC)**, the **rate-limiting enzyme in fatty acid synthesis**. - Activation of ACC leads to the production of **malonyl-CoA**, which commits acetyl-CoA to fatty acid synthesis, storing excess energy as fat.

Question 65: 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.

Question 66: A 25-year-old male flushes and feels ill after drinking small amounts of ethanol in alcoholic beverages. This is due to a genetic variation in an enzyme that metabolizes a liver metabolite of alcohol. Which metabolite accumulates?

A. Methanol
B. Acetone
C. Acetaldehyde (Correct Answer)
D. Hydrogen peroxide

Explanation: ***Acetaldehyde*** - The flushing and illness after consuming alcohol are characteristic symptoms of **acetaldehyde accumulation**. - This occurs due to a genetic polymorphism (often seen in individuals of East Asian descent) in **aldehyde dehydrogenase (ALDH2)**, which is responsible for converting acetaldehyde to acetate. *Methanol* - **Methanol** is metabolized to **formaldehyde** and then to **formic acid**, which are highly toxic and cause severe symptoms like metabolic acidosis, blindness, and death. - Methanol poisoning typically results from ingestion of denatured alcohol or adulterated spirits, not small amounts of ethanol-containing beverages. *Acetone* - **Acetone** is a ketone body produced during fat metabolism and is not a direct liver metabolite of ethanol. - While it can be found in the body, its metabolism is primarily via different pathways and does not cause the "alcohol flush reaction." *Hydrogen peroxide* - **Hydrogen peroxide** is a reactive oxygen species involved in oxidative stress and is not a direct metabolite of alcohol in the liver associated with flushing and illness from alcohol consumption. - It is primarily catabolized by **catalase** and **glutathione peroxidase**.

Question 67: What is a physiological effect of a high-protein diet on glucose metabolism?

A. Increased gluconeogenesis due to elevated glucogenic amino acids (Correct Answer)
B. Decreased insulin sensitivity
C. Increased glycogen breakdown
D. Increased glycolysis in muscle tissue

Explanation: ***Increased gluconeogenesis due to elevated glucogenic amino acids*** - A high-protein diet provides an abundance of **amino acids**, many of which are **glucogenic** (e.g., alanine, glutamine, serine, glycine). - These glucogenic amino acids serve as substrates for **gluconeogenesis** in the liver, leading to increased glucose production, especially during fasting or low carbohydrate intake. - This is the **primary effect on glucose metabolism** from a high-protein diet. *Decreased insulin sensitivity* - High-protein diets generally **improve** insulin sensitivity rather than decrease it. - Protein intake stimulates insulin secretion but also improves glycemic control and insulin sensitivity in most individuals. - Studies show that high-protein diets can enhance insulin sensitivity, especially in the context of weight loss. *Increased glycogen breakdown* - A high-protein diet generally aims to **spare glycogen stores**, not promote glycogenolysis. - Glycogen breakdown is primarily stimulated by hormones like **glucagon** and **epinephrine** in response to low blood glucose or stress. - Protein intake, through its effect on insulin and glucagon, tends to preserve rather than deplete glycogen. *Increased glycolysis in muscle tissue* - A high-protein diet does not primarily promote **glycolysis** (glucose breakdown for energy). - Glycolysis is enhanced when glucose availability is high and energy demand is present. - Protein metabolism focuses on amino acid utilization rather than increasing glucose breakdown pathways.

Question 68: Which of the following is a direct cause of ketosis in a patient with Von Gierke's disease?

A. Inadequate glucose availability
B. Increased ketone body production due to fatty acid oxidation
C. Increased fatty acid oxidation (Correct Answer)
D. Deficiency of glucose-6-phosphatase

Explanation: ***Increased fatty acid oxidation*** - In Von Gierke's disease, **glucose-6-phosphatase deficiency** leads to inability to release glucose from the liver, causing **hypoglycemia**. - The hypoglycemia triggers a hormonal response with **low insulin and high glucagon**, leading to lipolysis and fatty acid mobilization from adipose tissue. - These mobilized fatty acids undergo **β-oxidation in the liver**, generating excess **acetyl-CoA** that exceeds the capacity of the TCA cycle. - The excess acetyl-CoA is converted to **ketone bodies** (acetoacetate, β-hydroxybutyrate, acetone) - this is the **direct biochemical cause** of ketosis. *Inadequate glucose availability* - This is the **trigger** that initiates the metabolic shift, but not the direct biochemical cause of ketosis. - It creates the conditions that lead to fatty acid oxidation. *Deficiency of glucose-6-phosphatase* - This is the **primary enzyme defect** in Von Gierke's disease (GSD Type Ia). - It is the root cause but several metabolic steps removed from the actual production of ketone bodies. *Increased fatty acid mobilization* - This provides the **substrate** (fatty acids) that will be oxidized. - However, mobilization alone doesn't cause ketosis - the fatty acids must undergo **oxidation** in the liver to generate ketone bodies.

Practice by Chapter

Fed State Metabolism

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Fasting State Metabolism

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Starvation Adaptation

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Metabolic Adaptations During Exercise

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Interorgan Metabolite Exchange

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Metabolic Regulation: Hormonal Control

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Metabolic Regulation: Substrate Availability

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Metabolic Syndrome

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Obesity: Biochemical Aspects

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Liver as Metabolic Hub

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Muscle Metabolism

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Brain Metabolism and Ketone Bodies

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