Metabolic Integration — MCQs

Metabolic Integration Indian Medical PG Practice Questions and MCQs

Question 51: Which cytochrome P450 enzyme is also known as 11-beta hydroxylase?

A. CYP17A1
B. CYP21A2
C. CYP11B2
D. CYP11B1 (Correct Answer)

Explanation: ***CYP11B1*** - This enzyme is specifically known as **11β-hydroxylase**, responsible for converting 11-deoxycortisol to **cortisol** and 11-deoxycorticosterone to **corticosterone** in the adrenal cortex. - Deficiency leads to an accumulation of 11-deoxycortisol and 11-deoxycorticosterone, causing **congenital adrenal hyperplasia (CAH)** with hypertension due to mineralocorticoid activity. *CYP17A1* - This enzyme is **17α-hydroxylase/17,20-lyase**, involved in the synthesis of **androgens** and **estrogens** by converting pregnenolone and progesterone to their 17α-hydroxylated forms. - Its deficiency leads to impaired sex steroid synthesis and elevated mineralocorticoids, causing **hypertension** and **sexual infantilism**. *CYP21A2* - This enzyme is **21-hydroxylase**, critical for converting progesterone to 11-deoxycorticosterone and 17α-hydroxyprogesterone to 11-deoxycortisol. - It is the most common cause of **congenital adrenal hyperplasia (CAH)**, leading to a deficiency in cortisol and aldosterone, and an excess of androgens. *CYP11B2* - This enzyme is **aldosterone synthase**, responsible for the final steps in **aldosterone biosynthesis**, converting deoxycorticosterone to corticosterone, and then to 18-hydroxycorticosterone and aldosterone. - Its overexpression or mutations can lead to **primary aldosteronism**, while deficiency can cause **hypoaldosteronism**.

Question 52: Primary link between citric acid cycle and urea cycle is:

A. Fumarate (Correct Answer)
B. Malate
C. Succinate
D. Citrate

Explanation: ***Fumarate*** - **Fumarate** is the primary link between the urea cycle and the citric acid cycle, forming what is known as the **"Krebs bicycle"** or **"Krebs-Henseleit bicycle"**. - **Fumarate** is produced in the **cytosol** during the urea cycle when argininosuccinate is cleaved by argininosuccinate lyase to form arginine and fumarate. - This cytosolic fumarate can be converted to **malate** by cytosolic fumarase, which is then transported into the mitochondria to join the citric acid cycle. - This link allows the urea cycle to contribute intermediates to the TCA cycle and provides metabolic integration between amino acid catabolism and energy production. *Malate* - While **malate** is an intermediate in both pathways, it is **fumarate** that serves as the primary and direct link from the urea cycle. - Malate is formed from fumarate in the cytosol and then transported into the mitochondria, acting as a carrier but not the primary connecting molecule itself. *Succinate* - **Succinate** is an intermediate of the citric acid cycle but does not directly link to the urea cycle. - It is formed from succinyl CoA in the TCA cycle and typically remains within the **mitochondria**, with no role in the urea cycle. *Citrate* - **Citrate** is the first intermediate formed in the citric acid cycle and does not have a direct linkage to the urea cycle for substrate exchange. - While citrate can be transported out of mitochondria for fatty acid synthesis, it has no connection to the urea cycle.

Question 53: Gout results due to defective function of the following enzyme:

A. Glucose 6 phosphatase (Correct Answer)
B. Glucose 6 phosphate dehydrogenase
C. PRPP synthetase
D. Purine nucleotide phosphorylase

Explanation: ***Glucose 6 phosphatase*** - Deficiency of **glucose-6-phosphatase** causes **glycogen storage disease type I** (von Gierke disease), which leads to **secondary hyperuricemia** and recurrent **gout attacks**. - The mechanism involves impaired **gluconeogenesis** and **glycogenolysis**, causing increased production of **lactate** (which competes with uric acid for renal excretion) and accelerated **purine nucleotide degradation** to uric acid. - This is the **most common enzyme deficiency** among the options that results in gout as a clinical manifestation. *Glucose 6 phosphate dehydrogenase* - Deficiency of **G6PD** primarily causes **hemolytic anemia** due to oxidative stress on red blood cells, not gout. - It is crucial for the **pentose phosphate pathway** generating NADPH, but does not directly affect purine or uric acid metabolism. *PRPP synthetase* - **Overactivity** (gain-of-function mutation) of **PRPP synthetase** leads to gout through increased purine synthesis. - However, **defective function** (as stated in the question) would actually **decrease** purine synthesis and **lower** uric acid levels, not cause gout. - This is not the correct answer since the question specifically asks for "defective function." *Purine nucleotide phosphorylase* - Deficiency of **purine nucleoside phosphorylase** (PNP) causes severe **T-cell immunodeficiency**, not gout. - While it affects purine metabolism, the accumulated metabolites are immunotoxic rather than causing hyperuricemia and gout.

Question 54: Ammonia causes depletion of which of the following in TCA cycle?

A. Malate
B. Oxaloacetate
C. Alpha-ketoglutarate (Correct Answer)
D. Fumarate

Explanation: ***Alpha-ketoglutarate*** - Ammonia is detoxified in the brain by conversion to **glutamine**, a process that consumes **alpha-ketoglutarate** in the glutamate dehydrogenase reaction (alpha-ketoglutarate + NH3 + NADH <=> glutamate + NAD+). - The depletion of **alpha-ketoglutarate** in the TCA cycle impairs cellular respiration and ATP production, contributing to the neurological dysfunction seen in hyperammonemia. *Malate* - While malate is a component of the TCA cycle, its depletion is not a direct consequence of ammonia detoxification. - Ammonia metabolism primarily impacts the availability of alpha-ketoglutarate through the synthesis of glutamate and glutamine. *Oxaloacetate* - Although **oxaloacetate** is a key intermediate in the TCA cycle, its levels are not directly depleted by ammonia metabolism. - **Oxaloacetate** can be replenished through anaplerotic reactions, even if the TCA cycle is slightly inhibited due to alpha-ketoglutarate depletion. *Fumarate* - **Fumarate** is an intermediate of the TCA cycle and is not directly consumed or depleted by the ammonia detoxification pathway. - Its levels would only indirectly be affected if the overall flux of the TCA cycle is significantly reduced due to depletion of other intermediates.

Question 55: In G6PD deficiency, which enzyme's function is MOST directly impaired due to decreased NADPH availability, leading to reduced protection against oxidative stress?

A. Catalase
B. Pyruvate kinase
C. Superoxide dismutase
D. Glutathione reductase (Correct Answer)

Explanation: ***Glutathione reductase*** - **G6PD deficiency** impairs the production of **NADPH** through the pentose phosphate pathway - **Glutathione reductase** is NADPH-dependent and reduces oxidized glutathione (GSSG) back to reduced glutathione (GSH) - Without adequate NADPH, glutathione reductase cannot maintain sufficient **GSH levels**, which is the primary antioxidant protecting RBCs from oxidative damage - This explains why G6PD deficiency leads to **hemolysis** when exposed to oxidative stressors (antimalarials, sulfonamides, fava beans) *Catalase* - **Catalase** decomposes hydrogen peroxide to water and oxygen, protecting cells from oxidative damage - While important for antioxidant defense, catalase does **not require NADPH** for its function - Its activity is not directly impaired by decreased NADPH in G6PD deficiency *Pyruvate kinase* - **Pyruvate kinase** catalyzes the final step of **glycolysis**, producing ATP - Its function is **completely independent** of NADPH levels - Pyruvate kinase deficiency causes a separate hemolytic anemia unrelated to oxidative stress or G6PD deficiency *Superoxide dismutase* - **Superoxide dismutase (SOD)** converts superoxide radicals to hydrogen peroxide and oxygen - SOD functions **independently of NADPH** and uses metal cofactors (Cu/Zn or Mn) - While part of antioxidant defense, it is not directly affected by G6PD deficiency

Question 56: What happens to excess proteins in the body?

A. Converted to energy or stored as fat (Correct Answer)
B. Used to build structural proteins
C. Used to synthesize functional proteins
D. Stored as amino acids in tissues

Explanation: ***Converted to energy or stored as fat*** - When protein intake exceeds the body's needs for structural and functional protein synthesis, excess **amino acids** are deaminated. - The carbon skeletons can then be converted into **glucose** (via gluconeogenesis) or **fatty acids** (which are stored as triglycerides). *Used to build structural proteins* - While proteins are essential for building structural components like **collagen** and **keratin**, the body has a specific need for this, and excess intake doesn't lead to endless structural growth. - Building structural proteins is a regulated process that depends on tissue repair and growth demands, not solely on protein availability. *Used to synthesize functional proteins* - Functional proteins, such as **enzymes** and **hormones**, are synthesized as needed to maintain specific metabolic processes and cellular functions. - The body doesn't synthesize an unlimited amount of these proteins just because there's an excess of amino acids; synthesis is regulated based on physiological demand. *Stored as amino acids in tissues* - The body has a very limited capacity to store free amino acids; there is no dedicated storage compartment akin to **glycogen** for carbohydrates or **triglycerides** for fats. - Instead, unused amino acids are rapidly catabolized rather than accumulated in significant quantities.

Question 57: Pyruvate can be a substrate of all except

A. Lactate Dehydrogenase
B. Malic enzyme
C. Aspartate transaminase (Correct Answer)
D. Alanine transaminase

Explanation: ***Aspartate transaminase*** - **Aspartate transaminase (AST)** catalyzes the reversible transfer of an amino group from **aspartate** to **α-ketoglutarate**, producing **oxaloacetate** and **glutamate**. - **Pyruvate** is not a substrate for AST; its primary substrates are aspartate and α-ketoglutarate. *Lactate Dehydrogenase* - **Lactate dehydrogenase (LDH)** catalyzes the reversible conversion of **pyruvate** to **lactate** during anaerobic metabolism. - This reaction regenerates **NAD+** from **NADH**, crucial for continuing glycolysis in the absence of oxygen. - **Pyruvate** serves as a substrate for LDH. *Malic enzyme* - **Malic enzyme** catalyzes the oxidative decarboxylation of **malate** to **pyruvate**, generating **NADPH**. - In this reaction, **malate** (not pyruvate) is the substrate, and **pyruvate** is the product. - This is an essentially irreversible reaction under physiological conditions. *Alanine transaminase* - **Alanine transaminase (ALT)** catalyzes the reversible transfer of an amino group from **alanine** to **α-ketoglutarate**, producing **pyruvate** and **glutamate**. - In the reverse reaction, **pyruvate** serves as a substrate, accepting an amino group to form **alanine**.

Question 58: Autophagic vacuoles fuse with ?

A. Golgi complex
B. ER
C. Mitochondria
D. Lysosomes (Correct Answer)

Explanation: ***Lysosomes*** - **Autophagic vacuoles (autophagosomes)** are double-membraned vesicles that sequester cellular components destined for degradation. - The fusion of these autophagosomes with **lysosomes**, which contain hydrolytic enzymes, forms **autolysosomes**, where degradation occurs. *Golgi complex* - The **Golgi complex** is involved in modifying, sorting, and packaging proteins and lipids. - It does not directly participate in the final degradative step of autophagy. *ER* - The **endoplasmic reticulum (ER)** is involved in protein synthesis and lipid metabolism. - While it can be a source of membranes for autophagosome formation, it is not the destination for their fusion. *Mitochondria* - **Mitochondria** are primarily involved in energy production through cellular respiration. - They are often targets for **autophagy** (a process called mitophagy), but they do not fuse with autophagic vacuoles for degradation.

Question 59: Sirtuins are associated with?

A. Antioxidant mechanism in body
B. Regeneration of liver after partial resection
C. Carcinogenesis in human
D. Longevity of life span (Correct Answer)

Explanation: ***Longevity of life span*** - Sirtuins are a family of **NAD+-dependent deacetylases** that play a crucial role in regulating cellular processes related to **aging** and metabolism. - Activation of sirtuins, particularly **SIRT1**, has been shown in various model organisms like yeast, worms, and flies to extend their lifespan. *Antioxidant mechanism in body* - While sirtuins can indirectly influence antioxidant defenses by regulating enzymes like **superoxide dismutase** (SOD), their primary association is not directly with the antioxidant mechanism itself but with broader cellular stress responses. - Antioxidant mechanisms typically involve direct scavenging of **reactive oxygen species** (ROS) by specific enzymes or molecules. *Regeneration of liver after partial resection* - Sirtuins do have roles in **metabolism** and cellular repair, but their direct and primary association is not specifically with liver regeneration after partial resection. - Liver regeneration involves complex pathways including growth factors and cytokine signaling. *Carcinogenesis in human* - Sirtuins have a complex and dual role in cancer, acting as both **tumor suppressors** and **oncogenes** depending on the specific sirtuin isoform, cancer type, and cellular context. - Their association with carcinogenesis is multifaceted, not a singular, consistent mechanism like their link to longevity.

Question 60: In Krebs cycle and Urea cycle the linking amino acid is

A. Fumarate
B. Alanine
C. Aspartate (Correct Answer)
D. Arginine

Explanation: ***Aspartate*** - **Aspartate** acts as the crucial amino acid link between the two cycles - In the urea cycle, aspartate condenses with citrulline to form **argininosuccinate** (via argininosuccinate synthetase) - When argininosuccinate is cleaved, it produces **fumarate**, which enters the Krebs cycle - In the Krebs cycle, fumarate is converted to malate, then to **oxaloacetate**, which can be transaminated back to aspartate - This creates the **aspartate-argininosuccinate shunt**, linking both cycles through nitrogen metabolism *Fumarate* - While **fumarate** is a key metabolic intermediate connecting both cycles, it is **not an amino acid** (it's a dicarboxylic acid) - It is produced in the urea cycle from argininosuccinate cleavage and feeds into the Krebs cycle - This is a common distractor since fumarate does link the cycles, but the question specifically asks for an amino acid *Alanine* - **Alanine** participates in the glucose-alanine cycle for nitrogen transport from muscle to liver - It does not directly link the Krebs cycle and urea cycle in the same manner as aspartate *Arginine* - **Arginine** is a urea cycle intermediate that is cleaved by arginase to produce urea and ornithine - While it's an amino acid in the urea cycle, it does not serve as the linking amino acid between the Krebs cycle and urea cycle

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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