Vitamin B6 and Transamination — MCQs
Which amino acid is not involved in transamination?
In a patient with chronic alcoholism, which nutrient deficiency is most likely to cause neurological symptoms?
Which of the following statements about carbamoyl phosphate synthase is incorrect?
All are involved in non-toxic transport of ammonia except:
Pyridoxine is required in -
Pyridoxine deficiency leads to altered metabolism of?
Which vitamin is primarily associated with the antioxidant properties of glutathione?
Which of the following is the rich source of Vitamin B12?
Vitamin A is stored in
Maximum concentration of vitamin A is found in which organ?
Vitamin B6 and Transamination Indian Medical PG Practice Questions and MCQs
Question 1: Which amino acid is not involved in transamination?
- A. Alanine
- B. Aspartate
- C. Lysine (Correct Answer)
- D. Histidine
Explanation: ***Lysine*** - **Lysine** cannot undergo transamination because it lacks the structural requirements for typical transaminase enzymes. - While lysine has both an **α-amino group** and an **ε-amino group**, its metabolic pathway involves **oxidative deamination** rather than transamination. - Along with **threonine**, lysine is one of only two amino acids that do not participate in transamination reactions. *Alanine* - **Alanine** is a major substrate for transamination, readily converting to pyruvate via **alanine transaminase (ALT)**. - This reaction involves the transfer of its **α-amino group** to an α-keto acid, typically α-ketoglutarate, forming glutamate. *Aspartate* - **Aspartate** is actively involved in transamination, converting to oxaloacetate via **aspartate transaminase (AST)**. - Its **α-amino group** is easily transferred to α-ketoglutarate, forming glutamate. *Histidine* - **Histidine** can undergo transamination, though less commonly cited as a primary substrate compared to aspartate and alanine. - It can transfer its **α-amino group** to an α-keto acid, leading to the formation of imidazolepyruvate.
Question 2: In a patient with chronic alcoholism, which nutrient deficiency is most likely to cause neurological symptoms?
- A. Vitamin B6
- B. Thiamine (Correct Answer)
- C. Folate
- D. Vitamin B12
Explanation: ***Thiamine*** - **Thiamine (Vitamin B1)** deficiency is extremely common in chronic alcoholism due to poor nutrition and impaired absorption, leading to neurological disorders like **Wernicke-Korsakoff syndrome** [1]. - **Wernicke-Korsakoff syndrome** manifests with symptoms such as **ataxia**, **ophthalmoplegia**, **confusion**, and **memory impairment** [2]. *Vitamin B6* - While **Vitamin B6 (pyridoxine)** deficiency can occur in alcoholism, it is more commonly associated with peripheral neuropathy rather than the extensive neurological picture seen with thiamine deficiency. - Severe B6 deficiency can cause **seizures** and **encephalopathy**, but these are less common as primary neurological manifestations in typical chronic alcoholics compared to Wernicke-Korsakoff syndrome. *Folate* - **Folate deficiency** is very common in chronic alcoholism and primarily leads to **macrocytic anemia**. - While it can indirectly contribute to neurological issues due to anemia, it does not directly cause the classic acute neurological syndromes seen with thiamine deficiency. *Vitamin B12* - **Vitamin B12 deficiency** can cause neurological symptoms, including **peripheral neuropathy**, **ataxia**, and **cognitive impairment**, but it is less directly associated with alcoholism compared to thiamine deficiency. - B12 deficiency is more commonly seen in strict vegetarians, pernicious anemia, or malabsorption conditions involving the ileum.
Question 3: Which of the following statements about carbamoyl phosphate synthase is incorrect?
- A. Requires biotin as a cofactor (Correct Answer)
- B. Enzyme found in the cytosol
- C. Enzyme found in mitochondria
- D. Catalyzes a condensation reaction
Explanation: ***Requires biotin as a cofactor*** - This is the **incorrect** statement and therefore the correct answer to this question. - Carbamoyl phosphate synthase (both CPS I and CPS II) does **NOT require biotin** as a cofactor. - Biotin is a cofactor for **carboxylase enzymes** such as pyruvate carboxylase, acetyl-CoA carboxylase, propionyl-CoA carboxylase, and methylcrotonyl-CoA carboxylase. - Carbamoyl phosphate synthase requires **ATP** and **Mg²⁺** but not biotin. *Enzyme found in mitochondria* - This statement is **correct**. - **Carbamoyl phosphate synthase I (CPS I)** is located in the **mitochondrial matrix** and catalyzes the first step of the urea cycle. - CPS I uses free ammonia (NH₃) as the nitrogen source and is activated by N-acetylglutamate. *Enzyme found in the cytosol* - This statement is **correct**. - **Carbamoyl phosphate synthase II (CPS II)** is located in the **cytosol** and is involved in de novo pyrimidine biosynthesis. - CPS II uses the amide nitrogen of glutamine (not free ammonia) as the nitrogen source. *Catalyzes a condensation reaction* - This statement is **correct**. - Both CPS I and CPS II catalyze the condensation of CO₂ (as bicarbonate), ammonia/glutamine, and two molecules of ATP to form carbamoyl phosphate, 2 ADP, and inorganic phosphate. - This is a complex reaction involving phosphorylation and condensation steps.
Question 4: All are involved in non-toxic transport of ammonia except:
- A. Glutaminase (Correct Answer)
- B. Glutamine synthetase
- C. SGPT
- D. Alanine cycle
Explanation: ***Glutaminase*** - **Glutaminase** is an enzyme that **removes ammonia from glutamine**, producing glutamate and free ammonia. - While glutamine is a non-toxic carrier of ammonia, glutaminase releases the toxic ammonia, therefore it is not involved in the *non-toxic transport* itself. *Glutamine synthetase* - **Glutamine synthetase** catalyzes the ATP-dependent synthesis of **glutamine from glutamate and ammonia**. - This process effectively **traps free ammonia** into the non-toxic amino acid **glutamine**, making it a key component of non-toxic transport, especially in the brain. *SGPT* - **SGPT** (serum glutamic-pyruvic transaminase), also known as **Alanine transaminase (ALT)**, is involved in the transfer of an amino group from alanine to α-ketoglutarate, forming glutamate and pyruvate. - It plays a role in the **alanine cycle**, which is a significant mechanism for transporting ammonia from muscle to the liver, thereby contributing to non-toxic ammonia transport. *Alanine cycle* - The **alanine cycle** (or glucose-alanine cycle) is a pathway that **transports ammonia from muscle to the liver** in the form of alanine. - In muscle, pyruvate is transaminated to alanine using an amino group from glutamate, and alanine then travels to the liver for gluconeogenesis and urea cycle processing of the ammonia.
Question 5: Pyridoxine is required in -
- A. Glycolysis
- B. TCA cycle
- C. Glycogenesis
- D. Transamination (Correct Answer)
Explanation: ***Transamination*** - **Pyridoxal phosphate (PLP)**, the active form of pyridoxine (vitamin B6), is an essential **coenzyme for aminotransferases (transaminases)** - Transamination reactions involve the transfer of an **amino group** from an amino acid to a keto acid, which is crucial for amino acid metabolism - This is the classic biochemical function of vitamin B6 and a frequently tested concept *Glycolysis* - Glycolysis is a metabolic pathway that breaks down glucose into pyruvate - Key cofactors for glycolysis include **NAD+ and ATP**, not vitamin B6 - Does not require pyridoxine as a coenzyme *TCA cycle* - The **TCA cycle (Krebs cycle)** is a central metabolic pathway for energy production - Uses enzymes that require cofactors such as **NAD+, FAD, and Coenzyme A** (derived from pantothenic acid) - Pyridoxine is not directly involved as a coenzyme in TCA cycle reactions *Glycogenesis* - Glycogenesis is the process of synthesizing **glycogen from glucose** - Primarily involves enzymes like **glycogen synthase** and **branching enzyme** - Requires **UTP and glucose-1-phosphate**, not pyridoxine
Question 6: Pyridoxine deficiency leads to altered metabolism of?
- A. Phenylalanine
- B. Methionine
- C. Tyrosine
- D. Tryptophan (Correct Answer)
Explanation: ***Tryptophan*** - **Pyridoxine (vitamin B6)** is a critical coenzyme in the metabolism of **tryptophan**, particularly in its conversion to **niacin** and serotonin. - A deficiency leads to an accumulation of abnormal tryptophan metabolites, such as **xanthurenic acid**, which can be excreted in the urine. *Phenylalanine* - The metabolism of phenylalanine involves its conversion to tyrosine, a process catalyzed by **phenylalanine hydroxylase**, which does not directly require pyridoxine. - Deficiencies in phenylalanine metabolism often point to issues like **phenylketonuria**. *Methionine* - Methionine metabolism involves a cycle that generates **S-adenosylmethionine (SAM)** and then homocysteine. - While vitamin B6 is involved in the transsulfuration pathway (converting homocysteine to cysteine), its primary direct impact on methionine metabolism itself is less pronounced than on tryptophan. *Tyrosine* - Tyrosine is synthesized from phenylalanine and is a precursor for **catecholamines** and thyroid hormones. - Its metabolism does not directly rely on pyridoxine as a coenzyme in the main initial steps.
Question 7: Which vitamin is primarily associated with the antioxidant properties of glutathione?
- A. Vitamin E
- B. Niacin (Correct Answer)
- C. Vitamin C
- D. Vitamin A
Explanation: ***Niacin*** - **Niacin** (Vitamin B3) is the vitamin most directly associated with glutathione's antioxidant properties - Niacin is a precursor to **NAD+** and **NADP+**, which are converted to **NADPH** - **NADPH is the essential cofactor** for **glutathione reductase**, the primary enzyme that reduces oxidized glutathione (GSSG) back to its active reduced form (GSH) - This NADPH-dependent enzymatic pathway is the **main mechanism** for maintaining the body's glutathione antioxidant system - Without adequate niacin → NADPH, glutathione cannot be efficiently regenerated *Vitamin C* - **Vitamin C** can non-enzymatically reduce GSSG to GSH, providing a **secondary backup mechanism** - While vitamin C does support glutathione regeneration, this is an **indirect, non-enzymatic process** - It acts as an antioxidant itself but is not the primary vitamin associated with glutathione's antioxidant function *Vitamin E* - **Vitamin E** is a **lipid-soluble antioxidant** that primarily protects cell membranes from oxidative damage - Works synergistically with other antioxidants but has **no direct role** in glutathione synthesis or regeneration *Vitamin A* - **Vitamin A** (retinol) is crucial for vision, immune function, and cell differentiation - Has some antioxidant properties as a carotenoid derivative but **no direct involvement** in glutathione metabolism
Question 8: Which of the following is the rich source of Vitamin B12?
- A. Mango
- B. Carrot
- C. Meat (Correct Answer)
- D. Spinach
Explanation: ***Meat*** - **Vitamin B12** is primarily found in **animal products** because it is synthesized by bacteria in the digestive tracts of animals. - **Meat**, especially red meat and liver, is an excellent source of this vitamin. *Mango* - Mangoes are a good source of **Vitamin C** and **Vitamin A**, but they contain negligible amounts of Vitamin B12. - As a fruit, mangos are a plant-based food and generally **do not contain B12**. *Carrot* - Carrots are rich in **beta-carotene** (a precursor to Vitamin A) and **fiber**, but they are not a source of Vitamin B12. - Being a vegetable, carrots are a **plant-based food** and lack Vitamin B12. *Spinach* - Spinach is known for its high content of **iron**, **folate**, and **Vitamin K**, but it does not contain Vitamin B12. - Like other plant-based foods, spinach **naturally lacks Vitamin B12**.
Question 9: Vitamin A is stored in
- A. Cells of Ito (Correct Answer)
- B. Hepatocyte
- C. Endothelial cell
- D. Kupffer cell
Explanation: ***Cells of Ito*** - **Ito cells**, also known as **hepatic stellate cells**, are the primary storage site for **vitamin A** in the body, specifically in lipid droplets within their cytoplasm. - These cells play a crucial role in vitamin A homeostasis and are located in the **perisinusoidal space** (space of Disse) of the liver. *Hepatocyte* - **Hepatocytes** are the main functional cells of the liver and are involved in many metabolic processes, but their primary role is not **vitamin A storage**. - While they metabolize and process vitamin A, the bulk of its storage occurs in the adjacent Ito cells. *Endothelial cell* - **Endothelial cells** line the vascular system, including the hepatic sinusoids, and are involved in nutrient and waste exchange, but not in significant **vitamin A storage**. - Their primary function is to regulate vascular tone and permeability. *Kupffer cell* - **Kupffer cells** are specialized **macrophages** found in the liver, acting as antigen-presenting cells and clearing pathogens and debris. - They are involved in immune surveillance and not in the long-term storage of **vitamin A**.
Question 10: Maximum concentration of vitamin A is found in which organ?
- A. Liver (Correct Answer)
- B. Kidney
- C. Lung
- D. Heart
Explanation: ***Liver*** - The **liver** is the primary organ for **storage of vitamin A** (retinyl esters), accounting for 90% of the body's total vitamin A content. - **Hepatic stellate cells** within the liver are specialized for storing the majority of this fat-soluble vitamin. *Kidney* - The **kidney** plays a role in **vitamin D metabolism** and excretion, but not significant vitamin A storage. - While it helps regulate blood levels of various substances, it does not accumulate large quantities of vitamin A. *Lung* - The **lung** does not serve as a major storage site for **vitamin A**. - Its primary functions are related to **gas exchange**, not nutrient storage. *Heart* - The **heart** is responsible for **pumping blood** throughout the body and has minimal involvement in vitamin A storage. - It utilizes certain vitamins for its metabolic processes but does not act as a primary reservoir.
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