Vitamins and Coenzymes Practice Questions

Q: Thiamine deficiency is assessed by:

Erythrocyte transketolase activity. ***Erythrocyte transketolase activity*** - This is the **most reliable functional assay** for thiamine deficiency, as thiamine pyrophosphate (TPP) is a crucial cofactor for the enzyme **transketolase**. - A significant increase in transketolase activity after the addition of TPP *in vitro* indicates a deficiency, demonstrating the enzyme's reliance on exogenous thiamine. *Serum thiamine level* - **Serum thiamine levels** can fluctuate and do not accurately reflect the body's thiamine stores or functional status, as most thiamine is intracellular. - This measurement may be normal even in cases of functional deficiency or tissue depletion. *RBC thiamine levels* - While red blood cell (RBC) thiamine levels provide a better estimate of tissue stores compared to serum levels, they are **still less sensitive** than direct functional assays like transketolase activity. - RBC thiamine measurements do not directly assess the functional impact of thiamine deficiency on metabolic pathways. *RBC Glutathione reductase* - **Glutathione reductase** activity is used to assess **riboflavin (vitamin B2) deficiency**, not thiamine. - Riboflavin in its coenzyme form, **flavin adenine dinucleotide (FAD)**, is a necessary cofactor for glutathione reductase.

Q: Vitamin K in its coenzyme form is regenerated by which enzyme?

Epoxide reductase. ***Epoxide reductase*** - **Vitamin K epoxide reductase (VKOR)** regenerates the active form of vitamin K, **hydroquinone**, from its inactive **epoxide metabolite**, which is essential for the function of **gamma-glutamyl carboxylase**. - This enzyme is the target of **warfarin**, an anticoagulant drug that inhibits its activity, thus reducing the synthesis of active **clotting factors**. *Glutathione reductase* - This enzyme is involved in maintaining the cellular redox state by reducing **oxidized glutathione (GSSG)** back to **reduced glutathione (GSH)**, using **NADPH** as a cofactor. - It plays a crucial role in protecting cells from **oxidative stress**, but it is not directly involved in vitamin K metabolism. *Dihydrofolate reductase* - **Dihydrofolate reductase (DHFR)** catalyzes the reduction of **dihydrofolate** to **tetrahydrofolate (THF)**, a critical cofactor for **one-carbon transfer reactions**, including purine and pyrimidine synthesis. - It is the target of **methotrexate**, a chemotherapy drug, and **trimethoprim**, an antibiotic, but has no role in vitamin K recycling. *Pyruvate carboxylase* - **Pyruvate carboxylase** is a mitochondrial enzyme that converts **pyruvate** to **oxaloacetate**, a crucial step in **gluconeogenesis** and for replenishing intermediates of the **citric acid cycle**. - It requires **biotin** as a cofactor and is not involved in vitamin K metabolism or redox cycling.

Q: Which vitamin deficiency causes peripheral neuropathy first?

Vitamin B12. ***Vitamin B12*** - **Vitamin B12 deficiency** often presents initially with **peripheral neuropathy**, including sensory symptoms like numbness and tingling, due to its role in myelin sheath maintenance. - This deficiency can also lead to **subacute combined degeneration** of the spinal cord, causing gait disturbances and proprioception issues. *Vitamin B1* - **Vitamin B1 (thiamine) deficiency** primarily causes **beriberi**, which presents with either cardiovascular (wet beriberi) or neurological (dry beriberi) symptoms [2]. - While dry beriberi involves **peripheral neuropathy**, it often affects motor function more prominently and is not typically the very first symptom compared to B12 deficiency [2]. *Vitamin B6* - Both **deficiency and toxicity of vitamin B6 (pyridoxine)** can cause peripheral neuropathy, but it is less common as an initial presentation compared to B12 [3]. - Deficiency usually occurs in specific circumstances, such as alcoholism or drug interactions, and can also lead to **sideroblastic anemia** and seizures [3]. *Vitamin E* - **Vitamin E deficiency** can cause neurological symptoms, including **peripheral neuropathy**, ataxia, and muscle weakness, due to its antioxidant role [1]. - However, symptoms typically develop slowly and are often associated with genetic disorders affecting vitamin E absorption or metabolism, rather than being a common initial presentation [1].

Q: The transfer of amino groups between amino acids and α-keto acids requires which coenzyme?

PLP. ***PLP*** - **Pyridoxal phosphate (PLP)**, a derivative of **vitamin B6**, is an essential coenzyme for **aminotransferases** (transaminases). - These enzymes facilitate the reversible transfer of an **amino group** from an amino acid to an **α-keto acid**, and vice versa, in processes like amino acid synthesis and catabolism. *FAD* - **Flavin adenine dinucleotide (FAD)** is a derivative of **riboflavin (vitamin B2)** and primarily acts as a coenzyme in **redox reactions**. - It is involved in processes like the **Krebs cycle** and **fatty acid oxidation**, where it accepts electrons. *Thiamine* - **Thiamine pyrophosphate (TPP)**, derived from **thiamine (vitamin B1)**, is a crucial coenzyme for enzymes involved in **carbohydrate metabolism**. - It plays a role in reactions that involve the transfer of **aldehyde groups**, such as those catalyzed by pyruvate dehydrogenase and α-ketoglutarate dehydrogenase. *NAD+* - **Nicotinamide adenine dinucleotide (NAD+)** is a coenzyme derived from **niacin (vitamin B3)** and is predominantly involved in **redox reactions**. - It acts as an electron acceptor in many catabolic pathways, including **glycolysis** and the **Krebs cycle**, generating **NADH**.

Q: All of the following are true about Vitamin K EXCEPT:

Synthesized in large amounts by the liver. ***Synthesized in large amounts by the liver*** - Vitamin K is primarily obtained from **dietary sources** and synthesized by **gut bacteria**, not in large amounts by the liver. - The liver is crucial for the **storage** and **utilization** of Vitamin K in synthesizing clotting factors, but not its primary production. *Requires bile salts for absorption* - Vitamin K is a **fat-soluble vitamin**, and like other fat-soluble vitamins (A, D, E), its absorption in the intestines requires the presence of **bile salts**. - Conditions that impair bile production or flow (e.g., biliary obstruction, cholestasis) can lead to **Vitamin K deficiency**. *Functions in carboxylation reactions* - Vitamin K acts as a **cofactor** for the enzyme **gamma-glutamyl carboxylase**, which catalyzes the carboxylation of glutamic acid residues. - This carboxylation is essential for activating a variety of **proteins**, including clotting factors. *Essential for blood clotting* - Vitamin K is critical for the synthesis of several **coagulation factors** in the liver, specifically factors **II (prothrombin), VII, IX, and X**. - It works by enabling the carboxylation of these factors, which allows them to bind calcium and participate in the **clotting cascade**.

Q: Which vitamin deficiency leads to megaloblastic anemia?

Folate. ***Folate*** - **Folate** is essential for DNA synthesis; a deficiency impairs erythrocyte maturation, leading to the production of **large, immature red blood cells** (megaloblasts) [3]. - This vitamin deficiency also presents with symptoms like **fatigue, glossitis**, and neurologic manifestations are absent unlike vitamin B12 deficiency [1]. *Riboflavin* - **Riboflavin (Vitamin B2)** deficiency can cause **normocytic anemia**, but generally not megaloblastic anemia. - Its deficiency is mainly associated with **angular stomatitis, cheilosis**, and ocular symptoms. *Vitamin C* - **Vitamin C** deficiency (scurvy) is associated with impaired collagen synthesis, leading to **gingival bleeding, petechiae**, and poor wound healing. - While it can cause some anemia, it is typically **microcytic** due to impaired iron absorption if it affects iron metabolism, not megaloblastic [2]. *Niacin* - **Niacin (Vitamin B3)** deficiency causes **pellagra**, characterized by the "3 D's": **dermatitis, diarrhea, and dementia**. - It does not directly lead to megaloblastic anemia, as it is not involved in a critical step of DNA synthesis in the same way folate is.

Q: Which coenzyme is essential for pyruvate carboxylase activity?

Biotin. ***Biotin*** - **Biotin** (vitamin B7) is a crucial coenzyme for **carboxylase enzymes**, including **pyruvate carboxylase**. - It acts as a **CO2 carrier**, facilitating the carboxylation of pyruvate to **oxaloacetate** in gluconeogenesis. *Thiamine* - **Thiamine pyrophosphate** is a coenzyme for enzymes involved in **decarboxylation reactions**, such as pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase. - It is not directly involved in the carboxylation activity of pyruvate carboxylase. *NAD+* - **NAD+** (nicotinamide adenine dinucleotide) is a coenzyme primarily involved in **redox reactions** as an electron acceptor. - While important in many metabolic pathways, it does not function as a coenzyme for carboxylase activity. *CoA* - **Coenzyme A** is essential for carrying acyl groups, particularly in the formation of **acetyl-CoA** which enters the citric acid cycle. - Although acetyl-CoA is an allosteric activator of pyruvate carboxylase, CoA itself is not a coenzyme for its catalytic activity.

Q: Due to which of the following enzyme deficiency, vitamin C cannot be synthesised in humans?

L-Gulonolactone oxidase. ***L-Gulonolactone oxidase*** - This enzyme is **deficient in humans**, guinea pigs, and some primates, preventing the conversion of **L-gulono-γ-lactone to L-ascorbic acid (vitamin C)**, the final step in vitamin C synthesis. - The absence of functional L-gulonolactone oxidase (due to a mutation in the GULO gene) makes **ascorbic acid (vitamin C)** an essential dietary nutrient for humans. *L-Gulonolactone reductase* - This enzyme is not part of the vitamin C synthesis pathway and its deficiency is not the reason for the inability to synthesize vitamin C in humans. - Reductase enzymes typically add electrons/hydrogen, whereas the critical missing step requires an oxidase. *L-Glucuronic acid oxidase* - This enzyme is part of the **glucuronic acid pathway**, but its deficiency does not directly impair the synthesis of vitamin C. - The glucuronic acid pathway primarily detoxifies xenobiotics and endogenous compounds and contributes to **UDP-glucuronic acid** synthesis. *L-Gulonic acid reductase* - This enzyme catalyzes the conversion of **L-gulonate to L-gulonolactone**, a step *prior* to the one blocked in humans through the deficiency of L-gulonolactone oxidase. - While it is part of the overall pathway, its deficiency is not the ultimate reason humans cannot synthesize vitamin C; rather, it is the inability to convert L-gulono-γ-lactone to ascorbic acid in the final oxidation step.

Q: Which vitamin is required for glycogen phosphorylase?

PLP. ***PLP*** - **Pyridoxal phosphate (PLP)**, a derivative of vitamin **B6**, acts as a crucial coenzyme for **glycogen phosphorylase**. - It participates in the **phosphorolysis of α-1,4 glycosidic bonds** to release glucose-1-phosphate from glycogen. *Lipoic acid* - **Lipoic acid** is a coenzyme primarily involved in **oxidative decarboxylation** reactions, such as those catalyzed by pyruvate dehydrogenase and α-ketoglutarate dehydrogenase. - It plays no direct role as a coenzyme for **glycogen phosphorylase**. *TPP* - **Thiamine pyrophosphate (TPP)**, a derivative of vitamin **B1**, is essential for enzymes like **pyruvate dehydrogenase** and **α-ketoglutarate dehydrogenase**. - It is involved in transferring aldehyde groups, not in the phosphorolysis of glycogen. *Riboflavin* - **Riboflavin (vitamin B2)** is a precursor for **flavin adenine dinucleotide (FAD)** and **flavin mononucleotide (FMN)**, which are crucial for redox reactions in intermediary metabolism. - It is not a direct coenzyme for **glycogen phosphorylase**.

Q: Which nutrient is essential for collagen synthesis?

Vitamin C. ***Vitamin C*** - **Vitamin C (ascorbic acid)** is absolutely essential as a cofactor for **prolyl hydroxylase and lysyl hydroxylase** enzymes - These enzymes hydroxylate proline and lysine residues in procollagen, which is critical for **stable collagen triple helix formation** - Deficiency causes **scurvy** with defective collagen leading to bleeding gums, poor wound healing, and perifollicular hemorrhages - **Copper** (as a cofactor for lysyl oxidase) is also important for collagen cross-linking but Vitamin C is the primary essential nutrient for synthesis *Vitamin D* - Primarily involved in **calcium and phosphate metabolism** and bone mineralization - Deficiency leads to **rickets** (children) and **osteomalacia** (adults), not defective collagen - No direct role in collagen synthesis pathways *Folic acid* - Essential for **DNA synthesis, cell division**, and red blood cell formation - Plays no direct role in the **enzymatic mechanisms of collagen formation** - Deficiency causes megaloblastic anemia *Vitamin A* - Important for **vision, immune function**, epithelial integrity, and cell differentiation - While it influences tissue health, it is not a direct cofactor for **collagen synthesis enzymes**

Question 1

Thiamine deficiency is assessed by:

Accepted Answer

Erythrocyte transketolase activity

Answer

Serum thiamine level

Answer

RBC thiamine levels

Answer

RBC Glutathione reductase

Question 2

Vitamin K in its coenzyme form is regenerated by which enzyme?

Accepted Answer

Epoxide reductase

Answer

Glutathione reductase

Answer

Dihydrofolate reductase

Answer

Pyruvate carboxylase

Question 3

Which vitamin deficiency causes peripheral neuropathy first?

Accepted Answer

Vitamin B12

Answer

Vitamin B1

Answer

Vitamin B6

Answer

Vitamin E

Question 4

The transfer of amino groups between amino acids and α-keto acids requires which coenzyme?

Accepted Answer

PLP

Answer

FAD

Answer

Thiamine

Answer

NAD+

Question 5

All of the following are true about Vitamin K EXCEPT:

Accepted Answer

Synthesized in large amounts by the liver

Answer

Requires bile salts for absorption

Answer

Functions in carboxylation reactions

Answer

Essential for blood clotting

Question 6

Which vitamin deficiency leads to megaloblastic anemia?

Accepted Answer

Folate

Answer

Riboflavin

Answer

Vitamin C

Answer

Niacin

Question 7

Which coenzyme is essential for pyruvate carboxylase activity?

Accepted Answer

Biotin

Answer

Thiamine

Answer

NAD+

Answer

CoA

Question 8

Due to which of the following enzyme deficiency, vitamin C cannot be synthesised in humans?

Accepted Answer

L-Gulonolactone oxidase

Answer

L-Gulonolactone reductase

Answer

L-Glucuronic acid oxidase

Answer

L-Gulonic acid reductase

Question 9

Which vitamin is required for glycogen phosphorylase?

Accepted Answer

PLP

Answer

Lipoic acid

Answer

TPP

Answer

Riboflavin

Question 10

Which nutrient is essential for collagen synthesis?

Accepted Answer

Vitamin C

Answer

Vitamin D

Answer

Folic acid

Answer

Vitamin A

Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes — MCQs

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