Thiamine (B1) and Pyruvate Dehydrogenase — MCQs
All of the following amino acids form Acetyl CoA via pyruvate dehydrogenase except:
Which coenzyme is not required in the formation of glutamate?
Thiamine deficiency is known to occur in all of the following except.
Which reaction requires Vitamin B1?
Which of the following substances does not inhibit glycolysis?
In which condition is the utilization of pyruvate in tissues decreased?
Which enzyme level is tested in thiamine deficiency?
Riboflavin deficiency is assessed by?
In glycolysis, which of the following enzymes is not involved?
Wernicke's encephalopathy is due to deficiency of:
Thiamine (B1) and Pyruvate Dehydrogenase Indian Medical PG Practice Questions and MCQs
Question 1: All of the following amino acids form Acetyl CoA via pyruvate dehydrogenase except:
- A. Glycine
- B. Alanine
- C. Hydroxyproline
- D. Tyrosine (Correct Answer)
Explanation: ***Tyrosine*** - Tyrosine is both a **glucogenic and ketogenic amino acid** that is broken down into **fumarate (glucogenic) and acetoacetate (ketogenic)**. - It does not form pyruvate as an intermediate in its degradation pathway. - Therefore, it does NOT pass through the pyruvate dehydrogenase complex to form acetyl-CoA. - The acetyl-CoA from tyrosine comes from acetoacetate breakdown, not via pyruvate dehydrogenase. *Glycine* - Glycine can be converted to **serine**, which is then converted to **pyruvate**. - Pyruvate is then converted to acetyl-CoA by the **pyruvate dehydrogenase complex**. - Therefore, glycine forms acetyl-CoA via pyruvate dehydrogenase. *Hydroxyproline* - Hydroxyproline is metabolized through several intermediates and can eventually form **pyruvate**. - Once converted to pyruvate, it can then be converted to acetyl-CoA via **pyruvate dehydrogenase**. - Therefore, hydroxyproline can form acetyl-CoA via this pathway. *Alanine* - Alanine is directly converted to pyruvate through a **transamination reaction** catalyzed by alanine aminotransferase (ALT). - Pyruvate is then converted to acetyl-CoA by the **pyruvate dehydrogenase complex**. - This is one of the most direct pathways from amino acid to acetyl-CoA via pyruvate dehydrogenase.
Question 2: Which coenzyme is not required in the formation of glutamate?
- A. None of the above
- B. Pyridoxal phosphate
- C. Thiamine pyrophosphate (Correct Answer)
- D. Niacin
Explanation: ***Thiamine pyrophosphate*** - **Thiamine pyrophosphate (TPP)** is a coenzyme derived from **vitamin B1** that is essential for reactions involving decarboxylation, such as those catalyzed by pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase. - The formation of glutamate primarily involves transamination or reductive amination, which do not require TPP. *Pyridoxal phosphate* - **Pyridoxal phosphate (PLP)**, derived from **vitamin B6**, is a crucial coenzyme for **transamination reactions**, which are a major pathway for glutamate synthesis (e.g., from alpha-ketoglutarate). - It also plays a role in decarboxylation and deamination reactions of amino acids. *Niacin* - **Niacin (vitamin B3)** is a precursor for **NAD+** and **NADP+**, which are essential coenzymes in many metabolic pathways. - **NADPH**, derived from NADP+, is required as a reductant in the **reductive amination** of **alpha-ketoglutarate** to form glutamate, catalyzed by glutamate dehydrogenase. *None of the above* - This option is incorrect because **thiamine pyrophosphate** is indeed not required for the formation of glutamate. - The other two coenzymes, **pyridoxal phosphate** and **niacin (as NAD(P)H)**, are involved in glutamate synthesis.
Question 3: Thiamine deficiency is known to occur in all of the following except.
- A. Food faddist
- B. Chronic heart failure
- C. Homocystinemia (Correct Answer)
- D. Chronic alcoholic
Explanation: **Homocystinemia** - **Homocystinemia** is a metabolic disorder characterized by elevated levels of **homocysteine**, usually due to deficiencies in enzymes involved in methionine metabolism, not thiamine. - This condition is typically associated with deficiencies in vitamins **B6, B12, or folate**, which are cofactors for enzymes like cystathionine beta-synthase and methionine synthase. *Food faddist* - Individuals with highly restrictive or **unbalanced diets** may exclude thiamine-rich foods, leading to deficiency. - This dietary pattern can result in an inadequate intake of essential nutrients, including **thiamine**. *Chronic heart failure* - Patients with **chronic heart failure** are at increased risk for **thiamine deficiency** due to diuretic use, which can increase urinary excretion of water-soluble vitamins. - **Reduced absorption** in the gastrointestinal tract and **poor dietary intake** are also contributing factors in these patients. *Chronic alcoholic* - **Chronic alcohol abuse** is a major cause of thiamine deficiency due to **poor nutrition**, **impaired absorption** of thiamine, and **increased metabolic demand**. - Alcohol can directly interfere with thiamine transport and phosphorylation, leading to **Wernicke-Korsakoff syndrome** and other neurological complications.
Question 4: Which reaction requires Vitamin B1?
- A. None of the options
- B. Oxidative decarboxylation (Correct Answer)
- C. Carboxylation
- D. Transamination
Explanation: ***Oxidative decarboxylation*** - Vitamin B1, in its active form **thiamine pyrophosphate (TPP)**, is a crucial coenzyme for enzymes catalyzing **oxidative decarboxylation** reactions. - Key examples include the **pyruvate dehydrogenase complex** and **alpha-ketoglutarate dehydrogenase complex**, essential for cellular respiration and the citric acid cycle. *Transamination* - This type of reaction, involving the transfer of an **amino group**, primarily requires **pyridoxal phosphate (PLP)**, the active form of **Vitamin B6**. - It is vital for amino acid metabolism but does not utilize Vitamin B1. *Carboxylation* - **Carboxylation** reactions, which add a carboxyl group to a substrate, typically require **biotin** (Vitamin B7) as a coenzyme. - Examples include pyruvate carboxylase and acetyl-CoA carboxylase, which are not dependent on Vitamin B1. *None of the options* - As **oxidative decarboxylation** specifically requires Vitamin B1, this option is incorrect. - The other listed reactions depend on different vitamins as coenzymes.
Question 5: Which of the following substances does not inhibit glycolysis?
- A. Fluoride
- B. Arsenite
- C. Iodoacetate
- D. Fluoroacetate (Correct Answer)
Explanation: ***Fluoroacetate*** - **Fluoroacetate** is not a direct inhibitor of glycolysis. Instead, it is metabolized to **fluorocitrate**, which then acts as an inhibitor of **aconitase** in the **Krebs cycle (TCA cycle)**, thereby affecting cellular respiration at a later stage. - Its primary role in metabolic inhibition is within the **mitochondria**, impacting energy production via the TCA cycle rather than the glycolytic pathway. *Fluoride* - **Fluoride** is a known inhibitor of **enolase**, an enzyme in the penultimate step of glycolysis. - It forms a complex with **magnesium** and **phosphate** to block the active site of enolase, preventing the conversion of 2-phosphoglycerate to phosphoenolpyruvate. *Arsenite* - **Arsenite** inhibits glycolysis by targeting enzymes containing **sulfhydryl (–SH) groups**, particularly **glyceraldehyde-3-phosphate dehydrogenase (GAPDH)**, a critical enzyme in the glycolytic pathway. - It also inhibits the **pyruvate dehydrogenase complex** (linking glycolysis to the TCA cycle) and TCA cycle enzymes like **α-ketoglutarate dehydrogenase**, thereby affecting multiple stages of cellular respiration. *Iodoacetate* - **Iodoacetate** is a potent inhibitor of the enzyme **glyceraldehyde-3-phosphate dehydrogenase (GAPDH)**. - It specifically alkylates the **cysteine residue** at the active site of GAPDH, preventing the conversion of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate, thereby blocking glycolysis.
Question 6: In which condition is the utilization of pyruvate in tissues decreased?
- A. Pernicious anemia
- B. Scurvy
- C. Beriberi (Correct Answer)
- D. Pellagra
Explanation: ***Beriberi*** - Beriberi is caused by **thiamine (vitamin B1) deficiency**, which is a crucial cofactor for the **pyruvate dehydrogenase complex (PDH)**. - A dysfunctional PDH enzyme leads to a decreased conversion of **pyruvate to acetyl-CoA**, thus **decreasing pyruvate utilization** and causing its accumulation. *Pernicious anemia* - This condition is caused by a deficiency in **vitamin B12 (cobalamin)**, typically due to a lack of intrinsic factor, leading to **megaloblastic anemia**. - While vitamin B12 is essential for various metabolic pathways, it does not directly impair the utilization of **pyruvate** by PDH. *Scurvy* - Scurvy results from **vitamin C (ascorbic acid) deficiency**, which is essential for collagen synthesis and acts as an antioxidant. - Vitamin C deficiency does not directly impact the activity of the **pyruvate dehydrogenase complex** or the utilization of pyruvate. *Pellagra* - Pellagra is caused by a deficiency in **niacin (vitamin B3)**, or its precursor, tryptophan. - Niacin is a component of **NAD+ and NADP+**, which are crucial coenzymes in many metabolic reactions, but its deficiency does not primarily manifest as decreased **pyruvate utilization**.
Question 7: Which enzyme level is tested in thiamine deficiency?
- A. Transketolase (Correct Answer)
- B. PDH
- C. Kinase
- D. Pyruvate kinase
Explanation: ***Transketolase*** - The activity of **transketolase** in red blood cells is a reliable biochemical indicator of **thiamine deficiency (vitamin B1)**. - Thiamine pyrophosphate (TPP), the active form of thiamine, is a critical coenzyme for transketolase in the **pentose phosphate pathway**. *PDH* - **Pyruvate dehydrogenase (PDH)** is an enzyme complex that uses thiamine pyrophosphate as a cofactor, but its activity is not typically measured directly for diagnosing thiamine deficiency. - While PDH function is impaired in thiamine deficiency, direct assessment of transketolase activity is the standard diagnostic test. *Kinase* - **Kinase** is a general term for an enzyme that catalyzes the transfer of a phosphate group from a high-energy phosphate-donating molecule (like ATP) to a specific substrate. - This general class of enzymes is not specifically tested for thiamine deficiency. *Pyruvate kinase* - **Pyruvate kinase** is a key enzyme in **glycolysis** that catalyzes the final step of the pathway, converting phosphoenolpyruvate to pyruvate. - Its activity is not directly related to thiamine metabolism or deficiency.
Question 8: Riboflavin deficiency is assessed by?
- A. None of the options
- B. Glutathione reductase activity (Correct Answer)
- C. Pyruvate dehydrogenase activity
- D. Transketolase activity
Explanation: ***Glutathione reductase activity*** - Riboflavin is a precursor to **flavin adenine dinucleotide (FAD)**, a coenzyme for **glutathione reductase**. - Reduced glutathione reductase activity and its activation coefficient in erythrocytes are reliable indicators of **riboflavin deficiency**. *Pyruvate dehydrogenase activity* - **Pyruvate dehydrogenase** complex requires **thiamine pyrophosphate** (from thiamine, vitamin B1), not riboflavin, as a coenzyme. - Its activity is used to assess **thiamine status**, not riboflavin. *None of the options* - This option is incorrect as **glutathione reductase activity** is a valid method for assessing riboflavin deficiency. - The other options provided target different vitamin deficiencies. *Transketolase activity* - **Transketolase** activity is specifically used to assess **thiamine (vitamin B1) status**. - It requires **thiamine pyrophosphate** as a coenzyme, which is derived from thiamine.
Question 9: In glycolysis, which of the following enzymes is not involved?
- A. Pyruvate dehydrogenase (Correct Answer)
- B. Phosphofructokinase
- C. Glucokinase
- D. Pyruvate kinase
Explanation: ***Pyruvate dehydrogenase*** - **Pyruvate dehydrogenase** is a mitochondrial enzyme complex that converts **pyruvate** to **acetyl-CoA** in the link reaction, which occurs after glycolysis and prepares for the citric acid cycle. - It is not directly involved in the ten-step glycolytic pathway itself, which converts glucose to pyruvate. *Phosphofructokinase* - **Phosphofructokinase-1 (PFK-1)** is a key regulatory enzyme in glycolysis, catalyzing the phosphorylation of **fructose-6-phosphate** to **fructose-1,6-bisphosphate**. - This step is often considered the **rate-limiting step** of glycolysis. *Glucokinase* - **Glucokinase**, located primarily in the liver and pancreatic beta cells, phosphorylates glucose to **glucose-6-phosphate** in the first step of glycolysis. - It has a high **Km** (low affinity) for glucose, allowing it to respond to high glucose concentrations. *Pyruvate kinase* - **Pyruvate kinase** catalyzes the final step of glycolysis, transferring a phosphate group from **phosphoenolpyruvate (PEP)** to ADP to form **ATP** and **pyruvate**. - This is one of the **irreversible** steps in glycolysis and a point of regulation.
Question 10: Wernicke's encephalopathy is due to deficiency of:
- A. B6
- B. Thiamine (Correct Answer)
- C. B12
- D. Niacin
Explanation: ***Thiamine*** - **Wernicke's encephalopathy** is a serious neurological disorder directly caused by a severe deficiency of **thiamine (vitamin B1)**. - Thiamine is crucial for **glucose metabolism** in the brain; its deficiency impairs energy production, leading to neuronal damage and the characteristic symptoms of confusion, ataxia, and ophthalmoplegia. *B6* - Deficiency of **vitamin B6 (pyridoxine)** can cause peripheral neuropathy, seizures, and microcytic anemia. - It is not the primary cause of the acute neurological syndrome seen in Wernicke's encephalopathy. *B12* - Deficiency of **vitamin B12 (cobalamin)** is associated with megaloblastic anemia and subacute combined degeneration of the spinal cord. - While it can cause neurological symptoms, they differ from the specific triad of Wernicke's encephalopathy. *Niacin* - Deficiency of **niacin (vitamin B3)** causes **pellagra**, characterized by dermatitis, diarrhea, and dementia. - Although it involves neurological symptoms (dementia), the presentation is distinct from Wernicke's encephalopathy.
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