Synthesis of Biologically Important Compounds from Amino Acids — MCQs
Acute intermittent porphyria is due to deficiency of?
Most important amino acid which acts as a methyl group donor?
A child was fed on a staple diet of maize for a long time. Which of the following vitamin may get deficient in his body?
A 32-year-old male is on a weight-maintenance diet, so he does not want to lose or gain any weight. Which amino acid must be present in the diet to prevent the patient from going into a negative nitrogen balance?
All are cofactors for Dehydrogenase except:
In the first step of heme synthesis, what is required?
Melanin is derived from which amino acid?
Which amino acid is involved in the synthesis of creatinine, NO, and urea?
Pyridoxine deficiency leads to altered metabolism of?
Creatinine is formed from -
Synthesis of Biologically Important Compounds from Amino Acids Indian Medical PG Practice Questions and MCQs
Question 1: Acute intermittent porphyria is due to deficiency of?
- A. Porphobilinogen deaminase (Correct Answer)
- B. Uroporphyrinogen III synthase
- C. Ferrochelatase
- D. ALA synthase
Explanation: ***Porphobilinogen deaminase*** - **Acute intermittent porphyria (AIP)** results from a deficiency in **porphobilinogen deaminase** (also known as hydroxymethylbilane synthase). - This enzyme deficiency leads to the accumulation of **aminolevulinic acid (ALA)** and **porphobilinogen (PBG)**, which are neurotoxic and cause the characteristic symptoms of AIP. *Uroporphyrinogen III synthase* - A deficiency in **uroporphyrinogen III synthase** causes **congenital erythropoietic porphyria (Günther disease)**, which is characterized by severe photosensitivity and hemolytic anemia. - This enzyme defect leads to the accumulation of uroporphyrinogen I and coproporphyrinogen I, not the ALA and PBG associated with AIP. *Ferrochelatase* - Deficiency in **ferrochelatase** causes **erythropoietic protoporphyria (EPP)**, which presents with photosensitivity and chronic liver disease due to the accumulation of **protoporphyrin**. - This condition does not cause the acute neurological attacks seen in AIP. *ALA synthase* - **ALA synthase** is the **rate-limiting enzyme** in heme synthesis; while its activity is crucial, a congenital *deficiency* is not the cause of AIP. - Instead, the *upregulation* of ALA synthase activity in AIP (due to the PBG deaminase block) contributes to the accumulation of ALA and PBG.
Question 2: Most important amino acid which acts as a methyl group donor?
- A. Tryptophan
- B. Methionine (Correct Answer)
- C. Cysteine
- D. Tyrosine
Explanation: ***Methionine*** - **Methionine** is the precursor to **S-adenosylmethionine (SAM)**, which serves as the primary and most important **methyl group donor** in various biochemical reactions, including DNA methylation and neurotransmitter synthesis. - The methyl group of methionine is transferred to a wide range of acceptors via SAM, playing a crucial role in metabolism and gene regulation. *Tryptophan* - **Tryptophan** is an essential amino acid primarily known as a precursor for the synthesis of **serotonin** and **niacin**. - While it has various metabolic roles, it does not directly act as a significant methyl group donor. *Cysteine* - **Cysteine** is important for maintaining protein structure through **disulfide bonds** and is a precursor for **glutathione** synthesis. - It contains a sulfur-containing thiol group but does not donate methyl groups. *Tyrosine* - **Tyrosine** is a non-essential amino acid that is a precursor for **thyroid hormones**, **catecholamines** (dopamine, norepinephrine, epinephrine), and **melanin**. - It does not function as a methyl group donor in biochemical pathways.
Question 3: A child was fed on a staple diet of maize for a long time. Which of the following vitamin may get deficient in his body?
- A. Thiamine (Vitamin B1)
- B. Vitamin B6 (Pyridoxine)
- C. Cobalamin (Vitamin B12)
- D. Niacin (Vitamin B3) (Correct Answer)
Explanation: ***Niacin (Vitamin B3)*** - Maize is deficient in tryptophan (an amino acid that can be converted to **niacin**) and contains niacin in a bound form (**niacytin**) that is not bioavailable. - A staple diet of maize without adequate supplementation can lead to **pellagra**, characterized by the classic triad of dermatitis, diarrhea, and dementia (3 D's). - This is particularly common in populations relying heavily on untreated maize as a staple food. *Thiamine (Vitamin B1)* - While polishing rice can remove thiamine, maize itself is not primarily associated with **thiamine deficiency** as a staple. - **Beriberi** (thiamine deficiency) presents with neurological and cardiovascular symptoms (wet and dry beriberi), distinct from pellagra. *Vitamin B6 (Pyridoxine)* - Deficiency of **pyridoxine** is uncommon with maize-based diets unless there are other contributing factors like drug interactions (e.g., isoniazid). - Symptoms include peripheral neuropathy, seborrheic dermatitis, and sideroblastic anemia, which are not directly linked to a maize staple diet. *Cobalamin (Vitamin B12)* - **Vitamin B12** is found primarily in animal products, so a vegetarian or vegan diet poses a risk for deficiency, not specifically a maize-based diet. - Deficiency leads to megaloblastic anemia and neurological damage (subacute combined degeneration), unrelated to maize's nutritional profile.
Question 4: A 32-year-old male is on a weight-maintenance diet, so he does not want to lose or gain any weight. Which amino acid must be present in the diet to prevent the patient from going into a negative nitrogen balance?
- A. Alanine
- B. Arginine
- C. Glycine
- D. Threonine (Correct Answer)
Explanation: ***Threonine*** - **Threonine** is an **essential amino acid**, meaning the body cannot synthesize it and it must be obtained from the diet. - To maintain a **neutral nitrogen balance** and prevent a **negative nitrogen balance** (loss of body protein), all essential amino acids, including threonine, must be supplied in adequate amounts. *Alanine* - **Alanine** is a **non-essential amino acid**, which means the body can synthesize it from other compounds; therefore, its absence from the diet would not directly cause a negative nitrogen balance. - It plays a significant role in **gluconeogenesis** and the **glucose-alanine cycle**. *Arginine* - **Arginine** is considered a **conditionally essential amino acid**, meaning it can be synthesized by the body, but sometimes not in sufficient amounts to meet needs (e.g., during rapid growth, illness, or trauma). - Under normal weight-maintenance conditions, the body can typically synthesize enough arginine. *Glycine* - **Glycine** is a **non-essential amino acid** and is the smallest amino acid, often easily synthesized by the body. - Its presence in the diet, while important, is not critical for preventing negative nitrogen balance because the body can produce it.
Question 5: All are cofactors for Dehydrogenase except:
- A. SAM (Correct Answer)
- B. NADP
- C. NAD
- D. FAD
Explanation: ***SAM*** - **S-adenosylmethionine (SAM)** is a cofactor involved in **methyl group transfer reactions**, carried out by enzymes known as methyltransferases. - Dehydrogenase enzymes catalyze **redox reactions**, typically involving the transfer of hydride ions, and thus do not utilize SAM as a cofactor. *NADP* - **Nicotinamide adenine dinucleotide phosphate (NADP)** is a crucial coenzyme for many **dehydrogenase reactions**, particularly in **anabolic pathways** like fatty acid synthesis and the pentose phosphate pathway. - It acts as an **electron carrier**, accepting or donating hydride ions. *NAD* - **Nicotinamide adenine dinucleotide (NAD)** is a highly common coenzyme for numerous **dehydrogenase enzymes**, especially in **catabolic pathways** such as glycolysis, the Krebs cycle, and oxidative phosphorylation. - It functions as an **electron acceptor** or donor in redox reactions. *FAD* - **Flavin adenine dinucleotide (FAD)** is a coenzyme derived from **riboflavin (Vitamin B2)** and is associated with various dehydrogenase enzymes, particularly those involved in **electron transport** and fatty acid oxidation. - FAD can accept two hydrogen atoms (one hydride and one proton) to become FADH₂.
Question 6: In the first step of heme synthesis, what is required?
- A. Fe
- B. Folate
- C. Glycine (Correct Answer)
- D. Histidine
Explanation: ***Glycine*** - The first and **rate-limiting step** in heme synthesis involves the condensation of **succinyl CoA** and **glycine**. - This reaction is catalyzed by **ALA synthase (aminolevulinate synthase)**, forming **δ-aminolevulinate (ALA)**. *Fe* - **Iron (Fe)** is incorporated into the protoporphyrin ring in the **final step** of heme synthesis, catalyzed by **ferrochelatase**. - Its presence is crucial for the functional heme molecule, but it is not a requirement for the initial condensation reaction. *Folate* - **Folate (vitamin B9)** is essential for the synthesis of nucleotides and various methylation reactions, but it is **not directly involved** in the heme synthesis pathway. - Deficiency can lead to **macrocytic anemia**, but not due to a direct defect in heme synthesis. *Histidine* - **Histidine** is an amino acid that plays a role in various protein structures and enzyme functions, but it is **not a precursor** for heme synthesis. - It is not directly consumed or required in the initial step of the heme biosynthesis pathway.
Question 7: Melanin is derived from which amino acid?
- A. Tyrosine (Correct Answer)
- B. Phenylalanine
- C. Alanine
- D. Tryptophan
Explanation: ***Tyrosine*** - **Melanin synthesis** (melanogenesis) begins with the hydroxylation of **tyrosine** to DOPA (dihydroxyphenylalanine), catalyzed by the enzyme **tyrosinase**. - Subsequent steps involve the oxidation of DOPA and its derivatives to form various types of melanin, such as **eumelanin** (black/brown) and **pheomelanin** (red/yellow). *Phenylalanine* - **Phenylalanine** is an essential amino acid that can be converted to tyrosine in the body by the enzyme **phenylalanine hydroxylase**. - While it's a precursor to tyrosine, it's not directly converted to melanin; rather, it must first be hydroxylated to tyrosine. *Alanine* - **Alanine** is a non-essential amino acid involved in glucose metabolism and protein synthesis. - It is not a precursor for melanin synthesis. *Tryptophan* - **Tryptophan** is an essential amino acid that is a precursor for **serotonin**, **melatonin**, and **niacin** (Vitamin B3). - It is not involved in the biosynthesis of melanin.
Question 8: Which amino acid is involved in the synthesis of creatinine, NO, and urea?
- A. Arginine (Correct Answer)
- B. Glycine
- C. Aspartate
- D. Citrulline
Explanation: ***Arginine*** - **Arginine** is a precursor for **Nitric Oxide (NO)** synthesis via **nitric oxide synthase**. - It is also a substrate for **creatinine** synthesis (along with glycine and methionine) and plays a key role in the **urea cycle**. *Glycine* - **Glycine** is a precursor for **creatinine** synthesis, but not directly involved in NO or urea production as the primary amino acid. - It is also a component of glutathione and purine synthesis. *Aspartate* - **Aspartate** is a key intermediate in the **urea cycle**, contributing one nitrogen atom to urea. - It is not directly a precursor for NO or creatinine. *Citrulline* - **Citrulline** is an intermediate in the **urea cycle** and is converted to arginine. - It is not directly involved in the synthesis of creatinine or as the primary precursor for NO.
Question 9: 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 10: Creatinine is formed from -
- A. Creatine (Correct Answer)
- B. Lysine
- C. Leucine
- D. Histidine
Explanation: ***Creatine*** - **Creatinine** is a waste product formed from the non-enzymatic, irreversible degradation of **creatine** and **creatine phosphate**, primarily in muscles. - **Creatine** itself is synthesized endogenously from three amino acids: **glycine, arginine, and methionine** (as S-adenosylmethionine) through a two-step enzymatic process in the kidney and liver. - The amount of creatinine produced daily is relatively constant and directly proportional to an individual's **muscle mass**, making it a useful marker for renal function. *Lysine* - **Lysine** is an **essential amino acid** and a precursor for various compounds like **carnitine** but is not involved in creatinine or creatine formation. - Deficiency can lead to impaired protein synthesis but does not impact creatinine levels. *Leucine* - **Leucine** is another **essential amino acid** and a **branched-chain amino acid (BCAA)** crucial for muscle protein synthesis and repair. - It does not serve as a direct precursor for creatinine or creatine. *Histidine* - **Histidine** is an **essential amino acid** and a precursor for **histamine** and other important compounds, but not creatinine or creatine. - It plays roles in immune response and gastric acid secretion.
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