Amino Acid Metabolism — MCQs
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Of the following amino acids required by the human body, which one falls under the category of 'essential' amino acid?
In a patient with maple syrup urine disease, all of the following amino acids should be restricted in diet except?
Ammonia formed in the brain is converted into
Atherosclerosis is associated with:
Melanin is derived from which amino acid?
All are involved in non-toxic transport of ammonia except:
Nitric oxide is synthesised from:
Positive nitrogen balance is seen in -
Which of the following amino acids has maximum buffering capacity at pH 7?
Acetylcholine is the neurotransmitter in the cholinergic neurons. What is the precursor that acetylcholine is derived from?
Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs
Question 441: Of the following amino acids required by the human body, which one falls under the category of 'essential' amino acid?
- A. Arginine
- B. Threonine (Correct Answer)
- C. Proline
- D. Serine
Explanation: ***Threonine*** - **Threonine** is one of the nine **essential amino acids** that the human body cannot synthesize on its own. - It must be obtained through the **diet** to support protein synthesis and other vital bodily functions. *Arginine* - **Arginine** is generally considered a **conditionally essential** or **semi-essential amino acid**. - While the body can synthesize it, requirements may increase during periods of rapid growth, illness, or stress, making dietary intake important in those situations. *Proline* - **Proline** is a **non-essential amino acid**, meaning the human body can synthesize it from other precursors. - It plays a crucial role in the structure of **collagen** and connective tissue. *Serine* - **Serine** is also a **non-essential amino acid** that can be synthesized by the human body. - It is involved in the metabolism of fats, fatty acids, and the synthesis of various other biomolecules.
Question 442: In a patient with maple syrup urine disease, all of the following amino acids should be restricted in diet except?
- A. Methionine (Correct Answer)
- B. Isoleucine
- C. Leucine
- D. Valine
Explanation: ***Methionine*** - **Maple syrup urine disease (MSUD)** is a disorder affecting the metabolism of **branched-chain amino acids (BCAAs)**: leucine, isoleucine, and valine. - Therefore, methionine, which is not a BCAA, typically does not need to be restricted and is, in fact, an **essential amino acid** crucial for protein synthesis. *Isoleucine* - **Isoleucine** is a branched-chain amino acid (BCAA) whose metabolism is impaired in MSUD due to a deficiency in **branched-chain alpha-keto acid dehydrogenase complex**. - Accumulation of isoleucine and its corresponding alpha-keto acid is toxic and must be **restricted in the diet**. *Leucine* - **Leucine** is another branched-chain amino acid (BCAA) that cannot be properly metabolized in MSUD. - High levels of leucine and its metabolites are particularly **neurotoxic** and contribute to the characteristic neurological symptoms, necessitating strict dietary restriction. *Valine* - **Valine** is the third branched-chain amino acid (BCAA) whose breakdown is defective in MSUD. - Dietary restriction of valine is essential to prevent its accumulation, which can lead to metabolic crises and **developmental delays**.
Question 443: Ammonia formed in the brain is converted into
- A. Glycine
- B. Urea
- C. Cysteine
- D. Glutamine (Correct Answer)
Explanation: ***Glutamine*** - **Ammonia** is detoxified in the brain by combining with **glutamate** to form **glutamine** via the enzyme **glutamine synthetase**. - This conversion is crucial because **glutamine** is non-toxic and can be safely transported out of the brain to the liver for further processing. *Glycine* - **Glycine** is an amino acid that can function as a neurotransmitter, but it is not the primary product of ammonia detoxification in the brain. - While it can be synthesized in the brain, it does not serve as the molecule to which toxic ammonia is directly converted for transport. *Urea* - **Urea** is the primary end-product of ammonia detoxification in the **liver** through the **urea cycle**. - The brain lacks the complete set of enzymes required for the **urea cycle**, so it cannot convert ammonia into urea. *Cysteine* - **Cysteine** is a sulfur-containing amino acid involved in protein synthesis and antioxidant defense, but it is not directly involved in the detoxification pathway of ammonia in the brain. - Its synthesis and metabolism are distinct from the process of ammonia sequestration.
Question 444: Atherosclerosis is associated with:
- A. Histidine
- B. Lysine
- C. Homocysteine (Correct Answer)
- D. Leucine
Explanation: ***Homocysteine*** - Elevated plasma levels of **homocysteine**, known as hyperhomocysteinemia, are an independent **risk factor for atherosclerosis** and cardiovascular disease. - While the precise mechanism is complex, homocysteine is thought to promote endothelial damage, oxidative stress, and lipid peroxidation, contributing to **plaque formation**. *Histidine* - **Histidine** is an essential amino acid involved in various metabolic processes, including the synthesis of **histamine** and carnosine. - It is not directly associated with the pathogenesis or risk factors for atherosclerosis. *Lysine* - **Lysine** is an essential amino acid important for **protein synthesis** and many bodily functions. - There is no established direct link between lysine levels and the development or progression of atherosclerosis. *Leucine* - **Leucine** is a branched-chain amino acid (BCAA) crucial for **muscle protein synthesis** and energy metabolism. - While some studies hint at associations between BCAAs and metabolic health, leucine itself is not a known direct risk factor or etiologic agent for atherosclerosis.
Question 445: 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 446: 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 447: Nitric oxide is synthesised from:
- A. Cysteine
- B. Arginine (Correct Answer)
- C. Cystine
- D. Methionine
Explanation: ***Arginine*** - **Nitric oxide synthase (NOS)** enzymes catalyze the five-electron oxidation of **L-arginine** to L-citrulline and **nitric oxide (NO)**. - This reaction requires molecular oxygen (O2) and reduced nicotinamide adenine dinucleotide phosphate (**NADPH**) as co-substrates. *Cysteine* - **Cysteine** is a sulfur-containing amino acid involved in protein structure and antioxidant defense. - It is a precursor for **glutathione** and plays a role in **sulfur metabolism**, but not directly in NO synthesis. *Cystine* - **Cystine** is a dimer formed from two cysteine molecules linked by a disulfide bond. - It does not serve as a substrate for nitric oxide synthesis. *Methionine* - **Methionine** is an essential amino acid important for protein synthesis and as a precursor for **S-adenosylmethionine (SAM)**, a methyl donor. - It is not involved in the direct synthesis of nitric oxide.
Question 448: Positive nitrogen balance is seen in -
- A. Scurvy
- B. Growing children (Correct Answer)
- C. Osteomalacia
- D. Acute illness
Explanation: ***Growing children*** - **Positive nitrogen balance** occurs when nitrogen intake (from protein) exceeds nitrogen excretion, indicating net protein synthesis and tissue anabolism. - This state is essential for **growth and development** as children build new tissues like muscle, bone, and organs, requiring more protein synthesis than breakdown. *Scurvy* - Scurvy is a disease caused by **vitamin C deficiency**, which is crucial for collagen synthesis. - It leads to impaired wound healing, weakened connective tissue, and is not directly associated with positive nitrogen balance. *Osteomalacia* - Osteomalacia is characterized by **softening of bones** due to impaired mineralization, often caused by **vitamin D deficiency**. - It relates to bone metabolism rather than overall protein synthesis or nitrogen balance. *Acute illness* - Acute illness, especially severe illness or trauma, typically leads to a **negative nitrogen balance**. - This occurs due to increased **catabolism** (protein breakdown) to provide energy and amino acids for the acute stress response and inflammation.
Question 449: Which of the following amino acids has maximum buffering capacity at pH 7?
- A. Arginine
- B. Lysine
- C. Histidine (Correct Answer)
- D. Glycine
Explanation: ***Histidine*** - Histidine has a side chain with a **pKa** of approximately **6.0**, which is close to physiological pH (7.4). - This proximity allows histidine to effectively **donate and accept protons** at pH 7, thus providing significant buffering capacity. *Arginine* - Arginine has a side chain with a very **high pKa** of approximately 12.5, making it a strong base. - It would be ineffective as a buffer at pH 7 because it would exist almost entirely in its **protonated form**. *Lysine* - Lysine has a side chain with a **pKa** of approximately 10.5, making it a strong base. - At pH 7, it would also be almost completely **protonated** and therefore have very limited buffering capacity. *Glycine* - Glycine is the simplest amino acid with **no ionizable side chain** (only a hydrogen atom as its R group). - Its buffering capacity at pH 7 relies solely on its **amino and carboxyl groups**, which have pKa values far from 7 (around 2.3 and 9.6, respectively), making it a poor buffer in this range.
Question 450: Acetylcholine is the neurotransmitter in the cholinergic neurons. What is the precursor that acetylcholine is derived from?
- A. Choline (Correct Answer)
- B. Dicholine
- C. Tyrosine
- D. Cholic acid
Explanation: ***Choline*** - **Acetylcholine** is synthesized from **choline** and acetyl-CoA by the enzyme **choline acetyltransferase** in the cytoplasm of cholinergic neurons. - Choline is an essential nutrient obtained from the diet or synthesized endogenously, and it is actively transported into the neuron. *Dicholine* - **Dicholine** is not a naturally occurring biological precursor for acetylcholine synthesis. - This term does not refer to a standard biochemical compound involved in neurotransmitter synthesis. *Tyrosine* - **Tyrosine** is the precursor for **catecholamine neurotransmitters** such as dopamine, norepinephrine, and epinephrine, not acetylcholine. - The synthesis pathway for catecholamines involves enzymes like **tyrosine hydroxylase**, distinct from the acetylcholine pathway. *Cholic acid* - **Cholic acid** is a primary **bile acid** produced by the liver, involved in fat digestion and absorption in the small intestine. - It has no role in the synthesis of acetylcholine or other neurotransmitters.
Practice by Chapter
Protein Digestion and Absorption
Practice Questions
Transamination and Deamination
Practice Questions
Urea Cycle
Practice Questions
Disorders of Urea Cycle
Practice Questions
Metabolism of Individual Amino Acids
Practice Questions
Inborn Errors of Amino Acid Metabolism
Practice Questions
Phenylketonuria and Alkaptonuria
Practice Questions
Homocystinuria and Methionine Metabolism
Practice Questions
Synthesis of Biologically Important Compounds from Amino Acids
Practice Questions
Nitrogen Balance
Practice Questions
Ammonia Metabolism and Toxicity
Practice Questions
One-Carbon Transfer Reactions
Practice Questions
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