Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 451: Because free ammonia in the blood is toxic to the body, it is transported in which of the following non-toxic forms?

A. Glutamine and urea
B. Alanine and glutamine (Correct Answer)
C. Phenylalanine and methionine
D. Histidine and urea

Explanation: ***Alanine and glutamine*** - **Glutamine** is the primary transporter of ammonia from most peripheral tissues to the liver and kidneys, where it can be safely released and processed. - **Alanine** transports ammonia from muscles to the liver via the glucose-alanine cycle, allowing for hepatic urea synthesis and glucose production. *Glutamine and urea* - While **glutamine** is a major ammonia transporter, **urea** is the *end product* of ammonia detoxification and is primarily synthesized in the liver for excretion. - Urea itself is not a transport form of ammonia *within* the blood before its synthesis; rather, it is the waste product. *Phenylalanine and methionine* - **Phenylalanine** and **methionine** are essential amino acids involved in protein synthesis and various metabolic pathways. - They do not serve as significant carriers for the transport of toxic free ammonia in the bloodstream. *Histidine and urea* - **Histidine** is an amino acid involved in protein synthesis and histamine production, but not primarily in ammonia transport. - As mentioned, **urea** is the final excretory form of detoxified ammonia, not a transport form within the blood.

Question 452: Guanidine group is present in

A. Tryptophan
B. Arginine (Correct Answer)
C. Proline
D. Tyrosine

Explanation: ***Arginine*** - Arginine is a basic amino acid characterized by a **guanidinium group** at the end of its side chain. - The **guanidinium group** consists of three nitrogen atoms bonded to a central carbon atom, giving it a high pKa and making it positively charged at physiological pH. *Tryptophan* - Tryptophan contains an **indole ring** in its side chain, an aromatic heterocyclic organic compound. - It is known for being a precursor to neurotransmitters like **serotonin** and melatonin. *Proline* - Proline is unique among amino acids due to its **cyclic structure**, where the side chain is bonded to both the alpha-amino group and the alpha-carbon. - This forms a **pyrrolidine ring**, which imposes structural constraints on protein folding. *Tyrosine* - Tyrosine contains a **phenol group** in its side chain, which is an aromatic ring with a hydroxyl group attached. - It plays a crucial role as a precursor to several important neurotransmitters like **dopamine**, norepinephrine, and epinephrine.

Question 453: A 20-year-old male presents with weight loss, heat intolerance, bilateral exophthalmos, a lid lag, sweating, and tachycardia. These symptoms are due to an increased production and secretion of a hormone that is derived from which one of the following?

A. Tryptophan
B. Dopamine
C. Cholesterol
D. Tyrosine (Correct Answer)

Explanation: ***Tyrosine*** - The symptoms described (weight loss, heat intolerance, exophthalmos, lid lag, sweating, tachycardia) are characteristic of **hyperthyroidism**, specifically **Graves' disease**. - Thyroid hormones (**T3 and T4**) are synthesized from the amino acid **tyrosine** through iodination of tyrosine residues on thyroglobulin in the thyroid gland. - This makes tyrosine the direct precursor for thyroid hormone synthesis. *Tryptophan* - **Tryptophan** is a precursor for the synthesis of **serotonin** and **melatonin**, neurotransmitters involved in mood regulation and sleep-wake cycles, not thyroid function. - Deficiency or altered metabolism of tryptophan is associated with conditions like **depression** or **carcinoid syndrome**, which have different clinical presentations. *Dopamine* - **Dopamine** is a catecholamine neurotransmitter, not a precursor for thyroid hormones. - While dopamine is synthesized from tyrosine (via the pathway: tyrosine → L-DOPA → dopamine), this is a completely separate biochemical pathway from thyroid hormone synthesis. - Symptoms of altered dopamine levels are associated with conditions like **Parkinson's disease** (dopamine deficiency) or **schizophrenia** (dopamine dysregulation), which do not match this patient's hyperthyroid presentation. *Cholesterol* - **Cholesterol** is the precursor for **steroid hormones** (glucocorticoids, mineralocorticoids, androgens, estrogens) and vitamin D, but not thyroid hormones. - Conditions related to altered cholesterol-derived hormone metabolism include various **endocrine disorders** involving adrenal or gonadal dysfunction, which present with different clinical features.

Question 454: Amino acid derived hormones are all except:

A. None.
B. Adrenal medullary hormone.
C. FSH. (Correct Answer)
D. Thyroid hormone.

Explanation: ***FSH (Follicle-Stimulating Hormone)*** - **FSH is a glycoprotein/peptide hormone**, NOT an amino acid-derived hormone - While all proteins are made from amino acid chains, **amino acid-derived hormones** specifically refers to hormones derived from **single modified amino acids** (tyrosine or tryptophan) - FSH belongs to the **peptide/protein hormone** class, which is a distinct category - **This is the correct answer** to this "all EXCEPT" question *Adrenal medullary hormone* - Refers to **epinephrine** and **norepinephrine** (catecholamines) - These are derived from the single amino acid **tyrosine** through enzymatic modification - Classic examples of **amino acid-derived hormones** *Thyroid hormone* - **Thyroxine (T4)** and **triiodothyronine (T3)** are derived from **tyrosine** - Formed by coupling of iodinated tyrosine residues on thyroglobulin - Clear examples of **amino acid-derived hormones** *None* - This would imply all listed options are amino acid-derived hormones - This is incorrect since FSH is a peptide hormone, not an amino acid-derived hormone

Question 455: Amino acid with its pKa value within physiological range is:

A. Histidine (Correct Answer)
B. Arginine
C. Glycine
D. Lysine

Explanation: ***Histidine*** - Histidine's side chain has a **pKa of 6.0**, which is very close to physiological pH (7.4), allowing it to act as both a **proton donor and acceptor** at physiological conditions. - This property makes histidine crucial for **enzyme catalysis** and maintaining **pH buffering** in various biological systems. *Arginine* - Arginine has a **pKa of 12.5**, which is significantly higher than physiological pH, meaning its side chain is almost always **protonated and positively charged** at physiological conditions. - Its high pKa makes it a strong base and less likely to buffer effectively within the physiological range. *Glycine* - Glycine is a **non-polar amino acid** with no ionizable side chain; its pKa values are for the **alpha-carboxyl** (around 2.34) and **alpha-amino** (around 9.60) groups. - While these groups are ionized at physiological pH, they are part of the peptide backbone in proteins and do not provide buffering capacity within the physiological range. *Lysine* - Lysine has a **pKa of 10.5**, which is also significantly higher than physiological pH, meaning its side chain is predominantly **protonated and positively charged** at this pH. - Like arginine, its high pKa makes it a strong base that does not readily buffer within the physiological pH range.

Question 456: BOAA toxin responsible for Neurolathyrism contains which amino acid?

A. Methionine
B. Alanine (Correct Answer)
C. Lysine
D. Tryptophan

Explanation: ***Alanine*** - BOAA (beta-N-oxalylamino-L-alanine), the neurotoxin responsible for neurolathyrism, is a non-protein amino acid that is a **derivative of alanine**. - Its chemical structure is characterized by an **oxalyl group attached to the amino group of alanine**. *Methionine* - Methionine is an essential amino acid with a **sulfur-containing side chain**, not found in BOAA. - It is involved in various metabolic processes like **methylation**, but not directly related to neurolathyrism. *Lysine* - Lysine is a basic essential amino acid with a **long side chain ending in an amino group**. - It plays a role in **protein synthesis** and **collagen formation**, but its structure is distinct from BOAA. *Tryptophan* - Tryptophan is an essential amino acid known for its **indole ring structure**, serving as a precursor for serotonin and niacin. - Its complex cyclic structure is **not related to the simpler oxalyl-alanine structure** of BOAA.

Question 457: All of the following are true about glutathione, except:

A. It is co-factor of various enzymes
B. It converts hemoglobin to methemoglobin (Correct Answer)
C. It is a tripeptide
D. It conjugates xenobiotics

Explanation: ***It converts hemoglobin to methemoglobin*** - Glutathione is a **reducing agent** that helps protect hemoglobin from oxidation, thus **preventing** the formation of methemoglobin. - **Methemoglobin** occurs when the iron in hemoglobin is oxidized from the ferrous (Fe2+) to the ferric (Fe3+) state, which is a process glutathione actively counters. *It is co-factor of various enzymes* - Glutathione serves as a crucial **co-factor** for several enzymes, including **glutathione peroxidase**, which plays a vital role in antioxidant defense. - It participates in various **detoxification reactions** and catalyzes the reduction of harmful reactive oxygen species. *It is a tripeptide* - Glutathione is indeed a **tripeptide** composed of three amino acids: **glutamate**, **cysteine**, and **glycine**. - Its unique structure enables its diverse biological functions, including its prominent role as an antioxidant. *It conjugates xenobiotics* - Glutathione plays a critical role in **detoxifying xenobiotics** (foreign compounds) by conjugating with them, making them more water-soluble and easier to excrete. - This process is mediated by **glutathione S-transferases**, which attach glutathione to various toxic compounds.

Question 458: Ammonia in brain is trapped by

A. Alanine
B. Aspartate
C. Glutamine (Correct Answer)
D. Ornithine

Explanation: ***Glutamine*** - Ammonia in the brain is primarily detoxified by its conversion to **glutamine** through the enzyme **glutamine synthetase**. - This reaction combines **ammonia** with **glutamate**, effectively trapping the toxic ammonia in a non-toxic form that can be transported out of the brain. *Alanine* - **Alanine** plays a role in ammonia transport within the **glucose-alanine cycle** between muscle and liver, but it is not the primary mechanism for trapping ammonia in the brain. - While it can be formed from pyruvate and glutamate, its formation is not the main brain ammonia detoxification pathway. *Aspartate* - **Aspartate** is involved in the urea cycle and as a neurotransmitter, but it does not directly trap free ammonia in the brain. - It participates in transamination reactions with alpha-ketoglutarate, forming oxaloacetate and glutamate, but this isn't the main ammonia trapping mechanism. *Ornithine* - **Ornithine** is a key intermediate in the **urea cycle**, which primarily occurs in the liver for the detoxification of ammonia. - It is not directly involved in trapping ammonia within the brain tissue itself.

Question 459: In Cystinuria, all of the following amino acids are excreted in urine, except:-

A. Cystine
B. Leucine (Correct Answer)
C. Ornithine
D. Arginine

Explanation: ***Leucine*** - Cystinuria is a disorder characterized by impaired transport of **dibasic amino acids** and **cystine**, not neutral amino acids like leucine. - Therefore, **leucine** would be properly reabsorbed and not significantly excreted in the urine. *Cystine* - **Cystine** is one of the four amino acids whose reabsorption is impaired in cystinuria, leading to its excessive excretion in urine and potential **kidney stone** formation. - The defect is in the **renal tubular transport system** for dibasic amino acids and cystine. *Ornithine* - **Ornithine** is a **dibasic amino acid** and its renal reabsorption is defective in cystinuria. - Like cystine, ornithine is excessively excreted in the urine due to the shared transport system. *Arginine* - **Arginine** is also a **dibasic amino acid** whose renal reabsorption is impaired in cystinuria. - Its presence in the urine is increased, along with cystine, ornithine, and lysine, forming the classic pattern of amino acid excretion in this condition.

Question 460: Elevated levels of vanillylmandelic acid (VMA) in urine are characteristically found in which of the following conditions?

A. Phenyl ketonuria
B. Diabetic ketoacidosis
C. Pheochromocytoma (Correct Answer)
D. Alkaptonuria

Explanation: ***Pheochromocytoma*** - **Vanillylmandelic acid (VMA)** is a major urinary metabolite of the **catecholamines epinephrine and norepinephrine**. - **Pheochromocytoma** is a tumor of the adrenal medulla that secretes excessive amounts of these catecholamines, leading to significantly elevated VMA levels in urine. *Phenyl ketonuria* - Characterized by the inability to metabolize **phenylalanine** due to a deficiency of the enzyme **phenylalanine hydroxylase**. - Leads to accumulation of **phenylalanine** and its metabolites, such as **phenylpyruvic acid**, not VMA. *Diabetic ketoacidosis* - A severe complication of **diabetes mellitus** resulting from a profound insulin deficiency, leading to high blood glucose and **ketone body** production. - While it alters metabolism, it does not directly lead to elevated VMA levels. *Alkaptonuria* - A rare genetic disorder caused by a deficiency of the enzyme **homogentisate 1,2-dioxygenase**, involved in **tyrosine metabolism**. - Results in the accumulation of **homogentisic acid**, which is excreted in the urine and turns dark on exposure to air, but does not involve VMA.

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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