Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 461: Glutamine is increased in CSF, blood & urine WITHOUT elevated orotic acid in which defect:

A. Arginase
B. Argininosuccinate lyase deficiency
C. CPS-I (Correct Answer)
D. Arginosuccinate synthetase
E. OTC

Explanation: ***CPS-I*** - A deficiency in **Carbamoyl Phosphate Synthetase I (CPS-I)** leads to a severe block in the **urea cycle**, resulting in profound hyperammonemia. - The elevated ammonia is then shunted to produce more **glutamine** (via glutamine synthetase), which serves as a detoxification mechanism but also causes high levels of glutamine in CSF, blood, and urine. *Arginase* - **Arginase deficiency** primarily leads to elevated **arginine** levels and mild to moderate hyperammonemia, but not typically a dramatic increase in glutamine due to the block occurring later in the cycle. - Clinical features include progressive spasticity, growth retardation, and intellectual disability. *Argininosuccinate lyase deficiency* - This deficiency causes accumulation of **argininosuccinate** in body fluids, which is a diagnostic marker, rather than primarily increased glutamine. - It presents with severe hyperammonemia, neurological symptoms, and often hepatomegaly. *Arginosuccinate synthetase* - A deficiency in **argininosuccinate synthetase** (also known as citrullinemia type I) leads to a buildup of **citrulline** and severe hyperammonemia. - While hyperammonemia can indirectly increase glutamine, the primary diagnostic marker is elevated citrulline, and the glutamine increase is not as pronounced or directly symptomatic as in CPS-I deficiency. *OTC* - **Ornithine Transcarbamylase (OTC) deficiency** is the most common urea cycle disorder and leads to severe hyperammonemia, accompanied by elevated **orotic acid** due to carbamoyl phosphate shunting to pyrimidine synthesis. - While hyperammonemia drives glutamine synthesis, the presence of elevated orotic acid is a key differentiator from CPS-I deficiency, which does not have increased orotic acid.

Question 462: 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 463: Serotonin is also known as?

A. 3-Methoxytyramine
B. Phenethylamine
C. N-methyl phenylamine
D. 5-hydroxytryptamine (5-HT) (Correct Answer)

Explanation: ***5-hydroxytryptamine (5-HT)*** - **Serotonin** is the common name for the neurotransmitter **5-hydroxytryptamine (5-HT)**. - This chemical name reflects its structure, derived from tryptophan, indicating a **hydroxyl group** at the 5-position of the **indole ring** and an **ethylamine side chain**. *3-Methoxytyramine* - **3-Methoxytyramine** is a metabolite of **dopamine**, not serotonin. - It is formed by the action of **catechol-O-methyltransferase (COMT)** on dopamine. *Phenethylamine* - **Phenethylamine** is a naturally occurring trace amine that acts as a **neuromodulator** or neurotransmitter. - It is structurally similar to certain recreational drugs but is not serotonin. *N-methyl phenylamine* - **N-methyl phenylamine** or N-methylaniline is an **organic chemical** used in dyes and other industrial applications. - It is not a neurotransmitter and has no relation to serotonin.

Question 464: 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 465: Amino acid required for conversion of norepinephrine to epinephrine:-

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

Explanation: ***Methionine*** - **Norepinephrine** is converted to **epinephrine** by the enzyme **phenylethanolamine N-methyltransferase (PNMT)**. - This enzyme uses **S-adenosylmethionine (SAM)** as a **methyl donor**, which is derived from methionine. *Lysine* - **Lysine** is an essential amino acid primarily involved in **protein synthesis**, **calcium absorption**, and the production of **carnitine**. - It does not directly participate in the methylation reaction converting norepinephrine to epinephrine. *Tryptophan* - **Tryptophan** is a precursor for **serotonin** and **niacin** synthesis. - It is not involved in the catecholamine synthesis pathway from norepinephrine to epinephrine. *Phenylalanine* - **Phenylalanine** is the initial amino acid in the **catecholamine synthesis pathway**, being converted to **tyrosine**, then to DOPA, dopamine, and norepinephrine. - While it's crucial for the synthesis *up to* norepinephrine, it is not directly involved in the *conversion of norepinephrine to epinephrine*.

Question 466: A patient complains of knee pain. Routine investigations are unremarkable and still, the patient is unsatisfied. Urine turns black on standing, what is the enzyme involved?

A. Methylmalonyl-CoA mutase
B. Homogentisate oxidase (Correct Answer)
C. Xanthine oxidase
D. Tyrosine hydroxylase

Explanation: ***Homogentisate oxidase*** - **Homogentisate oxidase** deficiency leads to **alkaptonuria**, where homogentisic acid accumulates and is excreted in urine, turning it black upon standing or exposure to air. The knee pain could be due to **ochronosis**, the deposition of oxidized homogentisic acid in cartilage and connective tissues. - The unremarkable routine investigations with persistent knee pain suggest a less common metabolic disorder, which is consistent with **alkaptonuria**'s chronic and insidious presentation. *Methylmalonyl-CoA mutase* - A deficiency in **methylmalonyl-CoA mutase** causes **methylmalonic acidemia**, a metabolic disorder typically presenting in infancy with neurological symptoms, feeding difficulties, and developmental delay. - It does not cause urine to turn black on standing or lead to specific joint pain like that seen in ochronosis. *Xanthine oxidase* - **Xanthine oxidase** is involved in purine metabolism, and its inhibition (e.g., by allopurinol) is used to treat **gout**. - While gout can cause knee pain, it is characterized by elevated uric acid and typically responds to conventional treatments and the urine does not turn black. *Tyrosine hydroxylase* - **Tyrosine hydroxylase** is the rate-limiting enzyme in catecholamine synthesis, converting tyrosine to L-DOPA. - Defects in this enzyme are rare and result in neurological disorders, such as **DOPA-responsive dystonia**, and do not cause urine to blacken or lead to direct joint damage.

Question 467: NO is synthesized from:

A. L-citrulline
B. Glycine
C. Lysine
D. L-arginine (Correct Answer)

Explanation: ***L-arginine*** - **Nitric oxide (NO)** is synthesized from the amino acid **L-arginine** by the enzyme **nitric oxide synthase (NOS)**. - This reaction requires **molecular oxygen** and **NADPH** as cofactors, producing **L-citrulline** as a co-product. - **Three NOS isoforms** exist: neuronal (nNOS), inducible (iNOS), and endothelial (eNOS). - **Clinical significance**: NO is crucial for **vascular smooth muscle relaxation**, **neurotransmission**, and **immune defense**. *L-citrulline* - **L-citrulline** is the co-product (not precursor) of the NO synthesis reaction. - While it can be recycled back to **L-arginine** via the **citrulline-arginine cycle**, it does not directly yield NO without first being converted to arginine. - This recycling pathway helps sustain NO production in endothelial cells. *Glycine* - **Glycine** is involved in various metabolic processes, including **heme synthesis**, **purine synthesis**, and as a neurotransmitter. - It plays no direct role in **nitric oxide** biosynthesis. - Not a substrate for any **NOS isoform**. *Lysine* - **Lysine** is an essential amino acid important for **protein synthesis**, **collagen formation**, and **carnitine synthesis**. - It is structurally distinct from arginine and is not recognized by **nitric oxide synthase**. - Not involved in the NO synthesis pathway.

Question 468: A newborn presents with metabolic acidosis, high ammonia, and orotic acid in urine. Which enzyme deficiency is most likely?

A. Arginase
B. Glutamine synthetase
C. Ornithine transcarbamylase (Correct Answer)
D. CPS-1

Explanation: ***Ornithine transcarbamylase*** - Deficiency of **ornithine transcarbamylase (OTC)**, a key enzyme in the urea cycle, leads to the accumulation of **carbamoyl phosphate**. - **Carbamoyl phosphate** is then shunted to the pyrimidine synthesis pathway, leading to increased production and excretion of **orotic acid** in the urine, along with **hyperammonemia** and **metabolic acidosis**. *Arginase* - **Arginase deficiency** in the urea cycle primarily causes increased **arginine levels** and **hyperammonemia**, but it does not typically lead to the accumulation of orotic acid. - The effects are often more chronic with gradual onset of symptoms, rather than severe neonatal presentation with orotic aciduria. *Glutamine synthetase* - **Glutamine synthetase** catalyzes the synthesis of glutamine from glutamate and ammonia, playing a crucial role in **ammonia detoxification**. - A deficiency would impair ammonia detoxification and lead to **hyperammonemia**, but it would not directly cause **orotic aciduria**. *CPS-1* - **Carbamoyl phosphate synthetase I (CPS-1)** deficiency is the first step of the urea cycle and leads to severe **hyperammonemia** due to the inability to form carbamoyl phosphate. - Unlike OTC deficiency, **CPS-1 deficiency** does not involve the buildup of carbamoyl phosphate; hence, **orotic acid levels** would be low or normal, not high.

Question 469: The transfer of amino groups between amino acids and α-keto acids requires which coenzyme?

A. FAD
B. PLP (Correct Answer)
C. Thiamine
D. NAD+

Explanation: ***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**.

Question 470: Which amino acid is involved in nitric oxide synthesis?

A. Glutamine
B. Arginine (Correct Answer)
C. Proline
D. Lysine

Explanation: ***Arginine*** - **Arginine** is the direct precursor for the synthesis of **nitric oxide (NO)** via the enzyme nitric oxide synthase (NOS). - The conversion of arginine to citrulline liberates nitric oxide, a crucial signaling molecule involved in various physiological processes. *Glutamine* - **Glutamine** is an abundant amino acid involved in protein synthesis, acid-base balance, and as a precursor for glutathione and neurotransmitters. - It does not directly serve as a substrate for nitric oxide synthesis. *Proline* - **Proline** is essential for collagen synthesis and wound healing and plays a role in cellular stress responses. - It is not directly involved in the biochemical pathway leading to nitric oxide production. *Lysine* - **Lysine** is an essential amino acid important for protein synthesis, calcium absorption, and the production of hormones and enzymes. - It is not a precursor for nitric oxide synthesis.

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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