Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 531: In Cystinuria, which of the following amino acids is not affected by the reabsorption defect?

A. Arginine
B. Ornithine
C. Citrulline (Correct Answer)
D. Lysine

Explanation: ***Citrulline*** - **Citrulline** is the correct answer because it is **not a dibasic amino acid** and does not share the same renal tubular transporter as the affected amino acids. - In cystinuria, the defect involves the **rBAT-b0,+AT transporter system**, which specifically transports **cystine** and **dibasic amino acids** (ornithine, arginine, lysine) - remembered by the mnemonic **COAL**. - Citrulline uses a **different transport mechanism** and therefore its reabsorption remains **normal** in cystinuria. *Lysine* - **Lysine** is one of the four amino acids affected in cystinuria (part of the COAL group). - Being a **dibasic amino acid**, its reabsorption is **impaired**, leading to increased urinary excretion. - This is **incorrect** as the question asks for the amino acid NOT affected. *Arginine* - **Arginine** is a **dibasic amino acid** whose reabsorption is significantly reduced in cystinuria. - It is part of the COAL group and shows **elevated urinary concentration** in affected individuals. - This is **incorrect** as arginine IS affected by the reabsorption defect. *Ornithine* - **Ornithine** is another **dibasic amino acid** included in the COAL group. - Its renal tubular reabsorption is **defective** in cystinuria, resulting in increased urinary excretion. - This is **incorrect** as ornithine IS affected by the transport defect.

Question 532: Conversion of Norepinephrine to epinephrine is mainly by?

A. S-adenosyl methionine (Correct Answer)
B. Arginine
C. Phenylalanine
D. Dehydrogenase

Explanation: **S-adenosyl methionine (SAM)** - SAM acts as the **methyl donor** in the enzymatic conversion of **norepinephrine to epinephrine** by phenylethanolamine N-methyltransferase (PNMT). - This **methylation reaction** adds a methyl group to the nitrogen atom of norepinephrine, forming epinephrine. *Arginine* - Arginine is a precursor for **nitric oxide (NO)** synthesis, an important signaling molecule, and is also involved in the **urea cycle**. - It is not directly involved in the methylation of norepinephrine to epinephrine. *Phenylalanine* - Phenylalanine is an **essential amino acid** and a precursor for the synthesis of **tyrosine**, which is subsequently converted to **catecholamines** like dopamine, norepinephrine, and epinephrine. - However, it does not directly facilitate the final conversion step from norepinephrine to epinephrine. *Dehydrogenase* - Dehydrogenases are enzymes that catalyze **redox reactions** by removing hydrogen atoms from a substrate. - These enzymes are crucial in many metabolic pathways, but they are not involved in the **methylation reaction** that converts norepinephrine to epinephrine.

Question 533: What is the scientific name for the neurotransmitter serotonin?

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

Explanation: ***5-hydroxytryptamine (5-HT)*** - **Serotonin** is the common name for the neurotransmitter **5-hydroxytryptamine**, often abbreviated as **5-HT**. - It plays a crucial role in regulating mood, appetite, sleep, and other physiological and behavioral functions. *N-methylphenylamine* - **N-methylphenylamine** is not the chemical name for serotonin. - This compound is a **synthetic amine** and does not serve as a well-known neurotransmitter in the human body. *3-Methoxytyramine* - **3-Methoxytyramine** is a **dopamine metabolite** and a trace amine, not serotonin. - It is typically formed from dopamine through the action of **catechol-O-methyltransferase (COMT)**. *Phenethylamine* - **Phenethylamine** is a **trace amine** that can act as a neuromodulator, but it is not serotonin. - It is known for its stimulant effects and is found in some foods like chocolate.

Question 534: Tyrosinemia Type I is caused due to deficiency of which enzyme?

A. Homogentisate 1,2-dioxygenase
B. Fumarylacetoacetate hydrolase (Correct Answer)
C. Tyrosine aminotransferase
D. 4-hydroxyphenylpyruvate dioxygenase

Explanation: ***Fumarylacetoacetate hydrolase*** - **Tyrosinemia Type I**, also known as **hereditary tyrosinemia type 1 (HT1)**, is an **autosomal recessive** metabolic disorder caused by a deficiency of the enzyme **fumarylacetoacetate hydrolase (FAH)**. - This enzyme is crucial for the final step in the **tyrosine degradation pathway**, leading to the accumulation of toxic metabolites like fumarylacetoacetate and succinylacetone. *Tyrosine aminotransferase* - Deficiency of **tyrosine aminotransferase** causes **Tyrosinemia Type II**, a distinct disorder from Type I. - Type II tyrosinemia primarily affects the eyes and skin, presenting with **corneal ulcers** and painful **hyperkeratotic plaques**. *Homogentisate 1,2-dioxygenase* - Deficiency of **homogentisate 1,2-dioxygenase** leads to **alkaptonuria (black urine disease)**, a rare metabolic disorder. - This condition involves the accumulation of **homogentisic acid**, which causes dark urine, **ochronosis** (bluish-black pigmentation of connective tissues), and severe arthropathy. *4-hydroxyphenylpyruvate dioxygenase* - Deficiency of **4-hydroxyphenylpyruvate dioxygenase** results in **Tyrosinemia Type III**, another rare form of tyrosinemia. - This type is typically milder, often presenting with **neurological symptoms** such as intellectual disability and seizures, but without the severe liver and kidney damage seen in Type I.

Question 535: What is the limiting amino acid in cereals?

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

Explanation: ***Lysine*** - **Lysine** is the **first limiting amino acid in cereals** (wheat, rice, corn), meaning it is present in the lowest proportion relative to the body's needs. - This deficiency can impact **protein synthesis** if cereals are the sole or primary protein source without supplementation. - Complementing cereals with **legumes** (rich in lysine but low in methionine) provides complete protein nutrition. *Methionine* - **Methionine** is typically the limiting amino acid in **legumes**, not cereals. - It works with cysteine to provide **sulfur-containing amino acids** essential for various metabolic processes. - This is why cereal-legume combinations (rice and lentils, corn and beans) are nutritionally complementary. *Tryptophan* - **Tryptophan** is an essential amino acid, but it is generally **not the primary limiting amino acid in cereals**. - While corn can be relatively low in tryptophan, **lysine deficiency is more significant** across cereal grains. - It is a precursor to **serotonin** and **niacin**. *Cysteine* - **Cysteine** is a non-essential amino acid, meaning the body can synthesize it from methionine. - While important for protein structure and function, it is **not considered a limiting amino acid** since it can be produced endogenously.

Question 536: Which of the following is not a metabolic product of the urea cycle?

A. Citrulline
B. Arginine
C. Alanine (Correct Answer)
D. Ornithine

Explanation: ***Alanine*** - **Alanine** is an amino acid primarily involved in the **glucose-alanine cycle** for glucose production and ammonia transport, not as a direct metabolic product within the urea cycle. - While it plays a role in nitrogen metabolism, it is not synthesized or directly consumed as an intermediate in the reactions that convert ammonia to urea. *Citrulline* - **Citrulline** is a key intermediate formed during the second step of the urea cycle when **ornithine carbamoyltransferase** combines carbamoyl phosphate with ornithine. - It is then transported out of the mitochondrion into the cytosol to continue the cycle. *Ornithine* - **Ornithine** is an amino acid that acts as a **catalytic intermediate** in the urea cycle, being regenerated at the end of the cycle to combine with carbamoyl phosphate. - It does not directly contribute a nitrogen atom to urea but is essential for the cycle's continuation. *Arginine* - **Arginine** is an amino acid that is a direct precursor to urea in the penultimate step of the urea cycle, where **arginase** cleaves it into urea and ornithine. - It provides one of the nitrogen atoms and the carbon atom for the formation of urea.

Question 537: 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 538: Which amino acid is not involved in transamination?

A. Alanine
B. Aspartate
C. Lysine (Correct Answer)
D. Histidine

Explanation: ***Lysine*** - **Lysine** cannot undergo transamination because it lacks the structural requirements for typical transaminase enzymes. - While lysine has both an **α-amino group** and an **ε-amino group**, its metabolic pathway involves **oxidative deamination** rather than transamination. - Along with **threonine**, lysine is one of only two amino acids that do not participate in transamination reactions. *Alanine* - **Alanine** is a major substrate for transamination, readily converting to pyruvate via **alanine transaminase (ALT)**. - This reaction involves the transfer of its **α-amino group** to an α-keto acid, typically α-ketoglutarate, forming glutamate. *Aspartate* - **Aspartate** is actively involved in transamination, converting to oxaloacetate via **aspartate transaminase (AST)**. - Its **α-amino group** is easily transferred to α-ketoglutarate, forming glutamate. *Histidine* - **Histidine** can undergo transamination, though less commonly cited as a primary substrate compared to aspartate and alanine. - It can transfer its **α-amino group** to an α-keto acid, leading to the formation of imidazolepyruvate.

Question 539: Which of the following compounds is not formed with the involvement of glycine?

A. Purines
B. Glutathione
C. Thyroxine (Correct Answer)
D. Heme

Explanation: ***Thyroxine*** - **Thyroxine** (and other thyroid hormones) are derived from the amino acid **tyrosine**. - Their synthesis involves iodination and coupling reactions of tyrosine residues within the protein **thyroglobulin**. *Heme* - **Glycine** is a direct precursor for the initial step in **heme synthesis**. - It condenses with **succinyl CoA** to form α-amino-β-ketoadipate, which then decarboxylates to form δ-aminolevulinate (ALA). *Purines* - **Glycine** contributes part of its structure to the **purine ring**. - Specifically, the **nitrogen at position 7** and the **carbons at positions 4 and 5** of the purine ring are derived from glycine. *Glutathione* - **Glutathione** is a tripeptide composed of three amino acids: **glutamate**, **cysteine**, and **glycine**. - **Glycine** is the C-terminal amino acid of glutathione and is essential for its structure and function as an antioxidant.

Question 540: Boiled cabbage or rancid butter smelling urine is seen in

A. Tyrosinemia
B. Phenylketonuria
C. Isovaleric Acidaemia (Correct Answer)
D. Multiple carboxylase deficiency

Explanation: ***Isovaleric Acidaemia*** - **Boiled cabbage or rancid butter odor** in urine is a classic feature of isovaleric acidemia, caused by the accumulation of isovaleric acid. - This **inborn error of metabolism** affects **leucine metabolism** due to deficiency of isovaleryl-CoA dehydrogenase. *Tyrosinemia* - Does NOT present with boiled cabbage or rancid butter odor. The characteristic features are **liver dysfunction** and **renal tubular defects**. - Tyrosinemia Type I is caused by deficiency of **fumarylacetoacetate hydrolase**, leading to accumulation of tyrosine metabolites. *Phenylketonuria* - Characterized by a **mousy or musty odor** in urine, resulting from the accumulation of phenylacetic acid. - The defect is in the enzyme **phenylalanine hydroxylase**, not associated with boiled cabbage odor. *Multiple carboxylase deficiency* - Typically presents with a **"cat urine" smell** due to the accumulation of various organic acids. - The deficiency impairs the function of several **biotin-dependent carboxylases**, not specifically linked to the boiled cabbage odor.

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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