Amino Acid Metabolism Practice Questions

Q: Which amino acid in Jowar is responsible for its pellagragenic effect?

Leucine. ***Leucine*** - A high intake of **leucine**, an essential amino acid, interferes with the metabolism of **tryptophan** and niacin, leading to **pellagra**. - Jowar (sorghum) contains high levels of leucine, which, when it forms a major part of the diet, can induce **niacin deficiency**. *Lysine* - Lysine is an essential amino acid and is generally considered to be in **limited supply** in many cereal grains, making it a desirable amino acid to increase in diets. - It does not directly contribute to the pellagragenic effect; rather, a deficiency in lysine can be a nutritional concern. *Tryptophan* - Tryptophan is a **precursor to niacin (Vitamin B3)** in the body; a deficiency in tryptophan can lead to pellagra. - The high leucine content in jowar interferes with the conversion of tryptophan to niacin, thus exacerbating niacin deficiency. *Methionine* - Methionine is an **essential sulfur-containing amino acid** important for various metabolic functions and protein synthesis. - It is not directly implicated in the pellagragenic effect associated with high jowar consumption.

Q: Which enzyme is deficient in Isovaleric acidemia?

Isovaleryl CoA dehydrogenase. ***Isovaleryl CoA dehydrogenase*** - **Isovaleric acidemia** is an **autosomal recessive** metabolic disorder caused by a deficiency in the enzyme **isovaleryl-CoA dehydrogenase** - This enzyme is crucial for the metabolism of **leucine**, a branched-chain amino acid, leading to the accumulation of toxic byproducts like **isovaleryl-CoA** and **isovaleric acid** - Characteristic **sweaty feet odor** due to isovaleric acid accumulation *Phenylalanine hydroxylase* - A deficiency in **phenylalanine hydroxylase** is responsible for **phenylketonuria (PKU)**, a different metabolic disorder involving the metabolism of **phenylalanine** - This enzyme converts **phenylalanine to tyrosine**, and its deficiency leads to the accumulation of phenylalanine and its metabolites, causing neurological damage if untreated *Arginase* - A deficiency in **arginase** causes **argininemia (hyperargininemia)**, which is a disorder of the **urea cycle** - This enzyme converts **arginine into urea and ornithine**, and its deficiency leads to the buildup of arginine and ammonia in the blood, causing neurological symptoms and developmental delay *Methylmalonyl CoA mutase* - A deficiency in **methylmalonyl CoA mutase** causes **methylmalonic acidemia**, another organic acidemia distinct from isovaleric acidemia - This disorder involves **propionate metabolism** and can present with metabolic acidosis, but affects a different metabolic pathway than leucine catabolism

Q: Which amino acid is used by the liver in the urea cycle?

Aspartate. ***Aspartate*** - **Aspartate** provides the second nitrogen atom to the urea cycle, directly contributing to the formation of **argininosuccinate** through condensation with citrulline. - It is crucial for the efficient removal of **ammonia** in the form of urea. *Glutamine* - **Glutamine** transports ammonia from peripheral tissues to the liver and kidneys, but it is typically deamidated to **glutamate** before its nitrogen can enter the urea cycle. - While it's a major ammonia carrier, it's not directly incorporated into urea as an intact amino acid. *Glutamate* - **Glutamate** can donate its amino group to form **aspartate** (via transamination with oxaloacetate) or release ammonia directly (via glutamate dehydrogenase), both of which then enter the urea cycle. - However, glutamate itself is not directly incorporated into the urea molecule in the same way aspartate is. *Ornithine* - **Ornithine** is an amino acid that participates in the urea cycle as a carrier molecule, being regenerated at the end of each cycle. - While essential for the cycle to function, it is not "used" in the sense of being consumed or providing nitrogen for urea formation - rather it acts as a catalytic intermediate that is recycled.

Q: Taurine is biosynthesized from which amino acid?

Cysteine. ***Cysteine*** - **Taurine** is primarily synthesized from the amino acid **cysteine** through a pathway involving **cysteine sulfinic acid** and **hypotaurine**. - This pathway utilizes enzymes like **cysteine dioxygenase** and **cysteine sulfinic acid decarboxylase**. - The biosynthetic pathway: Cysteine → Cysteine sulfinic acid → Hypotaurine → Taurine. *Arginine* - **Arginine** is a precursor for **nitric oxide**, **urea**, and **creatine**, not taurine. - It is involved in various metabolic pathways, including the **urea cycle** and protein synthesis. *Valine* - **Valine** is a **branched-chain amino acid (BCAA)** involved in protein synthesis and energy production. - It is not a direct precursor for taurine biosynthesis. *Leucine* - **Leucine** is also a **branched-chain amino acid (BCAA)** crucial for protein synthesis and muscle metabolism. - It does not participate in the synthesis of taurine.

Q: Hydrolysis occurs at which step of urea cycle ?

Cleavage of arginine. ***Cleavage of arginine*** - The final step in the urea cycle, where **arginine** is hydrolyzed by the enzyme **arginase** to form **urea** and **ornithine**. - This reaction involves the addition of a **water molecule** across the guanidino group to release urea. *Formation of argininosuccinate* - This step involves the condensation of **citrulline** and **aspartate**, catalyzed by **argininosuccinate synthetase**. - It is an **ATP-dependent** reaction, not a hydrolysis. *Formation of citrulline* - Occurs when **carbamoyl phosphate** condenses with **ornithine**, catalyzed by **ornithine transcarbamylase**. - This reaction involves the removal of a phosphate group, not the addition of water. *Formation of ornithine* - **Ornithine** is a substrate for the formation of citrulline and is also regenerated at the end of the cycle from arginine. - Its formation from arginine is a **hydrolysis** reaction, but simply stating "formation of ornithine" is less specific than "cleavage of arginine," which directly describes the hydrolytic event.

Q: Transamination of Alanine results in formation of ?

Pyruvate. **Pyruvate** ✓ - **Transamination** involves the transfer of an amino group from an amino acid to an α-ketoglutarate (catalyzed by aminotransferases). - When **alanine** undergoes transamination via **ALT (alanine aminotransferase)**, its amino group is transferred to α-ketoglutarate, forming glutamate, while alanine is converted to its corresponding α-keto acid, which is **pyruvate**. - Reaction: Alanine + α-Ketoglutarate ⇄ Pyruvate + Glutamate *Oxaloacetate* - **Oxaloacetate** is the α-keto acid formed from the transamination of **aspartate** (via AST/GOT). - It is a key intermediate in the **citric acid cycle** and gluconeogenesis, not a product of alanine transamination. *Aspartate* - **Aspartate** is an amino acid, not an α-keto acid. - It can be formed from oxaloacetate via transamination (reverse reaction), and is involved in the **urea cycle** and nucleotide synthesis. *Arginine* - **Arginine** is a semi-essential amino acid, not an α-keto acid or a product of alanine transamination. - It plays roles in **protein synthesis**, the urea cycle, and nitric oxide production.

Q: Which will activate carbamoyl phosphate synthase I?

N-acetyl glutamate. **N-acetyl glutamate** - **N-acetyl glutamate** is an **allosteric activator** of **carbamoyl phosphate synthase I (CPS I)**, which is the mitochondrial enzyme that catalyzes the first committed step of the **urea cycle**. - Its synthesis is stimulated by high levels of **arginine**, linking nitrogen load to urea production. *Acetyl-CoA* - Acetyl-CoA is a common **substrate** and **product** in various metabolic pathways, but it is not a direct activator of CPS I. - It is a precursor for the synthesis of **N-acetyl glutamate**, but does not activate CPS I directly. *Ornithine* - **Ornithine** is a key intermediate of the **urea cycle**, but it does not directly activate CPS I. - It combines with carbamoyl phosphate (the product of CPS I) in the second step of the urea cycle to form citrulline. *ATP* - **ATP** is a **substrate** used by CPS I to provide energy for the synthesis of carbamoyl phosphate. - While essential for the reaction, ATP itself does not act as an allosteric activator of the enzyme.

Q: Which of the following is a non-essential amino acid?

Tyrosine. ***Tyrosine*** - **Tyrosine** is considered a **non-essential amino acid** because the human body can synthesize it from the essential amino acid **phenylalanine**. - This synthesis occurs via the enzyme **phenylalanine hydroxylase**, making its dietary intake not strictly necessary if phenylalanine is available. *Phenylalanine* - **Phenylalanine** is an **essential amino acid**, meaning the human body **cannot synthesize it** and it must be obtained through the diet. - It serves as a precursor for various important molecules, including tyrosine, contributing to neurotransmitter synthesis. *Lysine* - **Lysine** is an **essential amino acid** that the human body **cannot synthesize** and must be acquired from dietary sources. - It plays a crucial role in **protein synthesis**, calcium absorption, and the production of hormones and enzymes. *Threonine* - **Threonine** is another example of an **essential amino acid** that the human body is **unable to produce** on its own. - It is important for the formation of **collagen** and elastin, and contributes to immune function.

Q: Which of the following substances is not derived from tyrosine?

Nicotinic acid. ***Nicotinic acid*** - **Nicotinic acid** (niacin, vitamin B3) is synthesized from **tryptophan** in the body, not tyrosine. - It plays a crucial role in metabolism as a precursor for NAD+ and NADH, which are involved in various enzymatic reactions. *Thyroxine* - **Thyroxine** (T4), a thyroid hormone, is derived from the amino acid **tyrosine**. - **Iodine** is incorporated into tyrosine residues on thyroglobulin to form monoiodotyrosine (MIT) and diiodotyrosine (DIT), which then couple to form T4 (and T3). *Melanin* - **Melanin**, the pigment responsible for skin, hair, and eye color, is synthesized from **tyrosine** through a pathway involving the enzyme **tyrosinase**. - This process involves the hydroxylation of tyrosine to 3,4-dihydroxyphenylalanine (DOPA) and subsequent oxidation reactions. *Dopamine* - **Dopamine**, an important neurotransmitter, is synthesized from **tyrosine** in a two-step process in the brain and adrenal medulla. - Tyrosine is first hydroxylated to DOPA by **tyrosine hydroxylase**, and then DOPA is decarboxylated to dopamine by DOPA decarboxylase.

Q: Which amino acid requires ascorbic acid for its formation in the body?

Hydroxyproline. ***Hydroxyproline*** - **Ascorbic acid (Vitamin C)** is an essential cofactor for **prolyl hydroxylase** and **lysyl hydroxylase** enzymes - These enzymes catalyze the **post-translational hydroxylation** of proline and lysine residues within collagen chains to form hydroxyproline and hydroxylysine - This hydroxylation is crucial for **stabilization of the collagen triple helix** structure - Hydroxyproline is formed by **modification of proline after incorporation into collagen**, not as a free amino acid - **Scurvy** (Vitamin C deficiency) results in defective collagen due to inadequate hydroxyproline formation *Lysine* - Lysine is an **essential amino acid** obtained from diet - Does not require ascorbic acid for its synthesis or formation - While lysine residues in collagen can be hydroxylated (forming hydroxylysine), the question asks about the amino acid whose formation requires Vitamin C *Cysteine* - Cysteine is a **sulfur-containing amino acid** synthesized from methionine via transsulfuration pathway - Its synthesis does not involve ascorbic acid *Proline* - Proline is a **non-essential amino acid** synthesized from glutamate - **Proline synthesis does not require ascorbic acid** - Proline serves as the precursor that gets hydroxylated to hydroxyproline within collagen

Question 1

Which amino acid in Jowar is responsible for its pellagragenic effect?

Accepted Answer

Leucine

Answer

Lysine

Answer

Tryptophan

Answer

Methionine

Question 2

Which enzyme is deficient in Isovaleric acidemia?

Accepted Answer

Isovaleryl CoA dehydrogenase

Answer

Phenylalanine hydroxylase

Answer

Arginase

Answer

Methylmalonyl CoA mutase

Question 3

Which amino acid is used by the liver in the urea cycle?

Accepted Answer

Aspartate

Answer

Glutamine

Answer

Glutamate

Answer

Ornithine

Question 4

Taurine is biosynthesized from which amino acid?

Accepted Answer

Cysteine

Answer

Valine

Answer

Arginine

Answer

Leucine

Question 5

Hydrolysis occurs at which step of urea cycle ?

Accepted Answer

Cleavage of arginine

Answer

Formation of ornithine

Answer

Formation of argininosuccinate

Answer

Formation of citrulline

Question 6

Transamination of Alanine results in formation of ?

Accepted Answer

Pyruvate

Answer

Oxaloacetate

Answer

Aspartate

Answer

Arginine

Question 7

Which will activate carbamoyl phosphate synthase I?

Accepted Answer

N-acetyl glutamate

Answer

ATP

Answer

Acetyl-CoA

Answer

Ornithine

Question 8

Which of the following is a non-essential amino acid?

Accepted Answer

Tyrosine

Answer

Phenylalanine

Answer

Lysine

Answer

Threonine

Question 9

Which of the following substances is not derived from tyrosine?

Accepted Answer

Nicotinic acid

Answer

Thyroxine

Answer

Melanin

Answer

Dopamine

Question 10

Which amino acid requires ascorbic acid for its formation in the body?

Accepted Answer

Hydroxyproline

Answer

Lysine

Answer

Cysteine

Answer

Proline

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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