Amino Acid Metabolism Practice Questions

Q: In phenylketonuria, which substance should be restricted in the diet?

Phenylalanine. ***Phenylalanine*** - **Phenylketonuria (PKU)** is a genetic disorder where the body cannot effectively metabolize **phenylalanine** due to a deficiency in the enzyme **phenylalanine hydroxylase**. - Restricting dietary phenylalanine is crucial to prevent the accumulation of toxic byproducts that can lead to severe neurological damage and developmental delays. *Tyrosine* - Tyrosine is normally synthesized from phenylalanine. In PKU, this conversion is impaired. - While phenylalanine must be restricted, tyrosine supplementation is often necessary for individuals with PKU, as it becomes a conditionally essential amino acid. *Maize* - Maize (corn) is a carbohydrate-rich food and does not contain high levels of phenylalanine that would necessitate its restriction in PKU. - Dietary management in PKU focuses on regulating protein intake, as phenylalanine is an amino acid found in proteins. *None of the options* - This option is incorrect because phenylalanine must be strictly restricted in the diet of individuals with phenylketonuria to manage the condition effectively. - Without dietary restriction, the accumulation of phenylalanine can lead to severe and irreversible neurological damage.

Q: Coenzyme for phenylalanine hydroxylase is?

Tetrahydrobiopterin. ***Tetrahydrobiopterin*** - **Tetrahydrobiopterin (BH4)** is an essential coenzyme for aromatic amino acid hydroxylases, including **phenylalanine hydroxylase (PAH)**. - PAH converts **phenylalanine** to **tyrosine**, and deficiencies in BH4 or PAH itself lead to *phenylketonuria (PKU)*. *Tetrahydrofolate* - **Tetrahydrofolate (THF)** is a coenzyme derived from **folic acid** and is primarily involved in **one-carbon metabolism**, including **purine** and **pyrimidine synthesis**, and various amino acid interconversions. - It does not directly act as a coenzyme for phenylalanine hydroxylase. *Pyridoxal phosphate* - **Pyridoxal phosphate (PLP)**, a derivative of **vitamin B6**, is a crucial coenzyme for many enzymes involved in **amino acid metabolism**, particularly in **transamination**, **decarboxylation**, and side-chain cleavage reactions. - It is not the coenzyme for phenylalanine hydroxylase. *S-adenosyl methionine* - **S-adenosyl methionine (SAM)** is a major **methyl donor** in various biochemical reactions, important for the synthesis of **neurotransmitters**, **hormones**, and **phospholipids**. - While essential for many metabolic pathways, it is not involved as a coenzyme for phenylalanine hydroxylase.

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

Alanine. ***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.

Q: Which of the following conditions is primarily treated with Vitamin B6?

Homocystinuria. ***Homocystinuria*** - The most common form of **homocystinuria** is caused by **cystathionine β-synthase (CBS) deficiency**, which requires **pyridoxal phosphate (Vitamin B6)** as a cofactor. - Approximately **50% of patients** with CBS deficiency are **B6-responsive**, making **high-dose Vitamin B6 (100-500 mg/day)** a **first-line primary treatment** for these cases. - This is a **clinically significant condition** that requires treatment, manifesting with features like **ectopia lentis, marfanoid habitus, intellectual disability, and thromboembolism**. - B6 supplementation enhances residual CBS enzyme activity and reduces plasma homocysteine levels. *Cystathionuria* - Caused by **cystathionine γ-lyase deficiency**, which also uses **Vitamin B6** as a cofactor. - This is generally a **benign, asymptomatic condition** that does **NOT require treatment**. - While B6 can reduce cystathionine accumulation, it is **not a primary treatment indication** because the condition is clinically insignificant. *Xanthurenic aciduria* - Caused by **kynureninase deficiency** in the **tryptophan metabolism pathway**, which requires **pyridoxal phosphate**. - This is a **rare and usually benign condition** that does not typically require treatment. - Not a primary indication for B6 therapy. *None of the options* - Incorrect, as **Homocystinuria** (CBS deficiency) is a **primary indication** for high-dose Vitamin B6 therapy in B6-responsive patients.

Q: Which of the following statements about Maple Syrup Urine Disease (MSUD) is true?

Deficiency of branched chain amino acid enzymes. ***Deficiency of branched chain amino acid enzymes*** - MSUD is caused by a deficiency in the **branched-chain alpha-keto acid dehydrogenase complex (BCKDC)**, an enzyme responsible for metabolizing **leucine, isoleucine, and valine**. - This enzyme deficiency leads to the accumulation of these **branched-chain amino acids (BCAAs)** and their corresponding branched-chain alpha-keto acids in the body. *Asymptomatic* - MSUD is a severe metabolic disorder that is **not asymptomatic**; it typically presents with distinct neurological and metabolic symptoms shortly after birth. - Clinical manifestations can include **poor feeding, lethargy, seizures**, and a characteristic **maple syrup odor** in urine and earwax. *FeCl3 turns navy blue* - The **ferric chloride test (FeCl3 test)** in MSUD typically yields a **green-gray or dark-green color** when testing for the accumulation of alpha-keto acids (specifically alpha-ketoisocaproic acid). - A **navy blue color** with FeCl3 is more characteristic of **alkaptonuria** due to the presence of homogentisic acid. *Hyperaminoaciduria of aromatic amino acids* - MSUD is characterized by the accumulation and excretion of **branched-chain amino acids (leucine, isoleucine, valine)** and their corresponding keto acids. - **Hyperaminoaciduria of aromatic amino acids** (e.g., phenylalanine, tyrosine, tryptophan) is seen in other conditions like **phenylketonuria (PKU)**, not MSUD.

Q: Sweaty feet odor in urine is seen in which condition?

Isovaleric acidemia. ***Isovaleric acidemia*** - This condition is characterized by a distinctive "sweaty feet" odor in body fluids, including urine, due to the accumulation of **isovaleric acid**. - It results from a deficiency in the enzyme **isovaleryl-CoA dehydrogenase**, which is crucial for leucine metabolism. *Phenylketonuria* - Patients with **phenylketonuria (PKU)** typically have a "mousy" or "musty" odor in their urine, not a sweaty feet smell. - This is due to the accumulation of **phenylalanine** and its metabolites. *Maple syrup urine disease* - This metabolic disorder is named for the characteristic sweet, maple syrup-like odor of the urine, which is distinctly different from a sweaty feet odor. - It is caused by a defect in the metabolism of **branched-chain amino acids (leucine, isoleucine, and valine)**. *Alkaptonuria* - This condition is known for urine that turns **dark brown or black** upon standing or when exposed to air, due to the oxidation of **homogentisic acid**. - It does not produce a sweaty feet odor.

Q: Transamination of Aspartate forms which compound?

Oxaloacetate. ***Oxaloacetate*** - **Aspartate** is transaminated by **aspartate aminotransferase (AST)**, transferring its alpha-amino group to **alpha-ketoglutarate**. - This reaction converts aspartate into its corresponding alpha-keto acid, which is **oxaloacetate**. *Pyruvate* - **Pyruvate** is the alpha-keto acid corresponding to the amino acid **alanine**. - Transamination of **alanine** yields **pyruvate**, not aspartate. *Acetyl-CoA* - **Acetyl-CoA** is not a direct product of amino acid transamination; it is formed from **pyruvate** or fatty acid oxidation. - It functions as a key metabolic intermediate in energy production and biosynthesis. *Alanine* - **Alanine** is an amino acid, and therefore a reactant in transamination reactions to form pyruvate, rather than a product of aspartate transamination. - While it can be formed from pyruvate via transamination, it is not formed from aspartate.

Q: Which amino acids accumulate in maple syrup urine disease?

All branched-chain amino acids. ***All branched-chain amino acids*** - Maple syrup urine disease (MSUD) is characterized by a deficiency in the **branched-chain alpha-keto acid dehydrogenase complex**, which is responsible for the breakdown of branched-chain amino acids (BCAAs). - This deficiency leads to the accumulation of **leucine, isoleucine, and valine**, along with their corresponding alpha-keto acids, in the blood and urine. - The distinctive **maple syrup odor** in the urine is caused by the accumulation of branched-chain keto acids derived from all three BCAAs. *Leucine* - While leucine is one of the BCAAs that accumulates in MSUD, it is not the *only* amino acid involved. - The accumulation of **leucine** is particularly associated with the severe neurological symptoms seen in MSUD, as it is the most neurotoxic of the three BCAAs. *Valine* - Valine is another BCAA that accumulates due to the metabolic block in MSUD. - However, the disease involves the accumulation of all three BCAAs, not just valine in isolation. *Isoleucine* - Isoleucine is the third BCAA that accumulates in MSUD due to the defective enzyme. - Like leucine and valine, isoleucine and its corresponding keto acid accumulate in blood and urine when the branched-chain alpha-keto acid dehydrogenase complex is deficient.

Q: Which enzyme catalyzes oxidative deamination?

Glutamate dehydrogenase. ***Glutamate dehydrogenase*** - This enzyme catalyzes the conversion of **glutamate** to **α-ketoglutarate** and ammonia (NH₃), which is an oxidative deamination reaction. - It utilizes **NAD⁺ or NADP⁺** as a coenzyme to remove hydrogen atoms during the oxidation process. - Plays a crucial role in both **amino acid catabolism** and anabolism. *Glutaminase* - This enzyme hydrolyzes **glutamine** to glutamate and ammonia, which is a **hydrolytic deamidation** reaction, not an oxidative deamination. - It does not involve the oxidation of the substrate or require NAD⁺/NADP⁺ as cofactors. *Glutamine synthase* - This enzyme synthesizes **glutamine** from glutamate and ammonia, using ATP, which is a **biosynthetic** reaction, not a catabolic deamination. - It is involved in **ammonia detoxification** and amino acid synthesis, functioning in the opposite direction of deamination. *None of the options* - This option is incorrect because **glutamate dehydrogenase** is a valid correct answer. - Glutamate dehydrogenase is the primary enzyme responsible for oxidative deamination in human metabolism.

Q: Which defect in the urea cycle is an X-linked disease?

Ornithine transcarbamylase. ***Ornithine transcarbamylase*** - **Ornithine transcarbamylase (OTC) deficiency** is the only **X-linked recessive** disorder among the urea cycle defects. - Males are usually more severely affected, while females can be symptomatic carriers. *Carbamoyl phosphate synthetase I* - **Carbamoyl phosphate synthetase I (CPS1) deficiency** is an **autosomal recessive** disorder. - It is one of the more severe urea cycle defects, leading to profound hyperammonemia. *Arginase* - **Arginase deficiency** (hyperargininemia) is an **autosomal recessive** disorder. - It typically presents with a distinct neurological phenotype, including spasticity and developmental delay. *Argininosuccinate synthase* - **Argininosuccinate synthase deficiency**, also known as **Citrullinemia type I**, is an **autosomal recessive** disorder. - It leads to the accumulation of **citrulline** and **ammonia** in the blood.

Question 1

In phenylketonuria, which substance should be restricted in the diet?

Accepted Answer

Phenylalanine

Answer

Tyrosine

Answer

Maize

Answer

None of the options

Question 2

Coenzyme for phenylalanine hydroxylase is?

Accepted Answer

Tetrahydrobiopterin

Answer

Tetrahydrofolate

Answer

Pyridoxal phosphate

Answer

S-adenosyl methionine

Question 3

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

Accepted Answer

Alanine

Answer

Citrulline

Answer

Arginine

Answer

Ornithine

Question 4

Which of the following conditions is primarily treated with Vitamin B6?

Accepted Answer

Homocystinuria

Answer

Cystathionuria

Answer

None of the options

Answer

Xanthourenic aciduria

Question 5

Which of the following statements about Maple Syrup Urine Disease (MSUD) is true?

Accepted Answer

Deficiency of branched chain amino acid enzymes

Answer

FeCl3 turns navy blue

Answer

Asymptomatic

Answer

Hyperaminoaciduria of aromatic amino acids

Question 6

Sweaty feet odor in urine is seen in which condition?

Accepted Answer

Isovaleric acidemia

Answer

Phenylketonuria

Answer

Alkaptonuria

Answer

Maple syrup urine disease

Question 7

Transamination of Aspartate forms which compound?

Accepted Answer

Oxaloacetate

Answer

Pyruvate

Answer

Acetyl-CoA

Answer

Alanine

Question 8

Which amino acids accumulate in maple syrup urine disease?

Accepted Answer

All branched-chain amino acids

Answer

Valine

Answer

Leucine

Answer

Isoleucine

Question 9

Which enzyme catalyzes oxidative deamination?

Accepted Answer

Glutamate dehydrogenase

Answer

Glutaminase

Answer

Glutamine synthase

Answer

None of the options

Question 10

Which defect in the urea cycle is an X-linked disease?

Accepted Answer

Ornithine transcarbamylase

Answer

Arginase

Answer

Argininosuccinate synthase

Answer

Carbamoyl phosphate synthetase I

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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