Amino Acid Metabolism Practice Questions

Q: A 5-year-old girl with global developmental delay, hypotonia, and failure to thrive presents with severe hyperammonemia. Laboratory results show increased orotic acid, increased glutamine, and decreased BUN. Evaluate and determine the underlying metabolic disorder.

Ornithine transcarbamylase deficiency (OTC deficiency). ***Ornithine transcarbamylase deficiency (OTC deficiency)*** - **Hyperammonemia** with **increased orotic acid** and **decreased BUN** is the hallmark of OTC deficiency, an X-linked urea cycle disorder. - The deficiency in OTC prevents the conversion of carbamoyl phosphate and ornithine to citrulline, leading to the accumulation of carbamoyl phosphate which then enters the **pyrimidine synthesis pathway**, increasing orotic acid. *Maple syrup urine disease (MSUD)* - While it presents with **developmental delay** and **poor feeding**, MSUD is characterized by the accumulation of **branched-chain amino acids** (leucine, isoleucine, valine) and their ketoacids, not hyperammonemia with increased orotic acid. - The urine has a characteristic sweet smell, resembling **maple syrup**. *Phenylketonuria (PKU)* - PKU is caused by a deficiency in **phenylalanine hydroxylase**, leading to the accumulation of phenylalanine. - It does not present with **hyperammonemia** or **orotic aciduria**; instead, it causes severe intellectual disability if untreated and is often detected by newborn screening. *Methylmalonic acidemia (MMA)* - MMA is an organic aciduria characterized by the accumulation of **methylmalonic acid** due to a defect in methylmalonyl-CoA mutase or its cofactor vitamin B12 metabolism. - Patients typically present with metabolic acidosis, hypoglycemia, elevated propionic acid, and ketonuria, but **hyperammonemia and orotic aciduria are not primary features**.

Q: Ochronosis is due to the accumulation of?

Homogentisic acid. ***Homogentisic acid (Correct)*** - **Ochronosis** is a rare genetic disorder characterized by the accumulation of **homogentisic acid** in connective tissues. - This accumulation results from a deficiency of the enzyme **homogentisate 1,2-dioxygenase**, which is crucial in the catabolism of tyrosine and phenylalanine. - Clinically presents with dark pigmentation of cartilage, sclera, and other connective tissues. *Alkapton (Incorrect)* - While **alkaptonuria** is the disease caused by homogentisic acid accumulation, **alkapton** itself is not the substance that accumulates in tissues in ochronosis. - **Alkapton** refers to the dark-colored urine observed in patients with alkaptonuria, which is due to the oxidation of homogentisic acid in the urine. *Xanthurenate (Incorrect)* - **Xanthurenate** is an intermediate in the metabolism of tryptophan, and its accumulation is associated with certain vitamin B6 deficiencies. - It is not involved in the pathogenesis of ochronosis or alkaptonuria. *Glyoxylate (Incorrect)* - **Glyoxylate** is a metabolic intermediate involved in various pathways, including carbohydrate and amino acid metabolism. - Accumulation of glyoxylate is associated with **primary hyperoxaluria type 1**, but not with ochronosis.

Q: What is the primary methyl donor in the conversion of norepinephrine to epinephrine?

S-adenosyl methionine. ***S-adenosyl methionine*** (SAM) - **S-adenosyl methionine (SAM)** is the universal methyl donor in many biological methylation reactions, including the conversion of **norepinephrine to epinephrine**. - The enzyme **phenylethanolamine N-methyltransferase (PNMT)** uses SAM to transfer a methyl group to norepinephrine, forming epinephrine. *Arginine* - **Arginine** is an amino acid primarily involved in the synthesis of **nitric oxide**, urea, and creatine, and does not serve as a direct methyl donor. - Its metabolic roles are distinct from methylation processes involving catecholamines. *Phenylalanine* - **Phenylalanine** is an essential amino acid that is a **precursor to tyrosine**, which is then used to synthesize catecholamines like dopamine, norepinephrine, and epinephrine. - It is a building block in the synthesis pathway but does not directly donate methyl groups. *Methionine* - **Methionine** is an essential amino acid that is converted to **S-adenosyl methionine (SAM)** through the action of methionine adenosyltransferase. - While methionine is the precursor to SAM, it is **SAM itself that serves as the active methyl donor**, not free methionine.

Q: Phenylketonuria is due to deficiency of:

Phenylalanine hydroxylase (PAH). ***Phenylalanine hydroxylase (PAH)*** - **Phenylketonuria (PKU)** is an inherited metabolic disorder caused by a deficiency of the enzyme **phenylalanine hydroxylase (PAH)**. - This enzyme is crucial for converting the amino acid **phenylalanine** into **tyrosine**, and its deficiency leads to an accumulation of phenylalanine in the body. - PKU follows an **autosomal recessive** inheritance pattern. *Phenylalanine (amino acid)* - While **phenylalanine** accumulates to toxic levels in PKU, it is the substrate, not the deficient component. - The disease stems from the body's inability to metabolize this amino acid due to the enzyme deficiency. *Tyrosine* - **Tyrosine** is the product of the reaction catalyzed by PAH, and its production is limited in PKU. - However, the deficiency of tyrosine is a secondary effect, not the primary cause of PKU. *None of the options* - This option is incorrect because the deficiency of **phenylalanine hydroxylase (PAH)** is precisely the cause of Phenylketonuria.

Q: In which condition is Vanillylmandelic Acid (VMA) excreted in urine?

Pheochromocytoma. ***Pheochromocytoma*** - **Vanillylmandelic Acid (VMA)** is a breakdown product of catecholamines (epinephrine and norepinephrine), which are overproduced in **pheochromocytoma** [2]. - Elevated VMA levels in urine are a key diagnostic marker for this **adrenal medulla tumor** [3]. *Alkaptonuria* - This is an inherited disorder of **tyrosine metabolism**, leading to the accumulation of **homogentisic acid**. - It causes **ochronosis** (darkening of cartilage and connective tissue) and dark urine upon standing, but not elevated VMA. *Neuroblastoma* - While neuroblastoma also involves **catecholamine overproduction**, it typically leads to elevated levels of **homovanillic acid (HVA)** and **VMA**, but is primarily characterized by HVA in children. - Neuroblastoma is a childhood cancer, distinct from the adrenal tumor seen in adults with pheochromocytoma. *Cushing's syndrome* - This condition is characterized by chronic **excess cortisol** production, leading to symptoms like central obesity, moon facies, and hypertension [1]. - It does not involve alterations in catecholamine metabolism or elevated VMA levels.

Q: Amino acid metabolism is implicated in which disease?

Maple syrup urine disease. ***Maple syrup urine disease*** - This disease is a genetic disorder of **amino acid metabolism**, specifically the breakdown of **branched-chain amino acids** (leucine, isoleucine, and valine). - Its characteristic odor, resembling **maple syrup**, is due to the accumulation of these amino acids and their ketoacids in bodily fluids. *Reye's syndrome* - This is an acute encephalopathy and **liver dysfunction**, typically affecting children, often linked to aspirin use during viral infections. - It primarily involves **mitochondrial damage** and fatty infiltration of the liver, not a direct amino acid metabolism defect. *Von Gierke's disease* - This is a type of **glycogen storage disease (Type I)** caused by a deficiency in **glucose-6-phosphatase**. - It leads to an inability to release glucose from stored glycogen in the liver and kidneys, resulting in **hypoglycemia** and hepatomegaly, not amino acid metabolism issues. *McArdle's disease* - This is another **glycogen storage disease (Type V)**, characterized by a deficiency in **muscle phosphorylase**. - Patients experience exercise intolerance, muscle pain, and cramping due to the inability to break down muscle glycogen for energy, unrelated to amino acid metabolism.

Q: Albinism is due to deficiency of the following enzyme?

Tyrosinase. ***Tyrosinase*** - **Albinism** is characterized by a lack of melanin production, which is primarily due to a defect in the enzyme **tyrosinase**. - **Tyrosinase** is crucial for converting **tyrosine** into **DOPA**, a key step in the biosynthesis of melanin. *Phenylalanine hydroxylase* - Deficiency of **phenylalanine hydroxylase** leads to **phenylketonuria (PKU)**, an inherited metabolic disorder. - This enzyme is responsible for converting **phenylalanine to tyrosine**, not directly involved in melanin synthesis. *Homogentisic acid oxidase* - A deficiency in **homogentisic acid oxidase** causes **alkaptonuria**, a rare metabolic disorder. - This enzyme is involved in the breakdown pathway of **tyrosine and phenylalanine**, causing homogentisic acid accumulation, which results in dark urine and ochronosis. *Decarboxylase* - The term "decarboxylase" refers to a general class of enzymes that remove a **carboxyl group** from a compound. - While decarboxylation reactions occur in many metabolic pathways, no single decarboxylase deficiency is directly responsible for **albinism**.

Q: Which amino acid is the primary precursor for the synthesis of glutamate?

Glutamine. ***Glutamine*** - **Glutamine** is a crucial precursor for **glutamate synthesis**, primarily through the enzyme **glutaminase**, which hydrolyzes glutamine to glutamate and ammonia. - This conversion is vital in various tissues, including the brain and kidneys, playing roles in **neurotransmission** and **acid-base balance**. *alpha-ketoglutarate* - **Alpha-ketoglutarate** is an intermediate in the **Krebs cycle** and can be converted to glutamate via **transamination** or **reductive amination** (by glutamate dehydrogenase). - While it directly forms glutamate, it is often considered an immediate precursor in metabolic cycles rather than the primary *amino acid* precursor. *Proline* - **Proline** is a non-essential amino acid that can be synthesized from **glutamate**, but it is not a direct precursor for glutamate itself. - Its metabolic pathway involves converting glutamate to **glutamate-gamma-semialdehyde**, which then cyclizes to form proline. *Cysteine* - **Cysteine** is a sulfur-containing amino acid involved in the synthesis of **glutathione** and protein structure, but it is not directly involved in the synthesis of glutamate. - Its metabolic pathways are distinct and primarily revolve around sulfur metabolism and redox regulation.

Q: HHH syndrome is due to a defect in which transporter?

Ornithine transporter. ***Ornithine transporter*** - HHH syndrome, or **hyperornithinemia-hyperammonemia-homocitrullinuria syndrome**, is caused by a defect in the mitochondrial ornithine transporter, **ORNT1** (also known as SLC25A15). - This deficiency leads to impaired transport of **ornithine** into the mitochondria, disrupting the **urea cycle** and resulting in the characteristic metabolic abnormalities. *Tryptophan transporter* - Deficiencies in tryptophan transporters are associated with conditions like **Hartnup disease** (impaired absorption in the intestine and reabsorption in the kidney). - This condition presents with symptoms related to **niacin deficiency**, such as pellagra-like skin rashes, ataxia, and psychiatric disturbances, which are distinct from HHH syndrome. *Histidine transporter* - Defects in histidine transporters are not typically associated with HHH syndrome. - Impaired histidine transport is linked to conditions causing **histidinemia**, characterized by elevated histidine levels in blood and urine, but not the hyperammonemia or homocitrullinuria seen in HHH syndrome. *Branched chain amino acid transporter* - Deficiencies in branched-chain amino acid (BCAA) transporters are primarily associated with conditions like **Maple Syrup Urine Disease (MSUD)**. - MSUD involves the accumulation of BCAAs and their keto acids, leading to neurological damage, but does not present with the specific metabolic profile of HHH syndrome.

Q: What is the primary function of the enzyme tyrosinase?

Synthesis of melanin. ***Synthesis of melanin*** - **Tyrosinase** is a copper-containing enzyme that catalyzes the hydroxylation of **tyrosine** to DOPA and the oxidation of DOPA to dopaquinone. - These steps are crucial for the biosynthesis of **melanin**, the primary pigment responsible for skin, hair, and eye color. *Synthesis of norepinephrine* - **Norepinephrine** synthesis involves a series of enzymatic steps starting from **tyrosine**, but tyrosinase is not directly involved in its formation. - The conversion of **dopamine** to norepinephrine is catalyzed by **dopamine β-hydroxylase**. *Synthesis of dopamine* - **Dopamine** is synthesized from **L-DOPA** by the enzyme **DOPA decarboxylase**. - While DOPA is an intermediate in melanin synthesis, **tyrosinase** is not the primary enzyme for dopamine production, although it can produce DOPA from tyrosine. *Synthesis of thyroxine* - **Thyroxine (T4)** is a thyroid hormone synthesized from **tyrosine residues** on **thyroglobulin** by the enzyme **thyroid peroxidase**. - This process is distinct from tyrosinase's role in melanin synthesis.

Question 1

A 5-year-old girl with global developmental delay, hypotonia, and failure to thrive presents with severe hyperammonemia. Laboratory results show increased orotic acid, increased glutamine, and decreased BUN. Evaluate and determine the underlying metabolic disorder.

Accepted Answer

Ornithine transcarbamylase deficiency (OTC deficiency)

Answer

Maple syrup urine disease (MSUD)

Answer

Phenylketonuria (PKU)

Answer

Methylmalonic acidemia (MMA)

Question 2

Ochronosis is due to the accumulation of?

Accepted Answer

Homogentisic acid

Answer

Alkapton

Answer

Xanthurenate

Answer

Glyoxylate

Question 3

What is the primary methyl donor in the conversion of norepinephrine to epinephrine?

Accepted Answer

S-adenosyl methionine

Answer

Arginine

Answer

Phenylalanine

Answer

Methionine

Question 4

Phenylketonuria is due to deficiency of:

Accepted Answer

Phenylalanine hydroxylase (PAH)

Answer

Phenylalanine (amino acid)

Answer

Tyrosine

Answer

None of the options

Question 5

In which condition is Vanillylmandelic Acid (VMA) excreted in urine?

Accepted Answer

Pheochromocytoma

Answer

Alkaptonuria

Answer

Neuroblastoma

Answer

Cushing's syndrome

Question 6

Amino acid metabolism is implicated in which disease?

Accepted Answer

Maple syrup urine disease

Answer

Reye's syndrome

Answer

Von Gierke's disease

Answer

McArdle's disease

Question 7

Albinism is due to deficiency of the following enzyme?

Accepted Answer

Tyrosinase

Answer

Phenylalanine hydroxylase

Answer

Homogentisic acid oxidase

Answer

Decarboxylase

Question 8

Which amino acid is the primary precursor for the synthesis of glutamate?

Accepted Answer

Glutamine

Answer

Proline

Answer

Cysteine

Answer

alpha-ketoglutarate

Question 9

HHH syndrome is due to a defect in which transporter?

Accepted Answer

Ornithine transporter

Answer

Histidine transporter

Answer

Tryptophan transporter

Answer

Branched chain amino acid transporter

Question 10

What is the primary function of the enzyme tyrosinase?

Accepted Answer

Synthesis of melanin

Answer

Synthesis of norepinephrine

Answer

Synthesis of dopamine

Answer

Synthesis of thyroxine

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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