Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 491: What is the consequence of a defect in the enzyme branched-chain alpha-keto acid dehydrogenase?

A. Accumulation of branched-chain amino acids (Correct Answer)
B. Reduced synthesis of fatty acids
C. Increased glucose synthesis
D. Decreased cholesterol levels

Explanation: ***Accumulation of branched-chain amino acids*** - **Branched-chain alpha-keto acid dehydrogenase (BCKDH)** is responsible for the oxidative decarboxylation of alpha-keto acids, which are derived from branched-chain amino acids (BCAAs) like **leucine, isoleucine, and valine**. - A defect in this enzyme leads to the **buildup of BCAAs** and their corresponding alpha-keto acids in the blood and urine, a hallmark of **Maple Syrup Urine Disease**. *Reduced synthesis of fatty acids* - While BCAAs can be used for energy production and contribute to lipid metabolism, a direct and primary consequence of BCKDH defect is not a reduction in fatty acid synthesis. - The main issue lies in the **catabolism of BCAAs**, not necessarily downstream fatty acid production. *Increased glucose synthesis* - The elevated levels of BCAAs can actually inhibit several enzymes involved in **gluconeogenesis**. - Therefore, a defect in BCKDH would more likely lead to impaired, rather than increased, glucose synthesis due to the toxic accumulation of metabolites. *Decreased cholesterol levels* - The catabolism of BCAAs, particularly **leucine**, produces metabolites like **acetyl-CoA** which are precursors for cholesterol synthesis. - While a defect in BCKDH would impair this pathway, the primary and most significant consequence is not decreased cholesterol, but rather the toxic effects of accumulated BCAAs and their alpha-keto acids.

Question 492: A patient has elevated levels of both homocysteine and methionine. Which enzyme deficiency could be the cause?

A. Cystathionine β-synthase (Correct Answer)
B. Methionine synthase
C. Cystathionine γ-lyase
D. S-adenosylmethionine synthetase

Explanation: ***Cystathionine β-synthase*** - A deficiency in **cystathionine β-synthase (CBS)** leads to the accumulation of both **homocysteine** and **methionine** because homocysteine cannot be converted into cystathionine, causing it to build up. - The elevated homocysteine also inhibits methionine metabolism, leading to methionine accumulation as well. - This condition is known as **classical homocystinuria**, characterized by elevated levels of both these amino acids in blood and urine. *Methionine synthase* - **Methionine synthase** (also known as homocysteine methyltransferase) catalyzes the conversion of **homocysteine to methionine**, using methylcobalamin (vitamin B12) and methyltetrahydrofolate. - A deficiency in this enzyme would lead to **elevated homocysteine** but **decreased methionine**, as methionine cannot be regenerated from homocysteine. *Cystathionine γ-lyase* - **Cystathionine γ-lyase (cystathionase)** is the enzyme downstream of CBS that converts **cystathionine to cysteine** in the transsulfuration pathway. - A deficiency causes **cystathioninuria** with elevated cystathionine but typically **normal or mildly elevated homocysteine and methionine**. - This does not match the clinical picture of elevated both homocysteine and methionine. *S-adenosylmethionine synthetase* - **S-adenosylmethionine (SAM) synthetase** converts **methionine into SAM**, which is a crucial methyl donor in methylation reactions. - A deficiency would cause **elevated methionine** but **normal or decreased homocysteine**, as methionine would not be efficiently converted to SAM and subsequently to homocysteine. - This deficiency is rare and presents with hepatic dysfunction and neurological issues.

Question 493: Which process is primarily responsible for converting ammonia to a less toxic substance in the liver?

A. Glycolysis
B. The Urea Cycle (Correct Answer)
C. The Citric Acid Cycle
D. Beta-oxidation

Explanation: ***The Urea Cycle*** - The **urea cycle** is the primary metabolic pathway in the liver for converting highly toxic **ammonia** into less toxic **urea**, which can then be safely excreted by the kidneys. - This cycle is crucial for detoxification of ammonia, a byproduct of amino acid metabolism. - Key enzymes include **carbamoyl phosphate synthetase I**, ornithine transcarbamylase, argininosuccinate synthetase, argininosuccinate lyase, and arginase. *Glycolysis* - Glycolysis is a metabolic pathway that breaks down glucose into pyruvate, producing energy (ATP). - It does not play any direct role in ammonia detoxification. *The Citric Acid Cycle* - The Citric Acid Cycle (Krebs cycle) is a central metabolic pathway for generating ATP through the oxidation of acetyl-CoA. - While it interfaces with amino acid metabolism, it is not directly responsible for converting ammonia to urea. *Beta-oxidation* - Beta-oxidation is the process by which fatty acids are broken down into acetyl-CoA for energy production. - This process is involved in lipid metabolism and has no direct function in ammonia detoxification.

Question 494: A patient with hyperammonemia is found to have elevated levels of citrulline. Which enzyme deficiency is most likely?

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

Explanation: ***Argininosuccinate synthetase deficiency*** - A deficiency in **argininosuccinate synthetase** (ASS) prevents the conversion of **citrulline** and **aspartate** to argininosuccinate, leading to the accumulation of **citrulline**. - The accumulated citrulline diffuses out of the hepatocytes and into the bloodstream, resulting in **elevated plasma citrulline** levels, which is a hallmark of this disorder, also known as **Citrullinemia Type I**. *CPS I deficiency* - **Carbamoyl phosphate synthetase I (CPS I)** is the first committed step of the urea cycle; a deficiency results in the inability to synthesize **carbamoyl phosphate**. - This leads to severe **hyperammonemia** but very **low levels of citrulline** and arginine, as the cycle cannot progress beyond the initial stages. *OTC deficiency* - **Ornithine transcarbamylase (OTC)** deficiency prevents the conversion of **carbamoyl phosphate** and **ornithine** to citrulline. - This common X-linked disorder causes **severe hyperammonemia** with **low blood citrulline** levels, contrasting with the elevated citrulline seen in the patient. *Arginase deficiency* - **Arginase** catalyzes the final step of the urea cycle, converting **arginine** to ornithine and urea. - A deficiency leads to an accumulation of **arginine** and **hyperargininemia**, with **normal or only modestly elevated levels of citrulline**, along with hyperammonemia, which is distinct from the primary elevation of citrulline.

Question 495: A patient with a history of gout is advised to avoid foods high in purines. Which metabolic product is primarily responsible for this condition?

A. Uric acid (a product of purine metabolism) (Correct Answer)
B. Nitrogen (a component of purines)
C. Ammonia (a waste product of protein metabolism)
D. Pyruvate (an end product of glycolysis)

Explanation: ***Uric acid (a product of purine metabolism)*** - **Gout** is caused by the deposition of **uric acid crystals** in joints, leading to inflammation and pain. - **Purines**, found in certain foods, are metabolized into uric acid, thus increasing its levels in the body. *Nitrogen (a component of purines)* - While nitrogen is a **component of purine molecules**, it is not the direct metabolic product responsible for gout. - Nitrogen is present in various biological molecules and is excreted in different forms, primarily as urea, not directly causing gout. *Ammonia (a waste product of protein metabolism)* - **Ammonia** is a highly toxic waste product predominantly from **protein and amino acid metabolism**. - It is converted to **urea in the liver** for safe excretion and is not directly involved in the pathogenesis of gout. *Pyruvate (an end product of glycolysis)* - **Pyruvate** is a crucial intermediate metabolite in **carbohydrate metabolism** (glycolysis). - It plays a central role in energy production but has no direct connection to purine metabolism or gout.

Question 496: 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.

A. Maple syrup urine disease (MSUD)
B. Phenylketonuria (PKU)
C. Methylmalonic acidemia (MMA)
D. Ornithine transcarbamylase deficiency (OTC deficiency) (Correct Answer)

Explanation: ***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**.

Question 497: Ochronosis is due to the accumulation of?

A. Homogentisic acid (Correct Answer)
B. Alkapton
C. Xanthurenate
D. Glyoxylate

Explanation: ***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.

Question 498: What is the primary methyl donor in the conversion of norepinephrine to epinephrine?

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

Explanation: ***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.

Question 499: Phenylketonuria is due to deficiency of:

A. Phenylalanine (amino acid)
B. Tyrosine
C. Phenylalanine hydroxylase (PAH) (Correct Answer)
D. None of the options

Explanation: ***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.

Question 500: Amino acid metabolism is implicated in which disease?

A. Maple syrup urine disease (Correct Answer)
B. Reye's syndrome
C. Von Gierke's disease
D. McArdle's disease

Explanation: ***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.

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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