Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 481: Which enzyme deficiency is associated with increased levels of orotic acid and megaloblastic anemia?

A. UMP synthase (Correct Answer)
B. Dihydroorotate dehydrogenase
C. Ribonucleotide reductase
D. Adenylosuccinate lyase

Explanation: ***UMP synthase*** - A deficiency in **UMP synthase** (a bifunctional enzyme with orotate phosphoribosyltransferase and OMP decarboxylase activities) leads to the accumulation of its substrate, **orotic acid**. - **Orotic aciduria** impairs pyrimidine synthesis, which in turn affects DNA replication in rapidly dividing cells, leading to **megaloblastic anemia**. *Dihydroorotate dehydrogenase* - This enzyme is involved in an earlier step of **pyrimidine synthesis**, converting dihydroorotate to orotate. - A deficiency would cause **dihydroorotate accumulation**, not orotic acid, and is a different, though related, metabolic disorder. *Ribonucleotide reductase* - **Ribonucleotide reductase** converts ribonucleotides to deoxyribonucleotides, essential for DNA synthesis. - While its inhibition can cause **megaloblastic anemia** (e.g., hydroxyurea), it does not directly lead to **orotic acid accumulation**. *Adenylosuccinate lyase* - This enzyme is involved in **purine synthesis**, converting adenylosuccinate to AMP. - A deficiency would affect purine metabolism and would not cause **orotic acid accumulation** or megaloblastic anemia directly related to pyrimidine synthesis.

Question 482: What is the effect of a defect in the enzyme phenylalanine hydroxylase on neurotransmitter synthesis?

A. Decreased dopamine production (Correct Answer)
B. Decreased serotonin production
C. Decreased norepinephrine synthesis
D. Increased glutamate synthesis

Explanation: ***Decreased dopamine production*** - A defect in **phenylalanine hydroxylase (PAH)** leads to **phenylketonuria (PKU)**, characterized by an accumulation of phenylalanine. - High phenylalanine levels competitively inhibit the enzyme **tyrosine hydroxylase**, which is crucial for the conversion of tyrosine to **L-DOPA**, a precursor to **dopamine** and other catecholamines. - This represents the **most direct enzymatic effect** of PAH deficiency on neurotransmitter synthesis. *Decreased serotonin production* - While serotonin production is indeed decreased in PKU, this occurs through an **indirect mechanism**. - High phenylalanine competitively inhibits the **large neutral amino acid transporter (LAT1)** at the blood-brain barrier, reducing tryptophan entry into the brain. - However, this is a **competitive transport effect**, not a direct enzymatic block like with dopamine synthesis. *Decreased norepinephrine synthesis* - **Norepinephrine** is synthesized from **dopamine** via dopamine β-hydroxylase. - Decreased dopamine production will subsequently lead to decreased norepinephrine synthesis. - However, this is a **downstream consequence** of the primary defect in dopamine synthesis, making it less direct than the dopamine effect itself. *Increased glutamate synthesis* - **Glutamate** synthesis occurs primarily from **alpha-ketoglutarate** in the Krebs cycle or from glutamine via glutaminase. - There is no direct mechanism by which a defect in **phenylalanine hydroxylase** would lead to increased glutamate synthesis. - While chronic high phenylalanine can cause excitotoxicity and neuronal damage, this does not result in increased overall glutamate synthesis.

Question 483: In the metabolism of amino acids, what is the significance of the enzyme glutamate dehydrogenase?

A. It converts glutamate to alpha-ketoglutarate (Correct Answer)
B. It synthesizes glutamate from ammonia
C. It degrades excess amino acids
D. It facilitates the uptake of amino acids into cells

Explanation: ***It converts glutamate to alpha-ketoglutarate*** - **Glutamate dehydrogenase** catalyzes the **oxidative deamination** of glutamate, producing **alpha-ketoglutarate** and releasing ammonia. - This reaction is crucial for funneling nitrogen from amino acids into the **urea cycle** and carbon skeletons into the **TCA cycle**. *It synthesizes glutamate from ammonia* - While glutamate dehydrogenase can catalyze the reverse reaction (reductive amination) under certain conditions, its primary physiological role in amino acid catabolism is the **degradation of glutamate**, not its synthesis. - The synthesis of glutamate from ammonia can occur via glutamate dehydrogenase or **glutamate synthase (GOGAT)** in various tissues. *It degrades excess amino acids* - Glutamate dehydrogenase primarily acts on **glutamate**, not directly on a wide range of excess amino acids. - It plays a key role in the **final common pathway for nitrogen removal** from many amino acids through glutamate. *It facilitates the uptake of amino acids into cells* - **Glutamate dehydrogenase** is an intracellular enzyme involved in metabolism, not in membrane transport processes. - **Amino acid transporters** are responsible for facilitating the uptake of amino acids into cells.

Question 484: A patient with a history of recurrent kidney stones is diagnosed with cystinuria. Which amino acid transport defect is most likely?

A. Lysine
B. Histidine
C. Arginine
D. Cysteine (Correct Answer)

Explanation: ***Cysteine*** - Cystinuria is an inherited autosomal recessive disorder characterized by defective transport of **cystine** (the oxidized dimer of cysteine) and the dibasic amino acids (lysine, ornithine, arginine) in the renal proximal tubules and intestinal epithelium. - The transport defect leads to increased urinary excretion of **cystine**, which has poor solubility in acidic urine, resulting in formation of hexagonal **cystine stones** in the kidneys, ureters, and bladder. - The primary defect is in the **rBAT-b0,+AT amino acid transporter system**, which normally reabsorbs cysteine (which oxidizes to cystine in urine) and dibasic amino acids. - This explains the patient's recurrent kidney stones and makes **cysteine transport defect** the correct answer. *Lysine* - While **lysine** is one of the dibasic amino acids whose reabsorption is impaired in cystinuria, it does not precipitate to form stones. - Lysine remains soluble in urine despite increased urinary excretion, so it does not contribute to nephrolithiasis. - The clinical manifestations of cystinuria are due to **cystine stone formation**, not lysine accumulation. *Histidine* - **Histidine** is a basic amino acid, but it is not one of the primary amino acids affected by the transport defect in classic cystinuria. - The main amino acids involved are cystine and the dibasic amino acids: **lysine, ornithine, and arginine** (mnemonic: COLA). - Histidine transport is generally preserved in cystinuria. *Arginine* - **Arginine** is another dibasic amino acid whose tubular reabsorption is defective in cystinuria, along with lysine and ornithine. - However, like lysine, **arginine** remains soluble in urine and does not precipitate to form stones. - The pathology and clinical presentation are driven by **cystine precipitation**, not arginine accumulation.

Question 485: A patient with maple syrup urine disease has a deficiency in which of the following enzymes?

A. Branched-chain α-keto acid dehydrogenase (Correct Answer)
B. Phenylalanine hydroxylase
C. Homogentisate oxidase
D. Fumarylacetoacetate hydrolase

Explanation: ***Branched-chain α-keto acid dehydrogenase*** - **Maple syrup urine disease (MSUD)** is caused by a deficiency in the **branched-chain α-keto acid dehydrogenase (BCKDH)** complex. - This enzyme is crucial for the catabolism of **branched-chain amino acids (BCAAs)**: leucine, isoleucine, and valine, leading to their toxic accumulation. *Phenylalanine hydroxylase* - A deficiency in **phenylalanine hydroxylase** causes **phenylketonuria (PKU)**, not MSUD. - This enzyme converts phenylalanine to tyrosine; its absence leads to the accumulation of **phenylalanine** and its metabolites. *Homogentisate oxidase* - A deficiency in **homogentisate oxidase** is responsible for **alkaptonuria**. - This enzyme is involved in the breakdown of **tyrosine**, leading to the accumulation of **homogentisic acid**. *Fumarylacetoacetate hydrolase* - A deficiency in **fumarylacetoacetate hydrolase** causes **hereditary tyrosinemia type 1 (HT1)**. - This enzyme is the final step in the degradation of **tyrosine**, and its deficiency leads to the accumulation of toxic metabolites like succinylacetone.

Question 486: A newborn presents with vomiting, lethargy, and failure to thrive. Laboratory tests reveal metabolic acidosis and elevated levels of propionic acid. Which enzyme deficiency is most likely?

A. Phenylalanine hydroxylase
B. Propionyl-CoA carboxylase (Correct Answer)
C. Branched-chain α-ketoacid dehydrogenase
D. Ornithine transcarbamylase

Explanation: ***Propionyl-CoA carboxylase*** - **Propionyl-CoA carboxylase deficiency** (propionic acidemia) leads to the accumulation of **propionic acid** due to the inability to convert **propionyl-CoA** to **methylmalonyl-CoA**. - This accumulation results in **metabolic acidosis**, vomiting, lethargy, and **failure to thrive** in newborns. - This is a **biotin-dependent enzyme**, so some cases may respond to biotin supplementation. *Phenylalanine hydroxylase* - Deficiency of **phenylalanine hydroxylase** causes **phenylketonuria (PKU)**, leading to elevated phenylalanine levels. - PKU typically presents with intellectual disability, seizures, and a musty odor, not primarily with elevated propionic acid. *Branched-chain α-ketoacid dehydrogenase* - Deficiency in **branched-chain α-ketoacid dehydrogenase** causes **maple syrup urine disease (MSUD)**, characterized by the accumulation of branched-chain amino acids (leucine, isoleucine, valine). - MSUD presents with a characteristic maple syrup odor in urine, feeding difficulties, and neurological symptoms, not elevated propionic acid. *Ornithine transcarbamylase* - **Ornithine transcarbamylase (OTC) deficiency** is an **X-linked urea cycle disorder** resulting in hyperammonemia. - Symptoms include lethargy, coma, and respiratory alkalosis due to ammonia toxicity, not elevated propionic acid or primary metabolic acidosis.

Question 487: A child presents with developmental delay and a musty odor. Which enzyme deficiency is most likely?

A. Phenylalanine hydroxylase (Correct Answer)
B. Tyrosinase
C. Homogentisate oxidase
D. Dihydropteridine reductase

Explanation: ***Phenylalanine hydroxylase*** - Deficiency of **phenylalanine hydroxylase** leads to the accumulation of **phenylalanine** and its metabolites, causing **phenylketonuria (PKU)**. - The elevated phenylalanine metabolites, such as **phenylacetate**, are responsible for the characteristic **"musty" or "mousy" odor** in affected children, along with **developmental delay** if untreated. *Tyrosinase* - Deficiency of **tyrosinase** is associated with **albinism**, a condition characterized by a lack of melanin production leading to **hypopigmentation of the skin, hair, and eyes**. - It does not typically present with a musty odor or primarily with global developmental delay in the absence of other symptoms. *Homogentisate oxidase* - Deficiency of **homogentisate oxidase** causes **alkaptonuria**, a genetic disorder characterized by the accumulation of **homogentisic acid**. - This leads to **dark urine upon standing**, **ochronosis** (dark pigmentation of cartilage and connective tissues), and severe **osteoarthritis** in adulthood, not a musty odor or early developmental delay. *Dihydropteridine reductase* - Deficiency of **dihydropteridine reductase** leads to a **tetrahydrobiopterin (BH4) deficiency**, which is a co-factor for phenylalanine hydroxylase, as well as tyrosine hydroxylase and tryptophan hydroxylase. - This can cause a **"malignant" form of PKU** with **severe neurological symptoms** due to impaired neurotransmitter synthesis, but a distinct musty odor is more specifically associated with direct phenylalanine hydroxylase deficiency.

Question 488: What dietary modification is recommended to manage hyperammonemia in patients with ornithine transcarbamylase deficiency?

A. Low-carbohydrate diet
B. High-carbohydrate diet
C. Low-protein diet (Correct Answer)
D. High-protein diet

Explanation: ***Correct: Low-protein diet*** - **Ornithine transcarbamylase (OTC) deficiency** impairs the urea cycle's ability to convert **ammonia from protein catabolism** into urea. - A **low-protein diet** reduces the substrate for ammonia production, thereby helping to manage hyperammonemia. - This is the **primary dietary modification** for managing OTC deficiency. *Incorrect: High-carbohydrate diet* - While a **high-carbohydrate diet** might be used to provide calories and spare protein, it is not the primary dietary modification for *managing ammonia levels* directly. - Its role is secondary to protein restriction, focusing on adequate energy intake. *Incorrect: Low-carbohydrate diet* - A **low-carbohydrate diet** would typically lead to increased reliance on **fat and protein metabolism for energy**, potentially exacerbating hyperammonemia due to increased protein breakdown. - It is generally not recommended for managing OTC deficiency. *Incorrect: High-protein diet* - A **high-protein diet** would significantly *increase the nitrogen load* and, consequently, **ammonia production**, worsening hyperammonemia in patients with OTC deficiency. - This diet would be detrimental and is contraindicated in such patients.

Question 489: Which enzyme deficiency is directly responsible for the accumulation of phenylalanine in the blood of individuals with phenylketonuria?

A. Phenylalanine hydroxylase (Correct Answer)
B. Hexokinase
C. Glucokinase
D. Pyruvate kinase

Explanation: ***Correct: Phenylalanine hydroxylase*** - This enzyme is crucial for converting **phenylalanine** into **tyrosine** - Its deficiency leads to the buildup of phenylalanine in the blood, which is characteristic of **phenylketonuria (PKU)** - PKU is an **autosomal recessive disorder** and one of the most common inborn errors of metabolism - Without treatment, elevated phenylalanine causes **intellectual disability, seizures, and behavioral problems** *Incorrect: Hexokinase* - This enzyme is involved in the **first step of glycolysis**, phosphorylating glucose to glucose-6-phosphate - Deficiency of hexokinase does not directly cause an accumulation of phenylalanine - It is unrelated to amino acid metabolism *Incorrect: Glucokinase* - Glucokinase is an enzyme that facilitates the **phosphorylation of glucose to glucose-6-phosphate** in the liver and pancreatic beta cells - Its dysfunction is associated with certain types of **maturity-onset diabetes of the young (MODY)**, but not with phenylalanine metabolism disorders - It serves as the glucose sensor in pancreatic beta cells *Incorrect: Pyruvate kinase* - This enzyme catalyzes the **final step of glycolysis**, converting phosphoenolpyruvate to pyruvate - Deficiencies in pyruvate kinase can lead to **hemolytic anemia** but do not impact phenylalanine levels - It is unrelated to amino acid metabolism

Question 490: Which of the following is a key regulatory enzyme in the urea cycle?

A. Carbamoyl phosphate synthetase I (Correct Answer)
B. Ornithine transcarbamylase
C. Argininosuccinate synthetase
D. Arginase

Explanation: ***Carbamoyl phosphate synthetase I*** - This enzyme catalyzes the **first committed step** of the urea cycle, forming **carbamoyl phosphate** from ammonia, bicarbonate, and ATP. - Its activity is tightly regulated, primarily by **N-acetylglutamate**, reflecting changes in protein catabolism and ammonia levels. *Ornithine transcarbamylase* - This enzyme catalyzes the **second step** of the urea cycle, combining **carbamoyl phosphate** with **ornithine** to form citrulline. - While essential for the cycle, it is not considered the primary regulatory point, as its substrate, carbamoyl phosphate, is regulated by CPS I. *Argininosuccinate synthetase* - This enzyme catalyzes the **third step** of the urea cycle, converting **citrulline** and **aspartate** into argininosuccinate. - Its activity is dependent on the availability of citrulline and aspartate, but it does not exert the primary regulatory control over the overall cycle flux. *Arginase* - This enzyme catalyzes the **final step** of the urea cycle, hydrolyzing **arginine** to form urea and regenerating ornithine. - While critical for producing urea and recycling ornithine, its regulation is secondary to the initial rate-limiting step controlled by CPS I.

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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