Amino Acid Metabolism Practice Questions

Q: A 3-day-old newborn presents with hyperammonemia in the blood and an unknown inborn error of metabolism. What is the most likely diagnosis?

Urea cycle enzyme deficiency. ***Urea cycle enzyme deficiency*** - **Hyperammonemia** in a newborn is a hallmark of urea cycle disorders, as the urea cycle is essential for detoxifying ammonia. - The rapid onset of symptoms in a **3-day-old newborn** is consistent with severe forms of urea cycle enzyme deficiencies where ammonia rapidly accumulates after the initiation of protein feeding. *Maple syrup urine disease* - This condition involves impaired metabolism of **branched-chain amino acids** (leucine, isoleucine, valine), leading to their accumulation. - While it can cause neurological symptoms, **hyperammonemia** is not its primary or characteristic metabolic derangement; rather, a distinctive **maple syrup odor** in urine is typical. *Organic aciduria* - These disorders result from defects in pathways of **amino acid or fatty acid metabolism**, leading to the accumulation of various organic acids. - While some severe forms can present with metabolic acidosis and neurological symptoms, **hyperammonemia** is usually secondary and less prominent or central to the pathology compared to urea cycle disorders. *Phenylketonuria* - This is a disorder of **phenylalanine metabolism**, leading to excessive accumulation of phenylalanine in the blood. - It primarily causes developmental delay and neurological issues if untreated, but **hyperammonemia** is not a feature of phenylketonuria.

Q: Citrullinemia is due to deficiency of?

Argininosuccinate synthase. ***Argininosuccinate synthase*** - **Citrullinemia** is primarily caused by a deficiency in **argininosuccinate synthase (ASS)**, an enzyme crucial for the conversion of **citrulline** and **aspartate** to **argininosuccinate** in the urea cycle. - This deficiency leads to an accumulation of **citrulline** in the blood and urine, resulting in hyperammonemia and neurological symptoms. *Arginase* - A deficiency in **arginase** (also known as argininemia) leads to the accumulation of **arginine**, but not citrulline. - This enzyme is responsible for the final step of the urea cycle, converting **arginine** to **ornithine** and **urea**. *Ornithine transcarbamylase* - A deficiency in **ornithine transcarbamylase (OTC)** is another common urea cycle disorder, but it results in a buildup of **ammonia** and **carbamoyl phosphate**, not citrulline. - OTC catalyzes the conversion of **carbamoyl phosphate** and **ornithine** to **citrulline**. *Argininosuccinate lyase* - A deficiency in **argininosuccinate lyase (ASL)** causes **argininosuccinic aciduria**, characterized by the accumulation of **argininosuccinate** in the blood and urine. - This enzyme is responsible for converting **argininosuccinate** to **arginine** and **fumarate**.

Q: An infant is brought by his parents with complaints that his urine turns black on standing. Which of the following metabolic disorders is likely?

Alkaptonuria. ***Alkaptonuria*** - **Alkaptonuria** is an autosomal recessive disorder characterized by a deficiency of **homogentisate 1,2-dioxygenase**, an enzyme involved in the metabolism of tyrosine. - The accumulation of **homogentisic acid** in tissues and urine causes the urine to turn black on standing due to oxidation. *Phenylketonuria* - **Phenylketonuria (PKU)** is caused by a deficiency of **phenylalanine hydroxylase**, leading to the accumulation of phenylalanine. - While it can manifest with intellectual disability and neurological symptoms, it does not typically cause the urine to turn black. *Homocystinuria* - **Homocystinuria** is a disorder of methionine metabolism, typically due to a deficiency of **cystathionine beta-synthase**. - It is characterized by intellectual disability, skeletal abnormalities, and lens dislocation, but not black urine. *Maple syrup urine disease* - **Maple syrup urine disease (MSUD)** results from a deficiency of **branched-chain alpha-keto acid dehydrogenase complex**, leading to the accumulation of branched-chain amino acids. - The distinguishing feature is urine that smells like maple syrup, not turning black.

Q: Tyrosine enters gluconeogenesis by forming which substrate

Fumarate. ***Fumarate*** - Tyrosine is both **glucogenic and ketogenic**, producing two products during catabolism. - The glucogenic portion of tyrosine degradation yields **fumarate**, which enters the **citric acid cycle** and can be converted to oxaloacetate, the key substrate for gluconeogenesis. - Tyrosine → Homogentisate → Maleylacetoacetate → **Fumarate + Acetoacetate** (the latter being ketogenic). - Fumarate is subsequently converted to **malate → oxaloacetate → phosphoenolpyruvate**, initiating gluconeogenesis. *Pyruvate* - While pyruvate is an important gluconeogenic substrate, tyrosine does **not directly form pyruvate**. - Amino acids like **alanine, serine, and cysteine** are converted to pyruvate, not tyrosine. - Tyrosine's gluconeogenic pathway proceeds through **fumarate → oxaloacetate**, bypassing pyruvate formation. *Succinyl CoA* - Succinyl CoA is formed from the catabolism of **valine, isoleucine, methionine, and threonine**. - It is a **citric acid cycle intermediate** but not the product of tyrosine degradation. *Alpha-ketoglutarate* - **Glutamate, glutamine, proline, arginine, and histidine** are catabolized to alpha-ketoglutarate. - This is another **citric acid cycle intermediate** but is not formed from tyrosine metabolism. *Citrate* - Citrate is formed from **oxaloacetate and acetyl-CoA** in the citric acid cycle. - It is not a direct product of amino acid catabolism and does not serve as an entry point for gluconeogenesis.

Q: Cabbage-like odour is seen in ?

Tyrosinemia. ***Tyrosinemia*** - **Tyrosinemia type 1 (hereditary tyrosinemia)** leads to the accumulation of toxic metabolites due to a deficiency in **fumarylacetoacetate hydrolase**. - These metabolites, particularly **succinylacetone**, are responsible for the characteristic **cabbage-like (or rancid butter) odor** in urine and sweat. *Alkaptonuria* - This condition is characterized by a deficiency in **homogentisate-1,2-dioxygenase**, leading to the accumulation of **homogentisic acid**. - The urine turns **dark (black)** when exposed to air, but there is no specific odor associated with cabbage. *Phenylketonuria* - PKU results from a deficiency in **phenylalanine hydroxylase**, causing a buildup of **phenylalanine** and its metabolites. - The classic odor in PKU is described as **musty or mousy**, not cabbage-like. *Hartnup disease* - This is a disorder of **amino acid transport** across the intestinal and renal tubules, specifically for neutral amino acids like **tryptophan**. - It does not involve a distinct body or urine odor like the "cabbage-like" smell.

Q: Which amino acid is a precursor for the synthesis of nicotinic acid?

Tryptophan. ***Tryptophan*** - **Tryptophan** is an essential amino acid that serves as a precursor for various vital compounds, including **nicotinic acid** (niacin), serotonin, and melatonin, making it crucial for metabolic pathways. - The conversion of tryptophan to nicotinic acid involves a complex pathway (kynurenine pathway) that results in the synthesis of **NAD+** and **NADP+**, which are coenzymes vital for redox reactions in the body. - Approximately 60 mg of tryptophan can be converted to 1 mg of niacin equivalent. *Glutamine* - **Glutamine** is the most abundant amino acid in the body and plays an important role in immune function, gut health, and nitrogen transport. - While essential for many bodily functions, glutamine is not directly involved in the synthesis pathway of nicotinic acid. *Glycine* - **Glycine** is the simplest amino acid and plays important roles in protein synthesis, collagen formation, and as a neurotransmitter. - It serves as a precursor for heme, purines, and creatine, but is not involved in nicotinic acid synthesis. *Phenylalanine* - **Phenylalanine** is an essential amino acid that is a precursor for tyrosine, which in turn is a precursor for thyroid hormones, catecholamines (dopamine, norepinephrine, epinephrine), and melanin. - While it has significant metabolic roles, phenylalanine does not participate in the synthesis of nicotinic acid.

Q: In argininosuccinase deficiency, what should be supplemented to continue the urea cycle ?

Arginine. ***Correct: Arginine*** - **Arginine** supplementation is the primary treatment for argininosuccinase deficiency - Provides the product of the blocked reaction, allowing the urea cycle to continue - Maintains essential arginine levels and enables **nitrogen scavenger pathway** - Accumulated **argininosuccinate** is excreted in urine, removing excess nitrogen - This bypasses the deficient enzyme while facilitating alternative nitrogen disposal *Incorrect: Aspartate* - **Aspartate** combines with citrulline to form argininosuccinate via argininosuccinate synthetase - Supplementing aspartate alone would increase argininosuccinate accumulation without resolving the enzymatic block - Does not provide the end product (arginine) needed to continue the urea cycle - Not part of the primary therapeutic strategy for this deficiency *Incorrect: Citrulline* - **Citrulline** is actually used as adjunct therapy alongside arginine in argininosuccinase deficiency - However, citrulline supplementation alone cannot **continue the urea cycle** because the enzyme block prevents conversion of argininosuccinate to arginine - While it helps generate argininosuccinate for urinary excretion (nitrogen scavenging), it doesn't provide arginine needed for urea formation - **Arginine** remains the primary supplement as it directly supplies the missing product and enables cycle completion *Incorrect: Argininosuccinate* - **Argininosuccinate** accumulates in this deficiency due to the enzyme block - Supplementing the substrate of a deficient enzyme would worsen metabolic accumulation - The therapeutic goal is to facilitate its excretion, not increase its levels - Contraindicated in this condition

Q: Rate limiting step in urea cycle is catalyzed by ?

Carbamoyl-phosphate synthase I (CPS1). ***Carbamoyl-phosphate synthase I (CPS1)*** - **Carbamoyl-phosphate synthase I (CPS1)** is the enzyme that catalyzes the first committed step of the urea cycle, - This reaction involves the synthesis of **carbamoyl phosphate** from ammonia, carbon dioxide, and two molecules of ATP in the mitochondria. *Arginase* - **Arginase** is the enzyme that catalyzes the final step of the urea cycle, converting **arginine** to urea and ornithine. - This enzyme is responsible for generating urea for excretion but does not regulate the initiation of the cycle. *Argininosuccinase* - **Argininosuccinase (argininosuccinate lyase)** catalyzes the breakdown of argininosuccinate into fumarate and arginine. - This is an intermediate step in the urea cycle and not the rate-limiting step. *Ornithine transcarbamylase* - **Ornithine transcarbamylase (OTC)** catalyzes the reaction where **ornithine** reacts with carbamoyl phosphate to form citrulline. - While an essential mitochondrial enzyme in the urea cycle, it is not the rate-limiting step; CPS1 activity primarily controls the flux through the pathway.

Q: Urea is synthesized in all except:

Brain. ***Brain*** - The brain **does not possess the complete set of enzymes** required for the urea cycle, particularly lacking carbamoyl phosphate synthetase I (CPS I). - Due to the **blood-brain barrier**, the central nervous system maintains a tightly controlled metabolic environment and handles ammonia through alternative pathways (glutamine synthesis). - The brain **does not synthesize urea** and is the most commonly cited answer for organs lacking this capability. *Liver* - The **liver is the primary and main site of urea synthesis** in mammals, essential for detoxifying ammonia produced from amino acid catabolism. - All five enzymes of the **urea cycle** are predominantly expressed in hepatocytes (periportal region). *Kidney* - The kidney possesses some urea cycle enzymes and has **limited capacity for urea synthesis** in certain cell types. - However, its primary role in nitrogen metabolism is **urea excretion** and gluconeogenesis from amino acids, not significant urea production. - The kidney is NOT a major site of urea synthesis. *Spleen* - The **spleen is primarily involved in immune responses** and erythrocyte recycling (hemolysis and iron metabolism). - It **completely lacks the enzymes of the urea cycle** necessary for converting ammonia to urea.

Q: A 4-year-old boy from a first-degree consanguineous couple presents with darkening of the urine to an almost black color when left standing. He has a normal sibling and no other medical problems. His growth and development are normal. Which of the following metabolites is most likely to be elevated in this patient?

Homogentisate. ***Correct: Homogentisate*** - The clinical presentation of urine darkening to an almost black color upon standing, especially in a child from a consanguineous union, is classic for **alkaptonuria**. - **Alkaptonuria** is an autosomal recessive disorder caused by a deficiency of homogentisate 1,2-dioxygenase, leading to the accumulation of **homogentisate** in the body. - The homogentisic acid polymerizes upon exposure to air, causing the characteristic dark discoloration of urine. - Patients are typically asymptomatic in childhood; **ochronosis** (pigment deposition in connective tissues) and arthritis develop in adulthood. *Incorrect: Methylmalonate* - Elevated **methylmalonate** is characteristic of **methylmalonic aciduria**, a disorder of branched-chain amino acid metabolism or vitamin B12 deficiency. - This condition typically presents with metabolic acidosis, developmental delays, and failure to thrive, which are not described in this patient. *Incorrect: Phenylpyruvate* - Elevated **phenylpyruvate** is a hallmark of **phenylketonuria (PKU)**, an inborn error of metabolism affecting phenylalanine breakdown. - Untreated PKU leads to severe neurological impairment and intellectual disability, which are absent in this child. *Incorrect: α-Ketoisovalerate* - Elevated **α-Ketoisovalerate** is one of the branched-chain keto acids found in **maple syrup urine disease (MSUD)**. - MSUD presents in infancy with poor feeding, lethargy, and a characteristic maple syrup odor in the urine, not darkening urine.

Question 1

A 3-day-old newborn presents with hyperammonemia in the blood and an unknown inborn error of metabolism. What is the most likely diagnosis?

Accepted Answer

Urea cycle enzyme deficiency

Answer

Maple syrup urine disease

Answer

Organic aciduria

Answer

Phenylketonuria

Question 2

Citrullinemia is due to deficiency of?

Accepted Answer

Argininosuccinate synthase

Answer

Arginase

Answer

Ornithine transcarbamylase

Answer

Argininosuccinate lyase

Question 3

An infant is brought by his parents with complaints that his urine turns black on standing.    Which of the following metabolic disorders is likely?

Accepted Answer

Alkaptonuria

Answer

Phenylketonuria

Answer

Homocystinuria

Answer

Maple syrup urine disease

Question 4

Tyrosine enters gluconeogenesis by forming which substrate

Accepted Answer

Fumarate

Answer

Succinyl CoA

Answer

Alpha-ketoglutarate

Answer

Citrate

Question 5

Cabbage-like odour is seen in ?

Accepted Answer

Tyrosinemia

Answer

Alkaptonuria

Answer

Phenylketonuria

Answer

Hartnup disease

Question 6

Which amino acid is a precursor for the synthesis of nicotinic acid?

Accepted Answer

Tryptophan

Answer

Glutamine

Answer

Glycine

Answer

Phenylalanine

Question 7

In argininosuccinase deficiency, what should be supplemented to continue the urea cycle ?

Accepted Answer

Arginine

Answer

Aspartate

Answer

Argininosuccinate

Answer

Citrulline

Question 8

Rate limiting step in urea cycle is catalyzed by ?

Accepted Answer

Carbamoyl-phosphate synthase I (CPS1)

Answer

Arginase

Answer

Argininosuccinase

Answer

Ornithine transcarbamylase

Question 9

Urea is synthesized in all except:

Accepted Answer

Brain

Answer

Spleen

Answer

Liver

Answer

Kidney

Question 10

A 4-year-old boy from a first-degree consanguineous couple presents with darkening of the urine to an almost black color when left standing. He has a normal sibling and no other medical problems. His growth and development are normal. Which of the following metabolites is most likely to be elevated in this patient?

Accepted Answer

Homogentisate

Answer

Methylmalonate

Answer

Phenylpyruvate

Answer

α-Ketoisovalerate

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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