Amino Acid Metabolism Practice Questions

Q: Which enzyme catalyzes the transfer of an α-amino group from aspartate to α-ketoglutarate?

Aspartate Transaminase (AST). ***Aspartate Transaminase (AST)*** - **Aspartate transaminase (AST)**, also known as **glutamic-oxaloacetic transaminase (GOT)**, specifically catalyzes the reversible transfer of an **α-amino group** from an L-amino acid (like **aspartate**) to an α-keto acid (like **α-ketoglutarate**). - This reaction forms a new amino acid and a new α-keto acid; in this case, **oxaloacetate** and **glutamate** are formed. *Ornithine transcarbamylase (OTC)* - **OTC** is an enzyme involved in the **urea cycle**, catalyzing the reaction between **ornithine** and **carbamoyl phosphate** to form citrulline. - It does not catalyze the transfer of an α-amino group from aspartate to α-ketoglutarate. *Argininosuccinate lyase (ASL)* - **ASL**, also part of the **urea cycle**, catalyzes the cleavage of **argininosuccinate** into **arginine** and **fumarate**. - Its function is distinct from transamination reactions involving α-amino and α-keto acids. *Alanine Aminotransferase (ALT)* - **ALT**, also known as **glutamic-pyruvic transaminase (GPT)**, specifically catalyzes the transfer of an **α-amino group** from **alanine** to **α-ketoglutarate**. - While it performs a similar type of transamination, it acts on alanine, not aspartate.

Q: Match the enzyme with the disease caused due to its deficiency: **Enzymes:** 1. Fumarylacetoacetate hydrolase 2. Tyrosine transaminase 3. Tyrosinase 4. Homogentisate oxidase **Diseases:** A. Tyrosinemia Type I B. Tyrosinemia Type II C. Albinism D. Alkaptonuria

1 → A, 2 → B, 3 → C, 4 → D. **1 → A, 2 → B, 3 → C, 4 → D** - **Fumarylacetoacetate hydrolase** deficiency causes **Tyrosinemia Type I**, a severe metabolic disorder affecting the liver and kidneys with accumulation of toxic metabolites like succinylacetone. - **Tyrosine transaminase** deficiency leads to **Tyrosinemia Type II** (Richner-Hanhart syndrome), characterized by ocular and cutaneous lesions with elevated plasma tyrosine. - **Tyrosinase** deficiency results in **Albinism** (oculocutaneous albinism type 1), due to lack of melanin production from tyrosine. - **Homogentisate oxidase** deficiency causes **Alkaptonuria**, where homogentisic acid accumulates, turning urine black upon standing and causing ochronosis. *1 → A, 2 → C, 3 → D, 4 → B* - This incorrectly matches tyrosine transaminase with Albinism and tyrosinase with Alkaptonuria, reversing the melanin synthesis pathway enzyme with the tyrosine degradation pathway enzyme. *1 → C, 2 → D, 3 → A, 4 → B* - This incorrectly matches fumarylacetoacetate hydrolase with Albinism (which requires tyrosinase deficiency) and homogentisate oxidase with Tyrosinemia Type II (which requires tyrosine transaminase deficiency). *1 → C, 2 → A, 3 → D, 4 → B* - This incorrectly places the severe hepatorenal disease (Tyrosinemia Type I) with tyrosine transaminase instead of fumarylacetoacetate hydrolase, and mismatches the melanin pathway enzyme with alkaptonuria.

Q: Which of the following amino acids is not converted to alpha-ketoglutarate during catabolism?

Glycine. ***Glycine*** - Glycine catabolism typically produces **serine**, which can then be converted to **pyruvate**, not alpha-ketoglutarate. - Its degradation pathway leads to the formation of **carbon dioxide** and **ammonia** via the glycine cleavage system (GCS), or it can be converted to serine and subsequently pyruvate. *Glutamate* - Glutamate is directly deaminated by **glutamate dehydrogenase** to form **alpha-ketoglutarate**, a key intermediate in the **Krebs cycle**. - This is a reversible reaction that plays a crucial role in nitrogen metabolism. *Histidine* - Histidine catabolism involves several steps, including deamination by **histidase**, eventually leading to the formation of **N-formiminoglutamate (FIGLU)**. - FIGLU is then converted to **glutamate**, which subsequently forms **alpha-ketoglutarate**. *Proline* - Proline is catabolized through a series of oxidation steps to yield **glutamate**. - This glutamate is then converted to **alpha-ketoglutarate** by glutamate dehydrogenase.

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: 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: 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: 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: 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: 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.

Question 1

Which enzyme catalyzes the transfer of an α-amino group from aspartate to α-ketoglutarate?

Accepted Answer

Aspartate Transaminase (AST)

Answer

Ornithine transcarbamylase (OTC)

Answer

Argininosuccinate lyase (ASL)

Answer

Alanine Aminotransferase (ALT)

Question 2

Match the enzyme with the disease caused due to its deficiency:

**Enzymes:**
1. Fumarylacetoacetate hydrolase
2. Tyrosine transaminase
3. Tyrosinase
4. Homogentisate oxidase

**Diseases:**
A. Tyrosinemia Type I
B. Tyrosinemia Type II
C. Albinism
D. Alkaptonuria

Accepted Answer

1 → A, 2 → B, 3 → C, 4 → D

Answer

1 → A, 2 → C, 3 → D, 4 → B

Answer

1 → C, 2 → D, 3 → A, 4 → B

Answer

1 → C, 2 → A, 3 → D, 4 → B

Question 3

Which of the following amino acids is not converted to alpha-ketoglutarate during catabolism?

Accepted Answer

Glycine

Answer

Glutamate

Answer

Histidine

Answer

Proline

Question 4

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 5

Urea is synthesized in all except:

Accepted Answer

Brain

Answer

Spleen

Answer

Liver

Answer

Kidney

Question 6

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 7

Tyrosine enters gluconeogenesis by forming which substrate

Accepted Answer

Fumarate

Answer

Succinyl CoA

Answer

Alpha-ketoglutarate

Answer

Citrate

Question 8

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

Accepted Answer

Arginine

Answer

Aspartate

Answer

Argininosuccinate

Answer

Citrulline

Question 9

Citrullinemia is due to deficiency of?

Accepted Answer

Argininosuccinate synthase

Answer

Arginase

Answer

Ornithine transcarbamylase

Answer

Argininosuccinate lyase

Question 10

Cabbage-like odour is seen in ?

Accepted Answer

Tyrosinemia

Answer

Alkaptonuria

Answer

Phenylketonuria

Answer

Hartnup disease

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

On this page

Practice by Chapter

Want unlimited practice?