Amino Acid Metabolism Practice Questions

Q: Which one of the following disorders does not depend on pyridoxine for treatment?

Methylmalonic acidemia. ***Methylmalonic acidemia*** - This condition is caused by a deficiency in the enzyme **methylmalonyl CoA mutase** or its coenzyme, **adenosylcobalamin (a derivative of vitamin B12)**. - Therefore, treatment primarily involves dietary management and supplementation with **vitamin B12**, not pyridoxine (vitamin B6). *Homocystinuria* - Many forms of homocystinuria, particularly those involving a deficiency in **cystathionine beta-synthase**, respond to large doses of **pyridoxine (vitamin B6)**. - Pyridoxine acts as a coenzyme for cystathionine beta-synthase, helping to reduce **homocysteine** levels. *Cystathioninuria* - This metabolic disorder results from a deficiency in **cystathionine gamma-lyase**, an enzyme that requires **pyridoxal phosphate (active form of vitamin B6)** as a coenzyme. - Supplementation with **pyridoxine** can normalize or reduce the excretion of cystathionine in the urine for many affected individuals. *Xanthurenic aciduria* - This condition is often associated with a deficiency in **kynureninase**, an enzyme in the **tryptophan metabolism pathway** that requires **pyridoxal phosphate** as a coenzyme. - Supplementation with **pyridoxine** can typically correct the metabolic defect and normalize xanthurenic acid excretion.

Q: Which amino acid can be utilized in both gluconeogenesis and ketogenesis?

Tyrosine. ***Tyrosine (Correct Answer)*** - Tyrosine is **both glucogenic and ketogenic**, making it the correct answer. - It is **glucogenic** because its metabolism yields **fumarate**, which can enter the TCA cycle and contribute to **gluconeogenesis**. - It is also **ketogenic** because its degradation produces **acetoacetate**, a **ketone body**. *Leucine* - Leucine is a purely **ketogenic** amino acid, meaning its catabolism only produces **acetyl-CoA** and **acetoacetate**. - It cannot be converted into glucose precursors and therefore does not contribute to gluconeogenesis. *Valine* - Valine is a purely **glucogenic** amino acid, meaning its metabolism produces **succinyl-CoA**. - Succinyl-CoA can be converted into **oxaloacetate** and then to glucose via gluconeogenesis, but it does not produce ketone bodies. *Arginine* - Arginine is a purely **glucogenic** amino acid, serving as a precursor for **α-ketoglutarate** in the TCA cycle. - This pathway allows its carbon skeleton to be diverted into glucose production, but it does not yield ketone bodies.

Q: Dopamine beta-hydroxylase catalyzes which of the following reactions?

Dopamine to norepinephrine. ***Dopamine to norepinephrine*** - **Dopamine beta-hydroxylase** is the enzyme responsible for the **hydroxylation** of the beta-carbon of dopamine. - This reaction adds a hydroxyl group, converting **dopamine** into **norepinephrine** within **catecholamine synthesis**. *Dopa to dopamine* - This reaction is catalyzed by **Dopa decarboxylase** (also known as aromatic L-amino acid decarboxylase). - It involves the **decarboxylation** of Dopa, removing a carboxyl group to form dopamine. *Norepinephrine to epinephrine* - This conversion is catalyzed by **phenylethanolamine N-methyltransferase (PNMT)**. - **PNMT** adds a methyl group to the nitrogen atom of norepinephrine to produce epinephrine. *Phenylalanine to tyrosine* - The enzyme responsible for this conversion is **phenylalanine hydroxylase**. - This initial step in the catabolism of phenylalanine is crucial, and a deficiency leads to **phenylketonuria**.

Q: Which one of the following is synthesized from an essential amino acid?

Tyrosine. ***Tyrosine*** - Tyrosine is a **non-essential amino acid** that is synthesized from the essential amino acid **phenylalanine** via the enzyme phenylalanine hydroxylase. - Individuals with **phenylketonuria (PKU)** lack this enzyme, making tyrosine essential for them. *Alanine* - Alanine is a **non-essential amino acid** synthesized through transamination from **pyruvate**, a product of glycolysis. - It is not derived from an essential amino acid. *Glutamate* - Glutamate is a **non-essential amino acid** synthesized from **α-ketoglutarate** in the citric acid cycle. - It is not derived from an essential amino acid. *Proline* - Proline is a **non-essential amino acid** synthesized from **glutamate**. - It is not derived from an essential amino acid.

Q: The process in which the amino group of an amino acid is transferred to a keto acid and the keto group of a keto acid is transferred to an amino acid is called:

Transamination. ***Transamination*** - This process involves the **transfer of an amino group** from an amino acid to a **keto acid**, and simultaneously, the keto group from the keto acid to the amino acid [1], [2]. - **Aminotransferases** are a class of enzymes that catalyze these reactions, crucial for amino acid metabolism [2]. *Phosphorylation* - This is the addition of a **phosphate group** to a molecule, typically a protein or another organic compound [3]. - It is a key mechanism for regulating protein activity and energy transfer in the cell, and is not directly involved in nitrogen transfer between amino and keto acids [3]. *Deamination* - This process refers to the **removal of an amino group** from an amino acid, resulting in the formation of an alpha-keto acid and ammonia [4]. - Unlike transamination, deamination does not involve the transfer of the amino group to another molecule [1]. *Decarboxylation* - This is a chemical reaction that removes a **carboxyl group** (COOH) from a molecule, typically releasing carbon dioxide [3]. - It is involved in various metabolic pathways but does not involve the exchange of amino and keto groups.

Q: Citrullinemia type I results from a deficiency of the

Argininosuccinate synthase. ***Argininosuccinate synthase*** - **Citrullinemia type I** is an **autosomal recessive disorder** characterized by a deficiency of the enzyme **argininosuccinate synthase**. - This enzyme is crucial in the **urea cycle**, catalyzing the conversion of **citrulline** and **aspartate** into **argininosuccinate**. Its deficiency leads to the accumulation of citrulline and ammonia. *Isocitrate dehydrogenase* - **Isocitrate dehydrogenase** is an enzyme involved in the **citric acid cycle** (Krebs cycle) and plays a role in energy production, not the urea cycle. - A deficiency in this enzyme would affect cellular respiration and lead to different metabolic disorders, unrelated to ammonia detoxification. *Pyruvate dehydrogenase* - **Pyruvate dehydrogenase** is a multi-enzyme complex that converts **pyruvate** to **acetyl-CoA**, linking glycolysis to the citric acid cycle. - Deficiency in pyruvate dehydrogenase leads to lactic acidosis and neurological problems, distinct from the hyperammonemia seen in citrullinemia. *Succinyl CoA synthase* - **Succinyl CoA synthase** (or succinate thiokinase) is another enzyme of the **citric acid cycle** that catalyzes the reversible reaction of succinyl-CoA to succinate. - Its deficiency would impair energy metabolism within the mitochondria and would not directly lead to the accumulation of citrulline or hyperammonemia.

Q: Which of the following amino acids is purely ketogenic?

Leucine. ***Leucine*** - **Leucine** is one of the two purely ketogenic amino acids (the other being **lysine**), meaning its carbon skeleton is catabolized entirely into **acetyl-CoA** or **acetoacetate**. - This makes leucine an important precursor for the synthesis of **ketone bodies** but not glucose. *Phenylalanine* - **Phenylalanine** is both a **ketogenic** and **glucogenic** amino acid. - Its breakdown products can be converted into both ketone bodies and glucose precursors. *Proline* - **Proline** is a **glucogenic** amino acid. - Its degradation pathway leads to intermediates of the **citric acid cycle**, which can be used for glucose synthesis. *Tyrosine* - **Tyrosine** is both a **ketogenic** and **glucogenic** amino acid. - It is derived from phenylalanine and can be metabolized to produce both **fumarate** (glucogenic) and **acetoacetate** (ketogenic).

Q: Cysteine is synthesized from which amino acid?

Methionine. ***Methionine*** - Cysteine is synthesized from **methionine** through a series of reactions involving **S-adenosylmethionine (SAM)** and the transsulfuration pathway. - This pathway first converts **methionine to homocysteine**, which then condenses with **serine** to form **cystathionine**, and finally breaks down into **cysteine**. *Arginine* - Arginine is a precursor for **nitric oxide**, **urea**, **creatine**, and **polyamine synthesis**, not cysteine. - It plays a crucial role in the **urea cycle** to excrete ammonia. *Histidine* - Histidine is a precursor for **histamine** and is involved in various enzymatic reactions. - It is not directly involved in the synthesis of cysteine. *Lysine* - Lysine is an **essential amino acid** primarily involved in protein synthesis and the formation of **carnitine**. - It is not a precursor for cysteine.

Q: All of the following amino acids form Acetyl CoA via pyruvate dehydrogenase except:

Tyrosine. ***Tyrosine*** - Tyrosine is both a **glucogenic and ketogenic amino acid** that is broken down into **fumarate (glucogenic) and acetoacetate (ketogenic)**. - It does not form pyruvate as an intermediate in its degradation pathway. - Therefore, it does NOT pass through the pyruvate dehydrogenase complex to form acetyl-CoA. - The acetyl-CoA from tyrosine comes from acetoacetate breakdown, not via pyruvate dehydrogenase. *Glycine* - Glycine can be converted to **serine**, which is then converted to **pyruvate**. - Pyruvate is then converted to acetyl-CoA by the **pyruvate dehydrogenase complex**. - Therefore, glycine forms acetyl-CoA via pyruvate dehydrogenase. *Hydroxyproline* - Hydroxyproline is metabolized through several intermediates and can eventually form **pyruvate**. - Once converted to pyruvate, it can then be converted to acetyl-CoA via **pyruvate dehydrogenase**. - Therefore, hydroxyproline can form acetyl-CoA via this pathway. *Alanine* - Alanine is directly converted to pyruvate through a **transamination reaction** catalyzed by alanine aminotransferase (ALT). - Pyruvate is then converted to acetyl-CoA by the **pyruvate dehydrogenase complex**. - This is one of the most direct pathways from amino acid to acetyl-CoA via pyruvate dehydrogenase.

Question 1

Which of the following statements regarding phenylketonuria (PKU) is false?

Accepted Answer

Neurological symptoms are due to deficiency of phenylalanine hydroxylase.

Answer

Blood phenyl alanine level >20 mg/dl causes severe disease

Answer

Method of choice for screening is blood phenylalanine by Guthrie's test.

Answer

PKU is caused by a deficiency of phenylalanine hydroxylase.

Question 2

Which one of the following disorders does not depend on pyridoxine for treatment?

Accepted Answer

Methylmalonic acidemia

Answer

Homocystinuria

Answer

Cystathioninuria

Answer

Xanthurenic aciduria

Question 3

Which amino acid can be utilized in both gluconeogenesis and ketogenesis?

Accepted Answer

Tyrosine

Answer

Leucine

Answer

Valine

Answer

Arginine

Question 4

Dopamine beta-hydroxylase catalyzes which of the following reactions?

Accepted Answer

Dopamine to norepinephrine

Answer

Dopa to dopamine

Answer

Norepinephrine to epinephrine

Answer

Phenylalanine to tyrosine

Question 5

Which one of the following is synthesized from an essential amino acid?

Accepted Answer

Tyrosine

Answer

Alanine

Answer

Glutamate

Answer

Proline

Question 6

The process in which the amino group of an amino acid is transferred to a keto acid and the keto group of a keto acid is transferred to an amino acid is called:

Accepted Answer

Transamination

Answer

Phosphorylation

Answer

Deamination

Answer

Decarboxylation

Question 7

Citrullinemia type I results from a deficiency of the

Accepted Answer

Argininosuccinate synthase

Answer

Isocitrate dehydrogenase

Answer

Pyruvate dehydrogenase

Answer

Succinyl CoA synthase

Question 8

Which of the following amino acids is purely ketogenic?

Accepted Answer

Leucine

Answer

Phenylalanine

Answer

Proline

Answer

Tyrosine

Question 9

Cysteine is synthesized from which amino acid?

Accepted Answer

Methionine

Answer

Arginine

Answer

Histidine

Answer

Lysine

Question 10

All of the following amino acids form Acetyl CoA via pyruvate dehydrogenase except:

Accepted Answer

Tyrosine

Answer

Glycine

Answer

Alanine

Answer

Hydroxyproline

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

On this page

Practice by Chapter

Want unlimited practice?