Amino Acid Metabolism Practice Questions

Q: Which of these amino acids does not enter the Krebs cycle by forming acetyl-CoA from pyruvate?

Tyrosine. ***Tyrosine*** - **Tyrosine** is both ketogenic and glucogenic, degraded to **fumarate** (a Krebs cycle intermediate) and **acetoacetate** (converted to acetyl-CoA). - Importantly, tyrosine does **not form acetyl-CoA via pyruvate** - it bypasses pyruvate entirely in its degradation pathway. - Its catabolism involves homogentisate and maleylacetoacetate, eventually yielding fumarate and acetoacetate directly. *Glycine* - **Glycine** is converted to **serine**, which is then deaminated to **pyruvate**. - This pyruvate undergoes oxidative decarboxylation by pyruvate dehydrogenase to form **acetyl-CoA**. - Thus, glycine enters the Krebs cycle via the pyruvate → acetyl-CoA route. *Alanine* - **Alanine** is directly converted to **pyruvate** via transamination (alanine aminotransferase). - This pyruvate is then converted to **acetyl-CoA**, which enters the Krebs cycle. - This is one of the most direct amino acid → pyruvate → acetyl-CoA pathways. *Hydroxyproline* - **Hydroxyproline** is primarily degraded to **glyoxylate**, which can be converted to glycine. - A portion is metabolized to **α-ketoglutarate**, which enters the Krebs cycle **directly** as an intermediate. - While some metabolic routes may generate pyruvate indirectly (via glycine → serine → pyruvate), the predominant pathway does involve pyruvate formation, making it functionally similar to glycine in this context.

Q: Which of the following amino acids is essential for the synthesis of cysteine?

Methionine. ***Methionine*** - **Methionine** is an **essential amino acid** that serves as the sole source of the **sulfur atom** required for cysteine synthesis. - The transsulfuration pathway converts methionine → **S-adenosylmethionine (SAM)** → **homocysteine** → **cystathionine** → **cysteine**. - This pathway requires enzymes **cystathionine β-synthase** and **cystathionine γ-lyase**, both dependent on **vitamin B6 (pyridoxal phosphate)**. - Since methionine is an **essential amino acid** and the only source of sulfur for cysteine, it is the correct answer. *Serine* - While **serine** provides the **carbon skeleton** (C1, C2, C3) for cysteine synthesis and is absolutely required for the reaction, serine itself is a **non-essential amino acid** that can be synthesized endogenously from **3-phosphoglycerate**. - Serine condenses with homocysteine (derived from methionine) via cystathionine β-synthase to form cystathionine. - The question asks for an **essential** amino acid, which serine is not. *Glycine* - **Glycine** is not involved in cysteine synthesis. - It is a simple non-essential amino acid with various metabolic roles but does not contribute to the sulfur or carbon components of cysteine. *Phenylalanine* - **Phenylalanine** is an essential aromatic amino acid that is converted to **tyrosine** via phenylalanine hydroxylase. - It plays no role in cysteine synthesis and does not provide sulfur atoms or the carbon skeleton for cysteine formation.

Q: Which of the following amino acids is not converted to succinyl-CoA?

Histidine. ***Histidine*** - Histidine is exclusively converted to **α-ketoglutarate**, a different **TCA cycle intermediate**, via **formiminoglutamate (FIGLU)**. - Its catabolic pathway does not involve formation of **succinyl-CoA**. *Methionine* - Methionine is a glucogenic amino acid that is catabolized to **succinyl-CoA** through several intermediate steps. - These steps include the formation of **S-adenosylmethionine (SAM)** and subsequent conversion to **homocysteine**. *Isoleucine* - Isoleucine is both a glucogenic and ketogenic amino acid. - Its catabolism yields **acetyl-CoA** and **propionyl-CoA**, with the latter being converted to **succinyl-CoA**. *Valine* - Valine is a **branched-chain amino acid** that is exclusively **glucogenic**. - Its breakdown pathway produces **propionyl-CoA**, which is then further metabolized to **succinyl-CoA**.

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: 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 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: 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: 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: 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.

Question 1

Which of these amino acids does not enter the Krebs cycle by forming acetyl-CoA from pyruvate?

Accepted Answer

Tyrosine

Answer

Glycine

Answer

Alanine

Answer

Hydroxyproline

Question 2

Which of the following amino acids is essential for the synthesis of cysteine?

Accepted Answer

Methionine

Answer

Glycine

Answer

Serine

Answer

Phenylalanine

Question 3

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 4

Which of the following amino acids is not converted to succinyl-CoA?

Accepted Answer

Histidine

Answer

Methionine

Answer

Valine

Answer

Isoleucine

Question 5

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 6

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 7

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

Accepted Answer

Tyrosine

Answer

Leucine

Answer

Valine

Answer

Arginine

Question 8

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

Accepted Answer

Tyrosine

Answer

Alanine

Answer

Glutamate

Answer

Proline

Question 9

Citrullinemia type I results from a deficiency of the

Accepted Answer

Argininosuccinate synthase

Answer

Isocitrate dehydrogenase

Answer

Pyruvate dehydrogenase

Answer

Succinyl CoA synthase

Question 10

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

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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