Amino Acid Metabolism — MCQs
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Which of these amino acids does not enter the Krebs cycle by forming acetyl-CoA from pyruvate?
Which of the following amino acids is essential for the synthesis of cysteine?
Which of the following amino acids is not converted to succinyl-CoA?
Which amino acid can be utilized in both gluconeogenesis and ketogenesis?
Dopamine beta-hydroxylase catalyzes which of the following reactions?
Which one of the following disorders does not depend on pyridoxine for treatment?
Which one of the following is synthesized from an essential amino acid?
Which amino acid has an imino group?
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:
Which of the following amino acids is purely ketogenic?
Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs
Question 591: Which of these amino acids does not enter the Krebs cycle by forming acetyl-CoA from pyruvate?
- A. Glycine
- B. Tyrosine (Correct Answer)
- C. Alanine
- D. Hydroxyproline
Explanation: ***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.
Question 592: Which of the following amino acids is essential for the synthesis of cysteine?
- A. Glycine
- B. Methionine (Correct Answer)
- C. Serine
- D. Phenylalanine
Explanation: ***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.
Question 593: Which of the following amino acids is not converted to succinyl-CoA?
- A. Methionine
- B. Histidine (Correct Answer)
- C. Valine
- D. Isoleucine
Explanation: ***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**.
Question 594: Which amino acid can be utilized in both gluconeogenesis and ketogenesis?
- A. Leucine
- B. Valine
- C. Arginine
- D. Tyrosine (Correct Answer)
Explanation: ***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.
Question 595: Dopamine beta-hydroxylase catalyzes which of the following reactions?
- A. Dopa to dopamine
- B. Dopamine to norepinephrine (Correct Answer)
- C. Norepinephrine to epinephrine
- D. Phenylalanine to tyrosine
Explanation: ***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**.
Question 596: Which one of the following disorders does not depend on pyridoxine for treatment?
- A. Homocystinuria
- B. Cystathioninuria
- C. Xanthurenic aciduria
- D. Methylmalonic acidemia (Correct Answer)
Explanation: ***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.
Question 597: Which one of the following is synthesized from an essential amino acid?
- A. Alanine
- B. Glutamate
- C. Proline
- D. Tyrosine (Correct Answer)
Explanation: ***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.
Question 598: Which amino acid has an imino group?
- A. Proline (Correct Answer)
- B. Glycine
- C. Arginine
- D. Tryptophan
Explanation: ***Proline*** - **Proline** is unique among the 20 standard **amino acids** because its side chain forms a ring with the amino group, creating a secondary amine or **imino group**. - This cyclic structure gives proline distinct conformational properties, often leading to kinks in **protein structures**. *Glycine* - **Glycine** is the simplest amino acid, with a single hydrogen atom as its side chain. - It possesses a primary **amino group**, not an imino group. *Arginine* - **Arginine** has a complex side chain containing a **guanidinium group**. - This group is basic but is not an **imino group** in the structural sense of pyrrolidine ring found in proline. *Tryptophan* - **Tryptophan** possesses an **indole ring** within its side chain. - The **indole ring** contains a nitrogen atom that is part of a five-membered ring, but this nitrogen is part of a secondary amine within an aromatic system, not an **imino group** as seen in proline.
Question 599: 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:
- A. Phosphorylation
- B. Transamination (Correct Answer)
- C. Deamination
- D. Decarboxylation
Explanation: ***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 600: Which of the following amino acids is purely ketogenic?
- A. Phenylalanine
- B. Leucine (Correct Answer)
- C. Proline
- D. Tyrosine
Explanation: ***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).
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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