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: Creatine is formed from:

Arginine. ***Arginine*** - **Creatine** is synthesized from **three amino acids**: arginine, glycine, and methionine. - In the **first step** (kidney/pancreas), arginine donates its **guanidino group** to glycine, forming **guanidinoacetate** via the enzyme arginine:glycine amidinotransferase. - In the **second step** (liver), guanidinoacetate is methylated by **S-adenosylmethionine (SAM)** derived from methionine to form **creatine**. - Among the options given, **arginine** is a direct precursor for creatine synthesis. *Lysine* - **Lysine** is an essential amino acid involved in protein synthesis, collagen formation, and **calcium absorption**. - It is **not involved** in creatine biosynthesis. *Leucine* - **Leucine** is a branched-chain amino acid **(BCAA)** important for muscle protein synthesis and energy production. - It is **not a precursor** for creatine synthesis. *Histidine* - **Histidine** serves as a precursor for **histamine** and participates in various metabolic reactions. - It is **not involved** in creatine biosynthesis.

Q: Which of the following amino acids contains two amino groups?

Lysine. ***Lysine*** - Lysine is an **essential amino acid** characterized by a **side chain with a primary amine group** at its epsilon-carbon, in addition to the alpha-amino group present in all amino acids. - This makes lysine one of the **basic amino acids**, as the extra amino group is protonated at physiological pH. *Glycine* - Glycine is the **simplest amino acid**, with a single hydrogen atom as its side chain, thus containing only the standard alpha-amino group. - It is **neither acidic nor basic** due to the absence of additional ionizable groups in its side chain. *Arginine* - Arginine contains a **guanidinium group** in its side chain, which is highly basic and contains three nitrogen atoms, but it is not considered two distinct amino groups. - While it has multiple nitrogen atoms, they are part of a single **complex functional group**, not separate amino groups. *Asparagine* - Asparagine has an **amide group** on its side chain, which contains nitrogen but is not an amino group. - The amide group is **neutral** and does not contribute to the basicity of the amino acid in the same way an amino group would.

Q: Which branched-chain amino acid is the PRIMARY contributor to neurotoxicity in maple syrup urine disease?

Leucine. ***Leucine*** - **Leucine** is the primary branched-chain amino acid (BCAA) responsible for neurotoxicity in **maple syrup urine disease (MSUD)** due to its high concentration in the brain, especially in immature brains. - High levels of leucine and its ketoacid derivative **alpha-ketoisocaproate** interfere with neurotransmitter synthesis, myelin formation, and cerebral energy metabolism, leading to significant neurological damage. *Valine* - While **valine** is one of the BCAAs that accumulates in MSUD, its contribution to direct neurotoxicity is less pronounced compared to leucine. - **Valine** is essential for protein synthesis and tissue repair but does not exert the same level of direct inhibitory effects on cerebral metabolism as leucine. *Isoleucine* - **Isoleucine** is another BCAA whose levels are elevated in MSUD, but it plays a relatively minor role in the acute neurotoxicity compared to leucine. - Its levels must be monitored during dietary treatment of MSUD, as maintaining proper balance of all three BCAAs is essential for preventing metabolic decompensation. *Phenylalanine* - **Phenylalanine** is an aromatic amino acid, not a branched-chain amino acid, and its elevated levels are characteristic of **phenylketonuria (PKU)**, not MSUD. - High phenylalanine causes neurotoxicity in PKU through different mechanisms, distinct from the BCAA-related toxicity seen in MSUD.

Q: HHH syndrome is due to a defect in which transporter?

Ornithine transporter. ***Ornithine transporter*** - HHH syndrome, or **hyperornithinemia-hyperammonemia-homocitrullinuria syndrome**, is caused by a defect in the mitochondrial ornithine transporter, **ORNT1** (also known as SLC25A15). - This deficiency leads to impaired transport of **ornithine** into the mitochondria, disrupting the **urea cycle** and resulting in the characteristic metabolic abnormalities. *Tryptophan transporter* - Deficiencies in tryptophan transporters are associated with conditions like **Hartnup disease** (impaired absorption in the intestine and reabsorption in the kidney). - This condition presents with symptoms related to **niacin deficiency**, such as pellagra-like skin rashes, ataxia, and psychiatric disturbances, which are distinct from HHH syndrome. *Histidine transporter* - Defects in histidine transporters are not typically associated with HHH syndrome. - Impaired histidine transport is linked to conditions causing **histidinemia**, characterized by elevated histidine levels in blood and urine, but not the hyperammonemia or homocitrullinuria seen in HHH syndrome. *Branched chain amino acid transporter* - Deficiencies in branched-chain amino acid (BCAA) transporters are primarily associated with conditions like **Maple Syrup Urine Disease (MSUD)**. - MSUD involves the accumulation of BCAAs and their keto acids, leading to neurological damage, but does not present with the specific metabolic profile of HHH syndrome.

Q: Which vitamin is not used in the treatment of homocysteinuria?

Thiamine (Vitamin B1). ***Thiamine (Vitamin B1)*** - **Thiamine** is not directly involved in the metabolic pathways that process homocysteine. - Its primary role is in **carbohydrate metabolism** and nerve function, not homocysteine reduction. *Vitamin B6* - **Vitamin B6 (pyridoxine)** is a cofactor for the enzyme **cystathionine beta-synthase**, which converts homocysteine to cystathionine. - Supplementation with vitamin B6 can help reduce homocysteine levels in some patients with homocystinuria, particularly those with **pyridoxine-responsive forms**. *Vitamin B12* - **Vitamin B12 (cobalamin)** is a cofactor for **methionine synthase**, an enzyme that converts homocysteine back to methionine. - This pathway is crucial for lowering homocysteine levels, making B12 supplementation beneficial in homocystinuria. *Folate* - **Folate (Vitamin B9)**, in the form of 5-methyltetrahydrofolate, provides the methyl group necessary for **methionine synthase** to convert homocysteine to methionine. - Therefore, folate supplementation is essential in the treatment regimen for homocystinuria to support homocysteine metabolism.

Q: Rate limiting enzyme in catecholamine synthesis?

Tyrosine hydroxylase. ***Tyrosine hydroxylase*** - **Tyrosine hydroxylase** catalyzes the conversion of **tyrosine to L-DOPA**, which is the first and **rate-limiting step** in the synthesis of **catecholamines** (dopamine, norepinephrine, epinephrine). - Its activity is tightly regulated, making it a key control point for **catecholamine levels**. *Dopa decarboxylase* - **Dopa decarboxylase** converts **L-DOPA to dopamine**, which is a subsequent step in the pathway. - This enzyme is generally **not rate-limiting** and has high activity, quickly processing L-DOPA. *Dopamine hydroxylase* - **Dopamine hydroxylase** converts **dopamine to norepinephrine**. - This enzyme is active after the rate-limiting step and does not control the overall synthesis rate from tyrosine. *N-methyltransferase* - Also known as **phenylethanolamine N-methyltransferase (PNMT)**, this enzyme converts **norepinephrine to epinephrine**. - This is the final step in epinephrine synthesis and occurs after the rate-limiting step, primarily in the adrenal medulla.

Q: Which of the following is an acidic amino acid?

Glutamic acid. ***Glutamic acid*** - **Glutamic acid** is an **acidic amino acid** because its side chain (R-group) contains a carboxyl group (-COOH) that can donate a proton, lowering the pH. - At physiological pH, the carboxyl group is deprotonated, giving it a **negative charge**. - The two acidic amino acids are **glutamic acid** and **aspartic acid**. *Alanine* - **Alanine** is a **non-polar, aliphatic amino acid** with a methyl group (-CH₃) as its side chain. - It is **neutral** at physiological pH and does not have acidic or basic properties. *Lysine* - **Lysine** is a **basic amino acid** because its side chain contains an amine group (-NH₂) that can accept a proton. - At physiological pH, the amine group is protonated, giving it a **positive charge**. *None of the options* - This option is incorrect because **glutamic acid** is indeed an acidic amino acid. - Therefore, there is a correct option provided in the list.

Q: Which of the following is the simplest amino acid?

Glycine. ***Glycine*** - Glycine is the **simplest amino acid**, with a single hydrogen atom as its side chain. - Its small size and lack of a bulky side chain make it unique. *Alanine* - Alanine has a **methyl (CH3) group** as its side chain, making it larger than glycine. - It is a common amino acid but not the simplest. *Tryptophan* - Tryptophan possesses a large, **aromatic indole ring** as its side chain. - This complex structure is far from the simplest amino acid. *Cysteine* - Cysteine contains a **thiol (-SH) group** in its side chain, which is capable of forming disulfide bonds. - The presence of the sulfur atom makes it more complex than glycine.

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.

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

Creatine is formed from:

Accepted Answer

Arginine

Answer

Leucine

Answer

Lysine

Answer

Histidine

Question 3

Which of the following amino acids contains two amino groups?

Accepted Answer

Lysine

Answer

Asparagine

Answer

Arginine

Answer

Glycine

Question 4

Which branched-chain amino acid is the PRIMARY contributor to neurotoxicity in maple syrup urine disease?

Accepted Answer

Leucine

Answer

Isoleucine

Answer

Valine

Answer

Phenylalanine

Question 5

HHH syndrome is due to a defect in which transporter?

Accepted Answer

Ornithine transporter

Answer

Histidine transporter

Answer

Tryptophan transporter

Answer

Branched chain amino acid transporter

Question 6

Which vitamin is not used in the treatment of homocysteinuria?

Accepted Answer

Thiamine (Vitamin B1)

Answer

Vitamin B6

Answer

Vitamin B12

Answer

Folate

Question 7

Rate limiting enzyme in catecholamine synthesis?

Accepted Answer

Tyrosine hydroxylase

Answer

Dopa decarboxylase

Answer

N-methyltransferase

Answer

Dopamine hydroxylase

Question 8

Which of the following is an acidic amino acid?

Accepted Answer

Glutamic acid

Answer

Alanine

Answer

Lysine

Answer

None of the options

Question 9

Which of the following is the simplest amino acid?

Accepted Answer

Glycine

Answer

Alanine

Answer

Tryptophan

Answer

Cysteine

Question 10

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

Amino Acid Metabolism — MCQs

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