Amino Acid Metabolism Practice Questions

Q: In which condition is 5-Hydroxyindoleacetic acid (HIAA) elevated in urine?

Carcinoid. ***Carcinoid*** - **Carcinoid tumors** can synthesize and release large amounts of **serotonin**, which is metabolized in the body to **5-HIAA**. - Elevated **urinary 5-HIAA** is a key diagnostic marker for **carcinoid syndrome**, indicating excessive serotonin production. *Alkaptonuria* - This is a rare genetic disorder characterized by the accumulation of **homogentisic acid** in the body, leading to dark urine upon standing and ultimately **ochronosis**. - It does not involve abnormalities in serotonin metabolism or 5-HIAA levels. *Albinism* - Albinism is a group of inherited disorders characterized by very **little or no melanin pigment production**, commonly affecting the skin, hair, and eyes. - This condition is related to **tyrosine metabolism** and has no direct link to 5-HIAA levels. *Phenylketonuria* - **PKU** is a metabolic disorder caused by a deficiency in the enzyme **phenylalanine hydroxylase**, leading to a buildup of **phenylalanine** in the body. - This condition primarily affects phenylalanine metabolism and has no direct relationship with 5-HIAA levels, which are associated with serotonin metabolism.

Q: Conversion of Norepinephrine to epinephrine is mainly by?

S-adenosyl methionine. **S-adenosyl methionine (SAM)** - SAM acts as the **methyl donor** in the enzymatic conversion of **norepinephrine to epinephrine** by phenylethanolamine N-methyltransferase (PNMT). - This **methylation reaction** adds a methyl group to the nitrogen atom of norepinephrine, forming epinephrine. *Arginine* - Arginine is a precursor for **nitric oxide (NO)** synthesis, an important signaling molecule, and is also involved in the **urea cycle**. - It is not directly involved in the methylation of norepinephrine to epinephrine. *Phenylalanine* - Phenylalanine is an **essential amino acid** and a precursor for the synthesis of **tyrosine**, which is subsequently converted to **catecholamines** like dopamine, norepinephrine, and epinephrine. - However, it does not directly facilitate the final conversion step from norepinephrine to epinephrine. *Dehydrogenase* - Dehydrogenases are enzymes that catalyze **redox reactions** by removing hydrogen atoms from a substrate. - These enzymes are crucial in many metabolic pathways, but they are not involved in the **methylation reaction** that converts norepinephrine to epinephrine.

Q: Which of the following is classified as an essential amino acid?

Lysine. ***Lysine*** - **Essential amino acids** cannot be synthesized by the human body and must be obtained through the diet. - Lysine is one of the **nine essential amino acids** necessary for **protein synthesis**, collagen formation, calcium absorption, and other metabolic functions. - Remembered by the mnemonic: **PVT TIM HALL** (Phe, Val, Thr, Trp, Ile, Met, His, Arg*, Leu, Lys). *Aspartic acid* - Aspartic acid is a **non-essential amino acid**, meaning the body can synthesize it from oxaloacetate through transamination. - It plays a role in the **urea cycle** and **gluconeogenesis**. - Functions as an excitatory neurotransmitter in the central nervous system. *Glutamic acid* - Glutamic acid is also a **non-essential amino acid** that the body can readily produce from α-ketoglutarate. - It is a crucial excitatory **neurotransmitter** and a precursor for other amino acids like **glutamine** and **proline**. - Important in nitrogen metabolism and the urea cycle. *None of the options* - This option is incorrect because **lysine** is clearly an essential amino acid that must be obtained from dietary sources.

Q: Which coenzyme is not required in the formation of glutamate?

Thiamine pyrophosphate. ***Thiamine pyrophosphate*** - **Thiamine pyrophosphate (TPP)** is a coenzyme derived from **vitamin B1** that is essential for reactions involving decarboxylation, such as those catalyzed by pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase. - The formation of glutamate primarily involves transamination or reductive amination, which do not require TPP. *Pyridoxal phosphate* - **Pyridoxal phosphate (PLP)**, derived from **vitamin B6**, is a crucial coenzyme for **transamination reactions**, which are a major pathway for glutamate synthesis (e.g., from alpha-ketoglutarate). - It also plays a role in decarboxylation and deamination reactions of amino acids. *Niacin* - **Niacin (vitamin B3)** is a precursor for **NAD+** and **NADP+**, which are essential coenzymes in many metabolic pathways. - **NADPH**, derived from NADP+, is required as a reductant in the **reductive amination** of **alpha-ketoglutarate** to form glutamate, catalyzed by glutamate dehydrogenase. *None of the above* - This option is incorrect because **thiamine pyrophosphate** is indeed not required for the formation of glutamate. - The other two coenzymes, **pyridoxal phosphate** and **niacin (as NAD(P)H)**, are involved in glutamate synthesis.

Q: Creatinine is formed from -

Creatine. ***Creatine*** - **Creatinine** is a waste product formed from the non-enzymatic, irreversible degradation of **creatine** and **creatine phosphate**, primarily in muscles. - **Creatine** itself is synthesized endogenously from three amino acids: **glycine, arginine, and methionine** (as S-adenosylmethionine) through a two-step enzymatic process in the kidney and liver. - The amount of creatinine produced daily is relatively constant and directly proportional to an individual's **muscle mass**, making it a useful marker for renal function. *Lysine* - **Lysine** is an **essential amino acid** and a precursor for various compounds like **carnitine** but is not involved in creatinine or creatine formation. - Deficiency can lead to impaired protein synthesis but does not impact creatinine levels. *Leucine* - **Leucine** is another **essential amino acid** and a **branched-chain amino acid (BCAA)** crucial for muscle protein synthesis and repair. - It does not serve as a direct precursor for creatinine or creatine. *Histidine* - **Histidine** is an **essential amino acid** and a precursor for **histamine** and other important compounds, but not creatinine or creatine. - It plays roles in immune response and gastric acid secretion.

Q: Fumarate is formed from which amino acid?

Tyrosine. ***Tyrosine*** - **Tyrosine** is a **glucogenic and ketogenic amino acid** that is catabolized to acetoacetate and fumarate. - **Fumarate** then enters the **citric acid cycle (Krebs cycle)**, whereas acetoacetate is a ketone body. *Methionine* - **Methionine** is an **essential amino acid** and a precursor for **S-adenosylmethionine (SAM)**, a methyl donor in many reactions. - Its catabolism produces **succinyl CoA**, not fumarate, through a series of steps via propionyl CoA. *Valine* - **Valine** is a **branched-chain amino acid (BCAA)** that is exclusively **glucogenic**. - Its catabolism ultimately leads to the formation of **succinyl CoA**, which can enter the citric acid cycle. *Histidine* - **Histidine** is an **essential amino acid** that is catabolized to **formiminoglutamate (FIGLU)**. - FIGLU is then converted to **glutamate**, which can eventually be deaminated to α-ketoglutarate, a citric acid cycle intermediate, but not directly fumarate.

Q: Coenzyme for phenylalanine hydroxylase is?

Tetrahydrobiopterin. ***Tetrahydrobiopterin*** - **Tetrahydrobiopterin (BH4)** is an essential coenzyme for aromatic amino acid hydroxylases, including **phenylalanine hydroxylase (PAH)**. - PAH converts **phenylalanine** to **tyrosine**, and deficiencies in BH4 or PAH itself lead to *phenylketonuria (PKU)*. *Tetrahydrofolate* - **Tetrahydrofolate (THF)** is a coenzyme derived from **folic acid** and is primarily involved in **one-carbon metabolism**, including **purine** and **pyrimidine synthesis**, and various amino acid interconversions. - It does not directly act as a coenzyme for phenylalanine hydroxylase. *Pyridoxal phosphate* - **Pyridoxal phosphate (PLP)**, a derivative of **vitamin B6**, is a crucial coenzyme for many enzymes involved in **amino acid metabolism**, particularly in **transamination**, **decarboxylation**, and side-chain cleavage reactions. - It is not the coenzyme for phenylalanine hydroxylase. *S-adenosyl methionine* - **S-adenosyl methionine (SAM)** is a major **methyl donor** in various biochemical reactions, important for the synthesis of **neurotransmitters**, **hormones**, and **phospholipids**. - While essential for many metabolic pathways, it is not involved as a coenzyme for phenylalanine hydroxylase.

Q: In phenylketonuria, which substance should be restricted in the diet?

Phenylalanine. ***Phenylalanine*** - **Phenylketonuria (PKU)** is a genetic disorder where the body cannot effectively metabolize **phenylalanine** due to a deficiency in the enzyme **phenylalanine hydroxylase**. - Restricting dietary phenylalanine is crucial to prevent the accumulation of toxic byproducts that can lead to severe neurological damage and developmental delays. *Tyrosine* - Tyrosine is normally synthesized from phenylalanine. In PKU, this conversion is impaired. - While phenylalanine must be restricted, tyrosine supplementation is often necessary for individuals with PKU, as it becomes a conditionally essential amino acid. *Maize* - Maize (corn) is a carbohydrate-rich food and does not contain high levels of phenylalanine that would necessitate its restriction in PKU. - Dietary management in PKU focuses on regulating protein intake, as phenylalanine is an amino acid found in proteins. *None of the options* - This option is incorrect because phenylalanine must be strictly restricted in the diet of individuals with phenylketonuria to manage the condition effectively. - Without dietary restriction, the accumulation of phenylalanine can lead to severe and irreversible neurological damage.

Q: Which of the following is required in the synthesis of acetylcholine -

Choline. ***Choline*** - **Choline** is a **precursor** to acetylcholine, providing the **choline moiety** for its synthesis. - The enzyme **choline acetyltransferase** combines choline with acetyl-CoA (which provides the acetyl group) to form acetylcholine. *Inositol* - **Inositol** is a sugar alcohol that plays a role in **cell signaling** and as a secondary messenger in various pathways. - It is not directly involved as a substrate in the synthesis of acetylcholine. *Carnitine* - **Carnitine** is essential for the **transport of fatty acids** into the mitochondria for beta-oxidation. - It does not participate in the enzymatic reactions that produce acetylcholine. *Glycine* - **Glycine** is an amino acid that functions as an **inhibitory neurotransmitter** in the central nervous system. - While it is a neurotransmitter, it is not a component or precursor for acetylcholine synthesis.

Q: Which amino acid is not involved in transamination?

Lysine. ***Lysine*** - **Lysine** cannot undergo transamination because it lacks the structural requirements for typical transaminase enzymes. - While lysine has both an **α-amino group** and an **ε-amino group**, its metabolic pathway involves **oxidative deamination** rather than transamination. - Along with **threonine**, lysine is one of only two amino acids that do not participate in transamination reactions. *Alanine* - **Alanine** is a major substrate for transamination, readily converting to pyruvate via **alanine transaminase (ALT)**. - This reaction involves the transfer of its **α-amino group** to an α-keto acid, typically α-ketoglutarate, forming glutamate. *Aspartate* - **Aspartate** is actively involved in transamination, converting to oxaloacetate via **aspartate transaminase (AST)**. - Its **α-amino group** is easily transferred to α-ketoglutarate, forming glutamate. *Histidine* - **Histidine** can undergo transamination, though less commonly cited as a primary substrate compared to aspartate and alanine. - It can transfer its **α-amino group** to an α-keto acid, leading to the formation of imidazolepyruvate.

Question 1

In which condition is 5-Hydroxyindoleacetic acid (HIAA) elevated in urine?

Accepted Answer

Carcinoid

Answer

Alkaptonuria

Answer

Albinism

Answer

Phenylketonuria

Question 2

Conversion of Norepinephrine to epinephrine is mainly by?

Accepted Answer

S-adenosyl methionine

Answer

Arginine

Answer

Phenylalanine

Answer

Dehydrogenase

Question 3

Which of the following is classified as an essential amino acid?

Accepted Answer

Lysine

Answer

Aspartic acid

Answer

Glutamic acid

Answer

None of the options

Question 4

Which coenzyme is not required in the formation of glutamate?

Accepted Answer

Thiamine pyrophosphate

Answer

None of the above

Answer

Pyridoxal phosphate

Answer

Niacin

Question 5

Creatinine is formed from -

Accepted Answer

Creatine

Answer

Lysine

Answer

Leucine

Answer

Histidine

Question 6

Fumarate is formed from which amino acid?

Accepted Answer

Tyrosine

Answer

Methionine

Answer

Valine

Answer

Histidine

Question 7

Coenzyme for phenylalanine hydroxylase is?

Accepted Answer

Tetrahydrobiopterin

Answer

Tetrahydrofolate

Answer

Pyridoxal phosphate

Answer

S-adenosyl methionine

Question 8

In phenylketonuria, which substance should be restricted in the diet?

Accepted Answer

Phenylalanine

Answer

Tyrosine

Answer

Maize

Answer

None of the options

Question 9

Which of the following is required in the synthesis of acetylcholine -

Accepted Answer

Choline

Answer

Inositol

Answer

Carnitine

Answer

Glycine

Question 10

Which amino acid is not involved in transamination?

Accepted Answer

Lysine

Answer

Alanine

Answer

Aspartate

Answer

Histidine

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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