Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 541: In phenylketonuria, which substance should be restricted in the diet?

A. Tyrosine
B. Phenylalanine (Correct Answer)
C. Maize
D. None of the options

Explanation: ***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.

Question 542: Pyridoxine is required in -

A. Glycolysis
B. TCA cycle
C. Glycogenesis
D. Transamination (Correct Answer)

Explanation: ***Transamination*** - **Pyridoxal phosphate (PLP)**, the active form of pyridoxine (vitamin B6), is an essential **coenzyme for aminotransferases (transaminases)** - Transamination reactions involve the transfer of an **amino group** from an amino acid to a keto acid, which is crucial for amino acid metabolism - This is the classic biochemical function of vitamin B6 and a frequently tested concept *Glycolysis* - Glycolysis is a metabolic pathway that breaks down glucose into pyruvate - Key cofactors for glycolysis include **NAD+ and ATP**, not vitamin B6 - Does not require pyridoxine as a coenzyme *TCA cycle* - The **TCA cycle (Krebs cycle)** is a central metabolic pathway for energy production - Uses enzymes that require cofactors such as **NAD+, FAD, and Coenzyme A** (derived from pantothenic acid) - Pyridoxine is not directly involved as a coenzyme in TCA cycle reactions *Glycogenesis* - Glycogenesis is the process of synthesizing **glycogen from glucose** - Primarily involves enzymes like **glycogen synthase** and **branching enzyme** - Requires **UTP and glucose-1-phosphate**, not pyridoxine

Question 543: Which of the following is classified as an essential amino acid?

A. Aspartic acid
B. Glutamic acid
C. Lysine (Correct Answer)
D. None of the options

Explanation: ***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.

Question 544: Creatinine is formed from -

A. Creatine (Correct Answer)
B. Lysine
C. Leucine
D. Histidine

Explanation: ***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.

Question 545: Coenzyme for phenylalanine hydroxylase is?

A. Tetrahydrofolate
B. Pyridoxal phosphate
C. S-adenosyl methionine
D. Tetrahydrobiopterin (Correct Answer)

Explanation: ***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.

Question 546: Fumarate is formed from which amino acid?

A. Methionine
B. Valine
C. Histidine
D. Tyrosine (Correct Answer)

Explanation: ***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.

Question 547: Which of the following is required in the synthesis of acetylcholine -

A. Inositol
B. Carnitine
C. Glycine
D. Choline (Correct Answer)

Explanation: ***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.

Question 548: Which of the following statements about Maple Syrup Urine Disease (MSUD) is true?

A. FeCl3 turns navy blue
B. Asymptomatic
C. Deficiency of branched chain amino acid enzymes (Correct Answer)
D. Hyperaminoaciduria of aromatic amino acids

Explanation: ***Deficiency of branched chain amino acid enzymes*** - MSUD is caused by a deficiency in the **branched-chain alpha-keto acid dehydrogenase complex (BCKDC)**, an enzyme responsible for metabolizing **leucine, isoleucine, and valine**. - This enzyme deficiency leads to the accumulation of these **branched-chain amino acids (BCAAs)** and their corresponding branched-chain alpha-keto acids in the body. *Asymptomatic* - MSUD is a severe metabolic disorder that is **not asymptomatic**; it typically presents with distinct neurological and metabolic symptoms shortly after birth. - Clinical manifestations can include **poor feeding, lethargy, seizures**, and a characteristic **maple syrup odor** in urine and earwax. *FeCl3 turns navy blue* - The **ferric chloride test (FeCl3 test)** in MSUD typically yields a **green-gray or dark-green color** when testing for the accumulation of alpha-keto acids (specifically alpha-ketoisocaproic acid). - A **navy blue color** with FeCl3 is more characteristic of **alkaptonuria** due to the presence of homogentisic acid. *Hyperaminoaciduria of aromatic amino acids* - MSUD is characterized by the accumulation and excretion of **branched-chain amino acids (leucine, isoleucine, valine)** and their corresponding keto acids. - **Hyperaminoaciduria of aromatic amino acids** (e.g., phenylalanine, tyrosine, tryptophan) is seen in other conditions like **phenylketonuria (PKU)**, not MSUD.

Question 549: In type IA Maple Syrup Urine Disease, which gene mutation is responsible?

A. BCKDHB
B. DBT
C. DLD
D. BCKDHA (Correct Answer)

Explanation: ***BCKDHA*** - **Maple Syrup Urine Disease (MSUD)** type IA is caused by a mutation in the **BCKDHA gene**, which codes for the E1α subunit of the **branched-chain α-keto acid dehydrogenase (BCKD) complex**. - This **enzyme complex** is crucial for the metabolism of **branched-chain amino acids (BCAAs)**: leucine, isoleucine, and valine. *BCKDHB* - The **BCKDHB gene** codes for the E1β subunit of the **BCKD complex**. - Mutations in **BCKDHB** are associated with **type IB MSUD**, not type IA. *DBT* - The **DBT gene** codes for the E2 subunit (dihydrolipoyl transacylase) of the **BCKD complex**. - Mutations in **DBT** are responsible for **type II MSUD**. *DLD* - The **DLD gene** codes for the E3 subunit (dihydrolipoyl dehydrogenase), which is a component shared by several **α-keto acid dehydrogenase complexes**. - Mutations in the **DLD gene** lead to **type III MSUD** and other pyruvate dehydrogenase complex deficiencies, rather than type IA.

Question 550: Which of the following amino acids is not classified as polar?

A. Glutamic acid
B. Histidine
C. Glutamine
D. Methionine (Correct Answer)

Explanation: ***Methionine*** - Methionine is classified as a **nonpolar**, **hydrophobic** amino acid due to its side chain containing a **thioether group** (-S-CH3), which has no significant dipole moment. - Its nonpolar nature means it is less likely to interact with water and tends to be found in the interior of proteins. *Glutamic acid* - Glutamic acid is a **polar**, **acidic** amino acid because its side chain contains a **carboxyl group** (-COOH) that is deprotonated (COO-) at physiological pH, carrying a negative charge. - This charged group allows it to participate in **ionic interactions** and hydrogen bonding. *Histidine* - Histidine is a **polar**, **basic** amino acid due to its **imidazole ring** in the side chain, which can be protonated or deprotonated depending on the pH. - Its pKa is close to physiological pH, making it an important **buffer** and active site residue in many enzymes. *Glutamine* - Glutamine is a **polar**, **uncharged** amino acid with an **amide group** (-CONH2) in its side chain. - The carbonyl oxygen and the amine hydrogens in the amide group allow for significant **hydrogen bonding** with water and other polar molecules.

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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