Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 501: Pulses are deficient in which amino acid?

A. Lysine
B. Threonine
C. None of the options
D. Methionine (Correct Answer)

Explanation: **Methionine (Correct)** - Pulses (legumes) are generally **deficient in sulfur-containing amino acids**, with methionine being the primary limiting one. - This deficiency makes it important to combine pulses with other foods rich in methionine, such as **grains**, to achieve a complete protein profile. *Lysine (Incorrect)* - Lysine is a **limiting amino acid in grains**, but pulses are generally a good source of lysine. - Therefore, combining grains and pulses (e.g., rice and dal) can provide a **complete essential amino acid profile**. *Threonine (Incorrect)* - While threonine is an essential amino acid, it is **not typically the limiting amino acid in pulses**. - The primary limiting factor in pulses related to essential amino acids is **methionine**. *None of the options (Incorrect)* - This option is incorrect because pulses do have a **limiting amino acid**, which is methionine. - Identifying the limiting amino acid is crucial for understanding **dietary protein complementarity**.

Question 502: Which of the following amino acids is not converted to alpha-ketoglutarate during catabolism?

A. Glutamate
B. Histidine
C. Proline
D. Glycine (Correct Answer)

Explanation: ***Glycine*** - Glycine catabolism typically produces **serine**, which can then be converted to **pyruvate**, not alpha-ketoglutarate. - Its degradation pathway leads to the formation of **carbon dioxide** and **ammonia** via the glycine cleavage system (GCS), or it can be converted to serine and subsequently pyruvate. *Glutamate* - Glutamate is directly deaminated by **glutamate dehydrogenase** to form **alpha-ketoglutarate**, a key intermediate in the **Krebs cycle**. - This is a reversible reaction that plays a crucial role in nitrogen metabolism. *Histidine* - Histidine catabolism involves several steps, including deamination by **histidase**, eventually leading to the formation of **N-formiminoglutamate (FIGLU)**. - FIGLU is then converted to **glutamate**, which subsequently forms **alpha-ketoglutarate**. *Proline* - Proline is catabolized through a series of oxidation steps to yield **glutamate**. - This glutamate is then converted to **alpha-ketoglutarate** by glutamate dehydrogenase.

Question 503: Which vitamin is required for conversion of serine to glycine?

A. Pyridoxine (Correct Answer)
B. Vitamin C
C. Vitamin B12
D. Thiamine (Vitamin B1)

Explanation: ***Pyridoxine*** - **Pyridoxine (Vitamin B6)** is a crucial cofactor for the enzyme **serine hydroxymethyltransferase**, which catalyzes the reversible conversion of **serine to glycine and 5,10-methylenetetrahydrofolate**. - This reaction requires **pyridoxal phosphate (PLP)**, the active form of vitamin B6, emphasizing its role in **amino acid metabolism** and one-carbon unit transfer. *Vitamin C* - **Vitamin C** (ascorbic acid) is primarily involved in **collagen synthesis**, acting as a cofactor for hydroxylase enzymes. - It also functions as an **antioxidant** and is essential for immune function, but not directly in the serine-glycine interconversion. *Vitamin B12* - **Vitamin B12** (cobalamin) is a cofactor for enzymes involved in two main reactions: the conversion of **methylmalonyl CoA to succinyl CoA** and the synthesis of **methionine from homocysteine**. - It is not directly involved in the conversion of serine to glycine. *Thiamine (Vitamin B1)* - **Thiamine (Vitamin B1)** is critical for carbohydrate metabolism, acting as a cofactor for enzymes like **pyruvate dehydrogenase** and **α-ketoglutarate dehydrogenase**. - It plays no direct role in the metabolic pathway linking serine and glycine.

Question 504: HHH syndrome is due to a defect in which transporter?

A. Histidine transporter
B. Tryptophan transporter
C. Ornithine transporter (Correct Answer)
D. Branched chain amino acid transporter

Explanation: ***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.

Question 505: Which enzyme catalyzes the transfer of an α-amino group from aspartate to α-ketoglutarate?

A. Ornithine transcarbamylase (OTC)
B. Argininosuccinate lyase (ASL)
C. Aspartate Transaminase (AST) (Correct Answer)
D. Alanine Aminotransferase (ALT)

Explanation: ***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.

Question 506: Where is Carbamoyl phosphate synthetase I located?

A. Located in the lysosome
B. Located in the cytosol
C. Located in the mitochondria (Correct Answer)
D. Located in all of the above

Explanation: ***Located in the mitochondria*** - **Carbamoyl phosphate synthetase I (CPS I)** is a key enzyme in the **urea cycle**, which primarily occurs in the **mitochondrial matrix** of liver cells. - CPS I catalyzes the rate-limiting step of the urea cycle, combining ammonia and bicarbonate to form **carbamoyl phosphate**. *Located in the lysosome* - **Lysosomes** are cellular organelles responsible for waste breakdown and degradation, not for the synthesis of molecules like carbamoyl phosphate. - Enzymes involved in the urea cycle, such as CPS I, are not found in lysosomes. *Located in the cytosol* - While some steps of the urea cycle occur in the **cytosol** (e.g., argininosuccinate synthetase and lyase), the initial steps involving CPS I take place exclusively in the **mitochondria**. - CPS I is a mitochondrial enzyme, distinguishing it from **carbamoyl phosphate synthetase II (CPS II)**, which is cytosolic and involved in pyrimidine synthesis. *Located in all of the above* - CPS I is specifically located in the **mitochondria** and is not found in all cellular compartments mentioned. - Its specific mitochondrial location is crucial for its function in the urea cycle and regulation of ammonia detoxification.

Question 507: How does essential amino acid deficiency affect nitrogen balance?

A. Results in positive nitrogen balance due to compensatory mechanisms
B. No significant effect on nitrogen balance
C. Maintains nitrogen equilibrium through adaptive responses
D. Results in negative nitrogen balance due to decreased protein synthesis and increased protein degradation (Correct Answer)

Explanation: ***Results in negative nitrogen balance due to decreased protein synthesis and increased protein degradation*** - A deficiency in even one **essential amino acid** limits the body's ability to synthesize new proteins, as all necessary building blocks are not available. - This leads to a state where **protein degradation** exceeds **protein synthesis**, causing more nitrogen to be excreted than consumed. *No significant effect on nitrogen balance* - This statement is incorrect because essential amino acids are crucial for protein synthesis, and their deficiency directly impacts the body's protein metabolism. - The body cannot synthesize essential amino acids, so their absence significantly disrupts normal physiological processes that rely on protein turnover. *Results in positive nitrogen balance due to compensatory mechanisms* - A **positive nitrogen balance** signifies that the body is retaining more nitrogen than it is excreting, typically seen during periods of growth, recovery from illness, or muscle building. - An essential amino acid deficiency inhibits such growth and repair, making a positive nitrogen balance impossible under these conditions. *Maintains nitrogen equilibrium through adaptive responses* - While the body does have adaptive responses to nutrient shortages, a prolonged or severe **essential amino acid deficiency** will eventually overwhelm these mechanisms. - **Nitrogen equilibrium** (nitrogen intake equals nitrogen excretion) would not be maintained because protein synthesis would be impaired, leading to a net loss of nitrogen.

Question 508: Which of the following is the simplest amino acid?

A. Glycine (Correct Answer)
B. Alanine
C. Tryptophan
D. Cysteine

Explanation: ***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.

Question 509: Which amino acid is the primary precursor for the synthesis of glutamate?

A. Proline
B. Cysteine
C. alpha-ketoglutarate
D. Glutamine (Correct Answer)

Explanation: ***Glutamine*** - **Glutamine** is a crucial precursor for **glutamate synthesis**, primarily through the enzyme **glutaminase**, which hydrolyzes glutamine to glutamate and ammonia. - This conversion is vital in various tissues, including the brain and kidneys, playing roles in **neurotransmission** and **acid-base balance**. *alpha-ketoglutarate* - **Alpha-ketoglutarate** is an intermediate in the **Krebs cycle** and can be converted to glutamate via **transamination** or **reductive amination** (by glutamate dehydrogenase). - While it directly forms glutamate, it is often considered an immediate precursor in metabolic cycles rather than the primary *amino acid* precursor. *Proline* - **Proline** is a non-essential amino acid that can be synthesized from **glutamate**, but it is not a direct precursor for glutamate itself. - Its metabolic pathway involves converting glutamate to **glutamate-gamma-semialdehyde**, which then cyclizes to form proline. *Cysteine* - **Cysteine** is a sulfur-containing amino acid involved in the synthesis of **glutathione** and protein structure, but it is not directly involved in the synthesis of glutamate. - Its metabolic pathways are distinct and primarily revolve around sulfur metabolism and redox regulation.

Question 510: Which of the following is an allosteric stimulator of glutamate dehydrogenase?

A. NADH
B. NADPH
C. Acetyl CoA
D. ADP (Correct Answer)

Explanation: ***ADP*** - **ADP (adenosine diphosphate)** signals a low-energy state in the cell, thus stimulating **glutamate dehydrogenase** to increase the production of **α-ketoglutarate** for the TCA cycle and ATP synthesis. - This activation promotes the **deamination of glutamate**, linking amino acid catabolism to energy production. *NADH* - **NADH** is typically an **inhibitor** of glutamate dehydrogenase, as it indicates a high-energy state and abundant reducing equivalents. - Its presence suggests that the cell has sufficient energy, so further production of **α-ketoglutarate** through glutamate deamination is not immediately needed. *NADPH* - **NADPH** is also an **inhibitor** of glutamate dehydrogenase, especially in its role in biosynthesis. - High levels of **NADPH** signify a reductive cellular environment, signaling that the cell does not need to catabolize amino acids for energy or precursors for processes like fatty acid synthesis. *Acetyl CoA* - **Acetyl CoA** is a key metabolic intermediate but it does **not directly act as an allosteric stimulator of glutamate dehydrogenase**. - While it plays a role in energy metabolism, its direct allosteric regulation is primarily directed towards enzymes like pyruvate carboxylase or pyruvate dehydrogenase complex.

Practice by Chapter

Protein Digestion and Absorption

Practice Questions

Transamination and Deamination

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Urea Cycle

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Disorders of Urea Cycle

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Metabolism of Individual Amino Acids

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Inborn Errors of Amino Acid Metabolism

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Phenylketonuria and Alkaptonuria

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Homocystinuria and Methionine Metabolism

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Synthesis of Biologically Important Compounds from Amino Acids

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Nitrogen Balance

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Ammonia Metabolism and Toxicity

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One-Carbon Transfer Reactions

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