Amino Acid Metabolism Practice Questions

Q: Pulses are deficient in which amino acid?

Methionine. **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**.

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: 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 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: What is the precursor of proline in the Krebs cycle?

α-ketoglutarate. ***α-ketoglutarate*** - **α-ketoglutarate** is an intermediate of the **Krebs cycle** that can be transaminated to form **glutamate**. - **Glutamate** can then be converted to **glutamate-γ-semialdehyde** (also called glutamate 5-semialdehyde), which cyclizes to form **Δ¹-pyrroline-5-carboxylate**, the immediate precursor for **proline** synthesis. - This pathway links the **Krebs cycle** to amino acid biosynthesis. *Oxaloacetate* - **Oxaloacetate** is a precursor for the synthesis of **aspartate** and **asparagine** in amino acid metabolism. - It is also involved in gluconeogenesis and can be converted to **phosphoenolpyruvate**. *Succinyl CoA* - **Succinyl CoA** is an intermediate in the Krebs cycle that can contribute to the synthesis of **porphyrins** (e.g., heme). - It is derived from the metabolism of certain amino acids like **valine**, **isoleucine**, and **methionine**. *Fumarate* - **Fumarate** is another intermediate of the Krebs cycle that links the cycle to the **urea cycle**. - It is involved in the metabolism of **phenylalanine** and **tyrosine**.

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: Which of the following statements about Carbamoyl Phosphate Synthetase I (CPSI) is true?

N-Acetyl Glutamate is an allosteric activator of CPSI.. ***N-Acetyl Glutamate is an allosteric activator of CPSI.*** - **N-acetylglutamate (NAG)** is an essential allosteric activator for **Carbamoyl Phosphate Synthetase I (CPSI)**, signaling a high availability of arginine and a need for urea cycle activity. - Activation by NAG ensures that ammonia is incorporated into the urea cycle only when necessary, preventing its accumulation which can be toxic. *It is present in the cytoplasm.* - **CPSI** is located exclusively in the **mitochondrial matrix** of hepatocytes, where it initiates the urea cycle by converting ammonia to carbamoyl phosphate. - Its cytosolic counterpart, **CPSII**, is involved in pyrimidine synthesis, which differentiates their cellular locations and metabolic roles. *It is involved in pyrimidine synthesis.* - **CPSI** is the rate-limiting enzyme of the **urea cycle**, responsible for detoxifying ammonia by forming carbamoyl phosphate. - **Carbamoyl Phosphate Synthetase II (CPSII)**, located in the cytosol, is the enzyme involved in **pyrimidine synthesis**. *Glutamine is the amino group donor for CPSI.* - **CPSI** uses **free ammonia** (NH3) and **bicarbonate** as substrates to synthesize carbamoyl phosphate, not glutamine. - **Glutamine** is the nitrogen source for **Carbamoyl Phosphate Synthetase II (CPSII)** in pyrimidine synthesis.

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

Pyridoxine. ***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 1

Pulses are deficient in which amino acid?

Accepted Answer

Methionine

Answer

Lysine

Answer

Threonine

Answer

None of the options

Question 2

Which vitamin is not used in the treatment of homocysteinuria?

Accepted Answer

Thiamine (Vitamin B1)

Answer

Vitamin B6

Answer

Vitamin B12

Answer

Folate

Question 3

Which of the following is an acidic amino acid?

Accepted Answer

Glutamic acid

Answer

Alanine

Answer

Lysine

Answer

None of the options

Question 4

Which of the following amino acids contains two amino groups?

Accepted Answer

Lysine

Answer

Asparagine

Answer

Arginine

Answer

Glycine

Question 5

What is the precursor of proline in the Krebs cycle?

Accepted Answer

α-ketoglutarate

Answer

Oxaloacetate

Answer

Succinyl CoA

Answer

Fumarate

Question 6

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 7

Which of the following statements about Carbamoyl Phosphate Synthetase I (CPSI) is true?

Accepted Answer

N-Acetyl Glutamate is an allosteric activator of CPSI.

Answer

It is present in the cytoplasm.

Answer

It is involved in pyrimidine synthesis.

Answer

Glutamine is the amino group donor for CPSI.

Question 8

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 9

Which of the following is the simplest amino acid?

Accepted Answer

Glycine

Answer

Alanine

Answer

Tryptophan

Answer

Cysteine

Question 10

Which vitamin is required for conversion of serine to glycine?

Accepted Answer

Pyridoxine

Answer

Vitamin C

Answer

Vitamin B12

Answer

Thiamine (Vitamin B1)

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

On this page

Practice by Chapter

Want unlimited practice?