Amino Acid Metabolism Practice Questions

Q: Nitric oxide is synthesized from which of the following amino acids?

Arginine. ***Arginine*** - **L-arginine** is the sole biological precursor for nitric oxide (NO), a crucial signaling molecule involved in vasodilation, neurotransmission, and immune response. - The synthesis is catalyzed by the family of enzymes known as **nitric oxide synthases (NOS)**, which convert L-arginine into NO and **L-citrulline**. *Tryptophan* - Tryptophan is an essential amino acid that serves primarily as the precursor for the synthesis of the neurotransmitter **serotonin** and the hormone **melatonin**. - It is metabolized via the **kynurenine pathway** and is not utilized in the reaction catalyzed by NOS. *Lysine* - Lysine is an essential amino acid important for protein synthesis, and it is a precursor for **carnitine**, which is vital for fatty acid metabolism. - It is a basic amino acid and is not the substrate required by nitric oxide synthase for NO generation. *Tyrosine* - Tyrosine is the precursor amino acid necessary for the synthesis of **catecholamines** (dopamine, norepinephrine, epinephrine) and the **thyroid hormones**. - Though part of various metabolic pathways, it is not involved in the direct enzymatic conversion that yields nitric oxide.

Q: A patient has elevated phenylalanine levels (40 mg/dL), but phenylalanine hydroxylase enzyme levels are normal. Which cofactor deficiency is most likely responsible for this condition?

Tetrahydrobiopterin (BH4). ***Tetrahydrobiopterin (BH4)*** - The conversion of **phenylalanine to tyrosine** by phenylalanine hydroxylase (PAH) absolutely requires the cofactor **Tetrahydrobiopterin (BH4)**. - Normal PAH enzyme levels combined with high phenylalanine (hyperphenylalaninemia) strongly indicates a defect in the essential cofactor (BH4 deficiency), often leading to a potentially treatable condition known as **Malignant PKU**. *Tetrahydrofolate* - **Tetrahydrofolate (THF)** is primarily involved in **one-carbon metabolism** and the transfer of methyl groups, crucial for purine and pyrimidine synthesis. - Its deficiency typically leads to **megaloblastic anemia** and neurological symptoms but does not directly impair phenylalanine hydroxylase activity. *Thiamine* - The active form, **Thiamine Pyrophosphate (TPP)**, is an essential cofactor for **dehydrogenase enzymes**, such as pyruvate dehydrogenase and $\alpha$-ketoglutarate dehydrogenase. - Deficiency is classically associated with impaired carbohydrate metabolism and results in conditions like **Beriberi**. *Pyridoxine* - **Pyridoxal phosphate (PLP)**, derived from pyridoxine, is a cofactor for manifold reactions in amino acid metabolism, including **transaminases** and decarboxylases. - While vital for general amino acid handling, it is not the required cofactor for the specific hydroxylation reaction catalyzed by **phenylalanine hydroxylase**.

Q: The image shows a biochemical pathway. Name the enzyme marked as X.

Creatine Kinase. ***Creatine Kinase*** - The image shows the synthesis of creatine, and the final step involves the conversion of **creatine** to **creatine phosphate** using **ATP**, which is typical of a kinase enzyme. - **Creatine kinase** (CK) catalyzes the reversible transfer of a phosphate group from ATP to creatine, forming phosphocreatine and ADP, or vice versa, playing a crucial role in energy storage in muscle. *Creatine Phosphorylase* - This term is not a standard or commonly recognized name for an enzyme in biochemistry. While it might sound related to phosphorylation of creatine, **creatine kinase (CK)** is the correct and widely accepted enzyme name. - Enzymes are named based on their function and substrate; "phosphorylase" usually refers to enzymes that cleave bonds using inorganic phosphate, which is not the reaction shown here. *Argininosuccinase* - **Argininosuccinase (argininosuccinate lyase)** is an enzyme involved in the **urea cycle**, catalyzing the cleavage of argininosuccinate into arginine and fumarate. - This enzyme is not involved in creatine metabolism and its reaction mechanism is distinct from the phosphorylation shown in the diagram. *Ornithine transcarbamoylase* - **Ornithine transcarbamoylase** (OTC) is another enzyme of the **urea cycle**, catalyzing the reaction between carbamoyl phosphate and ornithine to form citrulline. - Its function is entirely unrelated to the synthesis or phosphorylation of creatine and does not involve ATP-ADP interconversion in this manner.

Q: The image shows dopamine metabolism. Which is the product named as $X$ ?

3,4-dihydroxyphenylacetic acid. ***3,4-dihydroxyphenylacetic acid*** - The image clearly labels the product formed from dopamine via monoamine oxidase (MAO) and aldehyde dehydrogenase as **3,4-dihydroxyphenylacetic acid (DOPAC)**. - This is a direct metabolic pathway for dopamine deamination. *Vanillylmandelic acid* - **Vanillylmandelic acid (VMA)** is the end-product of norepinephrine and epinephrine metabolism. - It involves the action of both COMT and MAO, but it's not a direct product of dopamine metabolism in this specific reaction shown. *Homovanillic Acid* - **Homovanillic acid (HVA)** is a major metabolite of dopamine, but it is formed downstream from DOPAC (via COMT reacting with DOPAC) or from 3-methoxytyramine (via MAO and aldehyde dehydrogenase). - The pathway shown for product X is the direct deamination of dopamine to DOPAC not HVA. *Norepinephrine* - **Norepinephrine** is formed from dopamine through the action of dopamine-beta-hydroxylase, an enzymatic conversion, not a breakdown product as shown in the diagram. - It is a neurotransmitter itself, not a final metabolite in the breakdown pathway depicted for 'X'.

Q: A patient presents with ochronosis. Which of the following substance accumulates in this condition?

Homogentisic acid. ***Homogentisic acid*** - Ochronosis (alkaptonuria) is caused by the accumulation of **homogentisic acid** in connective tissues, leading to a dark blue-black discoloration of cartilage, skin, and organs. - This accumulation results from a deficiency of **homogentisate 1,2-dioxygenase**, an enzyme involved in the metabolism of **tyrosine**. - The accumulated homogentisic acid polymerizes and deposits in tissues, causing the characteristic pigmentation. *Phenylalanine* - Accumulation of **phenylalanine** occurs in **phenylketonuria (PKU)**, not ochronosis. - PKU is caused by deficiency of phenylalanine hydroxylase and presents with intellectual disability, musty odor, and fair complexion if untreated. *Tyrosine* - While ochronosis involves a defect in **tyrosine metabolism**, tyrosine itself does not accumulate in this condition. - Tyrosine accumulation occurs in **tyrosinemia**, which presents with different clinical features including hepatic dysfunction and renal tubular defects. *Tryptophan* - Defects in **tryptophan metabolism** are associated with conditions like **Hartnup disease**, which presents with pellagra-like symptoms, not ochronosis. - Tryptophan is a precursor for **serotonin** and **niacin (vitamin B3)**.

Q: An infant presents with failure to thrive and recurrent vomiting episodes. Sepsis screen was negative, blood urea is elevated and urine shows high orotic acid level. Which of the following enzyme is absent?

3. ***Correct Option 3 (Ornithine Transcarbamylase)*** - The presence of **elevated urine orotic acid** is pathognomonic for **ornithine transcarbamylase (OTC) deficiency**. - This occurs because carbamoyl phosphate accumulates in mitochondria when it cannot be converted to citrulline, then leaks into the cytoplasm where it enters the **pyrimidine synthesis pathway**, producing orotic acid. - The elevated blood urea indicates hyperammonemia, and the clinical picture of failure to thrive with recurrent vomiting is classic for OTC deficiency. - **OTC deficiency is the only urea cycle disorder that causes orotic aciduria** because the block occurs after carbamoyl phosphate formation but before citrulline synthesis. *Incorrect Option 1 (N-acetylglutamate synthase)* - N-acetylglutamate synthase produces N-acetylglutamate, which is an **allosteric activator of carbamoyl phosphate synthetase I (CPS1)**. - Deficiency here would impair CPS1 activity, leading to **decreased carbamoyl phosphate formation**. - This would cause hyperammonemia but **not orotic aciduria**, as insufficient carbamoyl phosphate would be available to shunt into pyrimidine synthesis. *Incorrect Option 2 (Carbamoyl Phosphate Synthetase I)* - CPS1 deficiency prevents formation of carbamoyl phosphate from ammonia and CO₂. - This causes severe hyperammonemia with very low or absent carbamoyl phosphate levels. - **No orotic aciduria occurs** because the substrate (carbamoyl phosphate) needed for pyrimidine synthesis is not produced. *Incorrect Option 4 (Downstream urea cycle enzymes)* - Later enzymes in the cycle include argininosuccinate synthetase and argininosuccinate lyase. - Deficiencies cause accumulation of citrulline or argininosuccinate respectively, with hyperammonemia. - **No orotic aciduria occurs** because carbamoyl phosphate is successfully converted to citrulline and doesn't accumulate to shunt into pyrimidine synthesis.

Q: Deficiency of which of the following will lead to increase in levels of homocysteine?

Cystathionine synthase, PLP. ***Cystathionine synthase, PLP*** - The enzyme responsible for converting **homocysteine to cystathionine** is **cystathionine synthase**. - **Cystathionine synthase** requires **pyridoxal phosphate (PLP)**, which is derived from vitamin **B6**, as a cofactor. A deficiency in either the enzyme or its cofactor would lead to an accumulation of homocysteine. *Cystathionase, PLP* - **Cystathionase** is the enzyme that converts **cystathionine to cysteine**, and while it also uses **PLP** as a cofactor, its deficiency would lead to an accumulation of **cystathionine**, not directly homocysteine. - The direct conversion of homocysteine to cystathionine is catalyzed by cystathionine synthase. *Cystathionine synthase, B12* - While **cystathionine synthase** catalyzes the conversion of **homocysteine to cystathionine**, its cofactor is **PLP (Vit B6)**, not vitamin **B12**. - Vitamin **B12** is primarily involved in the **remethylation pathway** of homocysteine metabolism (converting homocysteine back to methionine), not the transsulfuration pathway directly depicted here. *Cystathionase, B12* - **Cystathionase** converts **cystathionine to cysteine**, and its deficiency would cause **cystathionine** to accumulate. - Furthermore, its cofactor is **PLP (Vit B6)**, not vitamin **B12**, which is involved in homocysteine remethylation.

Q: Identify the amino acid marked as X from which all the products highlighted below are derived.

Tyrosine. ***Tyrosine*** - The diagram shows **Tyrosine** as the precursor (X) for pathways leading to **DOPA**, which then forms **Dopamine**, **Norepinephrine**, **Epinephrine**, **Melanin**, and also directly to **Thyroxine**. - Tyrosine is a non-essential amino acid that is a precursor for several important biomolecules, including **catecholamines** (dopamine, norepinephrine, epinephrine), **thyroid hormones** (thyroxine), and **melanin**. *Tryptophan* - **Tryptophan** is a precursor to **serotonin**, **melatonin**, and niacin (Vitamin B3). - It is not involved in the synthesis of catecholamines, melanin, or thyroxine, as depicted in the diagram. *Threonine* - **Threonine** is an essential amino acid primarily involved in **protein synthesis** and the formation of O-linked glycosylation. - It does not serve as a precursor for any of the listed compounds like DOPA, dopamine, norepinephrine, melanin, or thyroxine. *Leucine* - **Leucine** is a branched-chain essential amino acid primarily involved in **protein synthesis** and regulation of muscle metabolism. - It is not a precursor for any of the indicated neurohormones, pigments, or thyroid hormones shown in the pathway.

Q: All are correct about the amino acid marked as X, EXCEPT:

Ketogenic. ***Correct Answer: Ketogenic*** ✓ - Histidine is **purely glucogenic**, NOT ketogenic - Its carbon skeleton is converted to α-ketoglutarate (a TCA cycle intermediate), which can be used for gluconeogenesis - It does NOT produce acetyl-CoA or acetoacetate (ketone body precursors) - **This statement is FALSE, making it the correct answer for this EXCEPT question** *Incorrect: Maximum buffering action* - This statement is TRUE (so not the answer) - Histidine contains an **imidazole ring** with pKa ~6.0, close to physiological pH - Provides crucial **buffering capacity** in blood and tissues - Most effective amino acid buffer at physiological pH *Incorrect: Contributes to one carbon pool* - This statement is TRUE (so not the answer) - Histidine breakdown produces **Formimino glutamic acid (FIGLU)** - FIGLU donates its formimino group to **Tetrahydrofolate (THF)** - Contributes to the **one-carbon pool** essential for biosynthetic pathways *Incorrect: Excess amount leads to mental retardation* - This statement is TRUE (so not the answer) - **Histidinemia** results from histidase enzyme deficiency - Elevated histidine levels can be associated with **developmental delays and speech defects** in some cases - Though many affected individuals remain asymptomatic

Q: Name the product marked as X in the image shown below:

Indolequinone. ***Indolequinone*** - The image depicts the **melanin biosynthesis pathway** starting from **tyrosine**. - Tyrosine is converted to **L-DOPA** by tyrosinase, which is then oxidized to **dopaquinone** (also called DOPA-quinone). - Dopaquinone undergoes intramolecular cyclization to form **leucodopachrome**, which is subsequently oxidized to **dopachrome**. - Dopachrome is then converted through a series of steps to **5,6-dihydroxyindole**, which is finally oxidized to **indole-5,6-quinone** (indolequinone). - **Indolequinone** is a key intermediate that polymerizes to form **melanin**, the pigment responsible for coloration in skin, hair, and eyes. - Based on the pathway shown, X represents indolequinone, an oxidized indole derivative in melanin synthesis. *Tetrabenazine* - **Tetrabenazine** is a pharmaceutical drug that inhibits **vesicular monoamine transporter 2 (VMAT2)**. - It is used therapeutically to treat hyperkinetic movement disorders such as **Huntington's disease** and tardive dyskinesia. - It is not a natural metabolic intermediate in the tyrosine-to-melanin biosynthetic pathway. *Homovanillic acid* - **Homovanillic acid (HVA)** is a major end metabolite of **dopamine** in the catecholamine degradation pathway. - It is formed by the sequential actions of **monoamine oxidase (MAO)** and **catechol-O-methyltransferase (COMT)** on dopamine. - This represents a completely different branch of tyrosine metabolism (catecholamine pathway), not the melanin synthesis pathway. *Kynurenine* - **Kynurenine** is an intermediate metabolite in the **kynurenine pathway**, which is the major route of **tryptophan** degradation. - The kynurenine pathway leads to the formation of NAD+ and various neuroactive metabolites. - This pathway is completely separate from tyrosine metabolism and is unrelated to the melanin synthesis pathway shown in the image.

Question 1

Nitric oxide is synthesized from which of the following amino acids?

Accepted Answer

Arginine

Answer

Tryptophan

Answer

Lysine

Answer

Tyrosine

Question 2

A patient has elevated phenylalanine levels (40 mg/dL), but phenylalanine hydroxylase enzyme levels are normal. Which cofactor deficiency is most likely responsible for this condition?

Accepted Answer

Tetrahydrobiopterin (BH4)

Answer

Tetrahydrofolate

Answer

Pyridoxine

Answer

Thiamine

Question 3

The image shows a biochemical pathway. Name the enzyme marked as X.

Accepted Answer

Creatine Kinase

Answer

Creatine Phosphorylase

Answer

Argininosuccinase

Answer

Ornithine transcarbamoylase

Question 4

The image shows dopamine metabolism. Which is the product named as $X$ ?

Accepted Answer

3,4-dihydroxyphenylacetic acid

Answer

Vanillylmandelic acid

Answer

Homovanillic acid

Answer

Norepinephrine

Question 5

A patient presents with ochronosis. Which of the following substance accumulates in this condition?

Accepted Answer

Homogentisic acid

Answer

Phenylalanine

Answer

Tyrosine

Answer

Tryptophan

Question 6

An infant presents with failure to thrive and recurrent vomiting episodes. Sepsis screen was negative, blood urea is elevated and urine shows high orotic acid level. Which of the following enzyme is absent?

Accepted Answer

3

Answer

1

Answer

2

Answer

4

Question 7

Deficiency of which of the following will lead to increase in levels of homocysteine?

Accepted Answer

Cystathionine synthase, PLP

Answer

Cystathionase, PLP

Answer

Cystathionine synthase, B12

Answer

Cystathionase, B12

Question 8

Identify the amino acid marked as X from which all the products highlighted below are derived.

Accepted Answer

Tyrosine

Answer

Tryptophan

Answer

Threonine

Answer

Leucine

Question 9

All are correct about the amino acid marked as X, EXCEPT:

Accepted Answer

Ketogenic

Answer

Maximum buffering action

Answer

Contributes to one carbon pool

Answer

Excess amount leads to mental retardation

Question 10

Name the product marked as X in the image shown below:

Accepted Answer

Indolequinone

Answer

Tetrabenazine

Answer

Homovanillic acid

Answer

Kynurenine

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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