Amino Acid Metabolism — MCQs

Consider the following with regard to Amino Acids : I. Essential Amino Acids are those that the body cannot synthesize and they must be obtained from dietary proteins. II. Not just the Essential Amino Acids, Non-essential Amino Acids are also needed by the body for synthesis of tissue proteins. III. Unless a protein contains all the Essential Amino Acids in amounts corresponding to human needs, a protein is said to be biologically incomplete. IV. Animal proteins are rated superior to vegetable proteins. Of the above statements, how many are correct?

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 431: Nitric oxide is synthesized from which of the following amino acids?

A. Tryptophan
B. Lysine
C. Arginine (Correct Answer)
D. Tyrosine

Explanation: ***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.

Question 432: 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?

A. Tetrahydrofolate
B. Tetrahydrobiopterin (BH4) (Correct Answer)
C. Pyridoxine
D. Thiamine

Explanation: ***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**.

Question 433: The image shows dopamine metabolism. Which is the product named as $X$ ?

A. Vanillylmandelic acid
B. Homovanillic acid
C. 3,4-dihydroxyphenylacetic acid (Correct Answer)
D. Norepinephrine

Explanation: ***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'.

Question 434: A patient presents with ochronosis. Which of the following substance accumulates in this condition?

A. Homogentisic acid (Correct Answer)
B. Phenylalanine
C. Tyrosine
D. Tryptophan

Explanation: ***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)**.

Question 435: 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?

A. 1
B. 2
C. 3 (Correct Answer)
D. 4

Explanation: ***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.

Question 436: Deficiency of which of the following will lead to increase in levels of homocysteine?

A. Cystathionine synthase, PLP (Correct Answer)
B. Cystathionase, PLP
C. Cystathionine synthase, B12
D. Cystathionase, B12

Explanation: ***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.

Question 437: Identify the amino acid marked as X from which all the products highlighted below are derived.

A. Tryptophan
B. Tyrosine (Correct Answer)
C. Threonine
D. Leucine

Explanation: ***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.

Question 438: All are correct about the amino acid marked as X, EXCEPT:

A. Maximum buffering action
B. Ketogenic (Correct Answer)
C. Contributes to one carbon pool
D. Excess amount leads to mental retardation

Explanation: ***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

Question 439: Which one among the following essential amino acids is usually the "limiting" amino acid in most of the pulses?

A. Valine
B. Threonine
C. Methionine (Correct Answer)
D. Lysine

Explanation: ***Methionine*** - In **pulses** (legumes), the sulfur-containing amino acids, primarily **methionine** and **cysteine**, are often the first limiting amino acids. - This means that the amount of protein synthesis that can occur is restricted by the availability of methionine, even if other essential amino acids are abundant. *Valine* - **Valine** is an essential branched-chain amino acid, but it is typically not the limiting amino acid in pulses. - It is more likely to be limiting in certain grains or in specific diets lacking a variety of protein sources. *Threonine* - **Threonine** is an essential amino acid that can be limiting in some cereals, but it is not typically the primary limiting amino acid in pulses. - Its deficiency is less common in a balanced diet including legumes. *Lysine* - **Lysine** is often the limiting amino acid in most **cereal grains** (e.g., wheat, corn, rice). - Pulses, however, are generally good sources of lysine, making its deficiency less likely when consumed.

Question 440: Consider the following with regard to Amino Acids : I. Essential Amino Acids are those that the body cannot synthesize and they must be obtained from dietary proteins. II. Not just the Essential Amino Acids, Non-essential Amino Acids are also needed by the body for synthesis of tissue proteins. III. Unless a protein contains all the Essential Amino Acids in amounts corresponding to human needs, a protein is said to be biologically incomplete. IV. Animal proteins are rated superior to vegetable proteins. Of the above statements, how many are correct?

A. Only two
B. Only three
C. Only one
D. All four (Correct Answer)

Explanation: ***All four*** - **Statement I is correct:** Essential amino acids cannot be synthesized by the body and must be obtained from dietary sources. The 9 essential amino acids include histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. - **Statement II is correct:** Both essential and non-essential amino acids are required for tissue protein synthesis. Non-essential amino acids (like alanine, aspartic acid, glutamic acid, etc.) are equally important for building proteins, even though the body can synthesize them. - **Statement III is correct:** A protein is considered **biologically incomplete** if it lacks one or more essential amino acids in adequate amounts. Complete proteins contain all essential amino acids in proportions that meet human nutritional requirements. - **Statement IV is correct:** Animal proteins (meat, fish, eggs, dairy) are traditionally rated as superior to most plant proteins because they are **complete proteins** containing all essential amino acids in adequate proportions with high biological value. Most plant proteins (except soy, quinoa) are incomplete or limiting in one or more essential amino acids. Since all four statements (I, II, III, and IV) are factually correct, the answer is **all four**. *Only three* - This option is incorrect because all four statements about amino acids are medically accurate, not just three of them. *Only two* - This option is incorrect as it significantly underestimates the number of correct statements. All four statements are valid principles in biochemistry and nutrition. *Only one* - This option is incorrect as multiple statements are correct. All four statements represent established knowledge about protein nutrition and amino acid classification.

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

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free