Amino Acid Metabolism Practice Questions

Q: All of the following are tryptophan derivatives except?

Melanin. **Explanation:** The correct answer is **Melanin** because it is derived from the amino acid **Tyrosine**, not Tryptophan. In melanocytes, the enzyme tyrosinase converts tyrosine into DOPA and subsequently into melanin. **Analysis of Options:** * **Melanin (Correct):** Derived from Tyrosine. A deficiency in the enzyme tyrosinase leads to Albinism. * **Serotonin (Incorrect):** Tryptophan is hydroxylated by tryptophan hydroxylase (requiring BH4) and then decarboxylated to form Serotonin (5-hydroxytryptamine), a key neurotransmitter. * **Melatonin (Incorrect):** This hormone, which regulates the sleep-wake cycle in the pineal gland, is synthesized via the acetylation and methylation of Serotonin (thus making it a Tryptophan derivative). * **Niacin (Incorrect):** Approximately 60 mg of Tryptophan can be converted into 1 mg of Nicotinic acid (Vitamin B3) via the kynurenine pathway. This process requires Vitamin B6 (Pyridoxine) as a cofactor. **High-Yield Clinical Pearls for NEET-PG:** 1. **Hartnup Disease:** A defect in the transport of neutral amino acids (Tryptophan) in the gut and kidneys. It presents with pellagra-like symptoms because Tryptophan is unavailable for Niacin synthesis. 2. **Carcinoid Syndrome:** Tumor cells divert up to 60% of Tryptophan to produce Serotonin. This "tryptophan steal" leads to a secondary Niacin deficiency (Pellagra). 3. **Cofactor Alert:** Tryptophan metabolism requires **BH4** (for hydroxylation), **B6** (for the kynurenine pathway), and **B2/B3** for subsequent steps.

Q: What amino acid is produced by adding a hydroxyl group to the phenyl side chain of phenylalanine?

Tyrosine. **Explanation:** The correct answer is **Tyrosine**. **1. Why Tyrosine is Correct:** The conversion of Phenylalanine to Tyrosine is a classic hydroxylation reaction. The enzyme **Phenylalanine Hydroxylase (PAH)** adds a hydroxyl (-OH) group to the para-position of the phenyl ring of Phenylalanine. This reaction requires **Tetrahydrobiopterin (BH4)** as a co-factor and molecular oxygen. Tyrosine is considered a "non-essential" amino acid only because it can be synthesized from the essential amino acid Phenylalanine. **2. Why the Other Options are Incorrect:** * **Threonine:** An essential amino acid containing a hydroxyl group on its aliphatic side chain, not derived from a phenyl ring. * **Histidine:** An essential amino acid characterized by an imidazole ring, not a phenyl ring. * **Serine:** A non-essential amino acid with a hydroxymethyl group. It is primarily synthesized from 3-phosphoglycerate (an intermediate of glycolysis), not from Phenylalanine. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Phenylketonuria (PKU):** A deficiency in Phenylalanine Hydroxylase (Classic PKU) or BH4 leads to the accumulation of Phenylalanine and its metabolites (phenylpyruvate, phenyllactate), causing intellectual disability and a "mousy" body odor. * **Essentiality:** If Phenylalanine Hydroxylase is deficient, Tyrosine becomes a **conditionally essential** amino acid. * **Precursor Role:** Tyrosine is the metabolic precursor for several vital molecules: **Catecholamines** (Dopamine, Epinephrine, Norepinephrine), **Thyroid hormones** (T3, T4), and **Melanin**. * **Alkaptonuria:** A defect in Homogentisate oxidase (part of the Tyrosine degradation pathway) leads to dark urine and ochronosis.

Q: Massive aminoaciduria without a corresponding increase in plasma amino acid level is characteristic of which of the following diseases?

Hartnup disease. ### Explanation The key to this question lies in distinguishing between **Overflow Aminoaciduria** and **Renal Aminoaciduria**. **1. Why Hartnup Disease is Correct:** Hartnup disease is a **Renal Aminoaciduria**. It is caused by a defect in the **SLC6A19 transporter**, which is responsible for the reabsorption of neutral amino acids (especially Tryptophan) in the proximal renal tubules and the intestinal mucosa. Because the defect is in the transport mechanism of the kidney, amino acids are not reabsorbed and are excreted in massive amounts in the urine. Crucially, the **plasma levels remain normal or low** because the primary pathology is a "leak" at the kidney level, not an overproduction in the blood. **2. Why the Other Options are Incorrect:** * **Homocystinuria, Tyrosinemia, and Maple Syrup Urine Disease (MSUD):** These are all examples of **Overflow Aminoaciduria**. In these metabolic disorders, an enzyme deficiency leads to the accumulation of specific amino acids in the **plasma** (e.g., Methionine/Homocysteine, Tyrosine, or Branched-chain amino acids). Once the plasma concentration exceeds the renal threshold for reabsorption, the excess "overflows" into the urine. Therefore, in these conditions, high urinary levels are always accompanied by high plasma levels. **3. High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Hartnup disease mimics **Pellagra** (Dermatitis, Diarrhea, Dementia) because Tryptophan is a precursor for Niacin (Vitamin B3) synthesis. * **Diagnosis:** Look for the "Neutral Aminoaciduria" pattern in urine chromatography. * **Treatment:** High-protein diet and Nicotinamide supplementation. * **Other Renal Aminoacidurias:** Cystinuria (defect in COAL: Cystine, Ornithine, Arginine, Lysine) and Fanconi Syndrome (generalized proximal tubule dysfunction).

Q: All of the following regarding the urea cycle are true, except:

The rate-limiting enzyme is ornithine transcarbamoylase.. ### Explanation The urea cycle (Krebs-Henseleit cycle) is the primary mechanism for detoxifying ammonia into urea in the liver. **Why Option C is the correct answer (The Exception):** The rate-limiting and committed step of the urea cycle is catalyzed by **Carbamoyl Phosphate Synthetase I (CPS-I)**, not Ornithine Transcarbamoylase (OTC). CPS-I requires **N-acetylglutamate (NAG)** as an essential allosteric activator. While OTC deficiency is the most common urea cycle disorder, it does not serve as the rate-limiting step. **Analysis of other options:** * **Option A:** Urea production is directly proportional to protein intake. High protein consumption increases the breakdown of amino acids, leading to increased ammonia production, which induces the enzymes of the urea cycle. * **Option B:** The synthesis of argininosuccinate from citrulline and aspartate is catalyzed by Argininosuccinate Synthetase. This step consumes **one molecule of ATP**, which is hydrolyzed to AMP and PPi (equivalent to 2 high-energy phosphates). * **Option C:** Urea ($NH_2-CO-NH_2$) derives its components from three sources: one nitrogen from **free ammonia**, one nitrogen from **aspartate**, and the carbon atom from **bicarbonate ($CO_2$)**. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** The first two steps occur in the **mitochondria**, while the remaining steps occur in the **cytosol**. * **OTC Deficiency:** It is the only urea cycle disorder that is **X-linked recessive**; all others are autosomal recessive. It presents with orotic aciduria. * **Fumarate Link:** Argininosuccinate lyase cleaves argininosuccinate into arginine and **fumarate**, providing a link to the TCA cycle (the "Urea Bicycle"). * **Hyperammonemia:** Management includes protein restriction and ammonia scavengers like Sodium Benzoate or Phenylbutyrate.

Q: A fair-skinned child may be suffering with which of the following diseases?

Albinism. ### Explanation **Correct Option: C. Albinism** Albinism is a group of genetic disorders characterized by a deficit in **melanin production**. The most common cause is a deficiency of the enzyme **Tyrosinase**, which converts Tyrosine to DOPA and subsequently to Melanin. Since melanin is the primary pigment responsible for the color of the skin, hair, and eyes, its absence results in a characteristic **fair-skinned** appearance (hypopigmentation), white hair, and light-colored eyes. **Analysis of Incorrect Options:** * **A. Von Gierke Disease:** This is a Type I Glycogen Storage Disease (GSD) caused by a deficiency of Glucose-6-Phosphatase. Clinical features include hypoglycemia, hepatomegaly, and "doll-like" facies, but it does not typically affect skin pigmentation. * **B. Alkaptonuria:** Caused by a deficiency of **Homogentisate oxidase**. It leads to the accumulation of homogentisic acid, which causes **darkening** of urine upon standing and **ochronosis** (dark pigmentation of connective tissues/cartilage)—the opposite of fair skin. * **C. Tyrosinemia:** This involves defects in the catabolic pathway of tyrosine (e.g., Fumarylacetoacetate hydrolase deficiency in Type I). It primarily presents with liver failure, renal tubular dysfunction (Fanconi syndrome), and a "cabbage-like" odor, rather than generalized hypopigmentation. **High-Yield Clinical Pearls for NEET-PG:** * **Phenylketonuria (PKU):** Also presents with fair skin and blue eyes because high phenylalanine levels competitively inhibit tyrosinase, leading to decreased melanin. * **Ocular Albinism:** Often associated with nystagmus and photophobia due to lack of pigment in the iris and retina. * **Chediak-Higashi Syndrome:** A rare cause of partial albinism (silvery hair) associated with immunodeficiency and giant lysosomal granules.

Q: HHH syndrome is due to deficiency of which enzyme?

Ornithine transcarbamylase. **Explanation** **HHH Syndrome** (Hyperammonemia-Hyperornithinemia-Homocitrullinuria) is a rare autosomal recessive urea cycle disorder. It is caused by a defect in the **SLC25E1 gene**, which encodes the **mitochondrial ornithine transporter (ORNT1)**. This transporter is responsible for moving ornithine from the cytosol into the mitochondrial matrix. **Why the Correct Answer is "None of the above" (Contextual Correction):** While the provided key indicates *Ornithine transcarbamylase* (OTC), this is technically a common distractor. HHH syndrome is specifically due to a **transporter defect**, not a primary enzyme deficiency. However, in the context of NEET-PG questions where the transporter is not listed, it is often grouped with urea cycle defects. If "None of the above" is an option, it is the most accurate choice because the primary pathology is the **ORNT1 transporter deficiency**. **Analysis of Options:** * **A. Arginase:** Deficiency leads to Argininemia. It presents with spastic diplegia and high arginine levels, but not the specific HHH triad. * **B. Arginosuccinase:** Deficiency causes Arginosuccinic Aciduria, characterized by friable hair (Trichorrhexis nodosa). * **C. Ornithine transcarbamylase (OTC):** This is the most common urea cycle disorder (X-linked). While it involves ornithine, it leads to **Orotic aciduria**, not hyperornithinemia. **Clinical Pearls for NEET-PG:** 1. **The Triad:** Hyperammonemia (due to urea cycle failure), Hyperornithinemia (ornithine cannot enter mitochondria and builds up in cytosol), and Homocitrullinuria (excess carbamoyl phosphate reacts with lysine). 2. **Genetics:** Autosomal Recessive (unlike OTC, which is X-linked). 3. **High-Yield Fact:** In HHH syndrome, the urea cycle is blocked because **Ornithine transcarbamylase** lacks its substrate (ornithine) inside the mitochondria, even though the enzyme itself is functional.

Q: Which of the following is NOT a feature of Phenylketonuria?

Reduced tendon reflexes. **Explanation:** Phenylketonuria (PKU) is an autosomal recessive disorder caused by a deficiency of the enzyme **Phenylalanine Hydroxylase (PAH)** or its cofactor **Tetrahydrobiopterin (BH4)**. This leads to the accumulation of phenylalanine and its metabolites (phenylpyruvate, phenyllactate) in the body. **Why "Reduced tendon reflexes" is the correct answer:** In PKU, the neurological manifestation is typically **hyperreflexia** (increased tendon reflexes) and spasticity, rather than reduced reflexes. The accumulation of phenylalanine is neurotoxic, interfering with myelination and neurotransmitter synthesis, which leads to upper motor neuron signs and tremors. **Analysis of incorrect options:** * **Severe mental retardation:** This is a hallmark of untreated PKU. High levels of phenylalanine interfere with the transport of other large neutral amino acids into the brain, disrupting protein synthesis and brain development. * **Hypopigmentation of skin:** Phenylalanine is a competitive inhibitor of **Tyrosinase**, the key enzyme in melanin synthesis. Furthermore, the lack of Tyrosine (which becomes an essential amino acid in PKU) leads to decreased melanin, resulting in fair skin and blue eyes. * **Hair loss:** While not the primary feature, thinning of hair and light-colored (blonde) hair are common due to the systemic deficiency of melanin. **NEET-PG High-Yield Pearls:** * **Mousy/Musty Odor:** Caused by Phenylacetic acid in sweat and urine. * **Guthrie Test:** A bacterial inhibition assay used for neonatal screening. * **Dietary Management:** Restriction of Phenylalanine and supplementation of **Tyrosine** (which becomes "conditionally essential"). * **Maternal PKU:** If a mother doesn't control her Phe levels, the fetus may develop microcephaly and congenital heart defects (even if the fetus is heterozygous).

Q: Which branched-chain amino acid is both ketogenic and glucogenic?

Isoleucine. **Explanation:** The classification of amino acids as glucogenic, ketogenic, or both depends on the metabolic intermediates produced during their catabolism. Branched-chain amino acids (BCAAs) follow distinct pathways: 1. **Isoleucine (Correct):** Isoleucine is unique among BCAAs because its carbon skeleton is cleaved into two fragments: **Succinyl-CoA** (which enters the TCA cycle for gluconeogenesis) and **Acetyl-CoA** (which is used for ketogenesis). Therefore, it is both glucogenic and ketogenic. 2. **Leucine (Incorrect):** Leucine is strictly **ketogenic**. Its catabolism yields Acetyl-CoA and Acetoacetate. It cannot contribute to glucose synthesis. 3. **Valine (Incorrect):** Valine is strictly **glucogenic**. It is converted entirely into Succinyl-CoA. 4. **Tryptophan (Incorrect):** While Tryptophan is indeed both glucogenic and ketogenic, it is an **aromatic** amino acid, not a branched-chain amino acid. The question specifically asks for a BCAA. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Dual Nature:** "PhITTT" (Phenylalanine, Isoleucine, Tyrosine, Threonine, Tryptophan) are both glucogenic and ketogenic. * **Purely Ketogenic:** Leucine and Lysine (The only two). * **Maple Syrup Urine Disease (MSUD):** Caused by a deficiency in the **Branched-chain α-keto acid dehydrogenase** complex, leading to the accumulation of Leucine, Isoleucine, and Valine. * **Metabolic Site:** Unlike most amino acids metabolized in the liver, BCAAs are primarily oxidized in **extrahepatic tissues**, specifically skeletal muscle.

Q: Methionine is synthesized in the human body from which precursor?

Homocysteine. **Explanation:** **Why Homocysteine is Correct:** Methionine is an **essential amino acid**, meaning the carbon skeleton cannot be synthesized de novo in humans. However, it can be **regenerated** from **Homocysteine** via a remethylation pathway. This reaction is catalyzed by the enzyme **Methionine Synthase**, which requires two critical cofactors: **Vitamin B12 (Cobalamin)** and **N5-methyltetrahydrofolate** (a derivative of Folic Acid). In this reaction, homocysteine accepts a methyl group to become methionine, effectively recycling the sulfur-containing amino acid. **Why Other Options are Incorrect:** * **Cysteine & Cystine:** These are synthesized *from* methionine via the transsulfuration pathway (Methionine → Homocysteine → Cystathionine → Cysteine). The process is unidirectional; humans cannot convert cysteine back into methionine. Cystine is simply the oxidized dimer of cysteine. * **Tryptophan:** This is an aromatic essential amino acid and is not involved in the sulfur-containing amino acid metabolic pathway. **High-Yield Clinical Pearls for NEET-PG:** * **The Methyl Trap:** A deficiency in Vitamin B12 leads to folate being "trapped" as N5-methyl-THF, because Methionine Synthase is the only enzyme that can utilize it. This results in functional folate deficiency and megaloblastic anemia. * **Hyperhomocysteinemia:** Deficiencies in B12, Folate, or the enzyme MTHFR lead to elevated homocysteine levels, which is a significant risk factor for coronary artery disease, deep vein thrombosis (DVT), and neural tube defects. * **S-Adenosylmethionine (SAM):** Methionine is the precursor for SAM, the body's universal methyl donor.

Q: Sapropterin is used in the treatment of which of the following conditions?

Phenylketonuria. **Explanation:** **Sapropterin** is a synthetic form of **Tetrahydrobiopterin (BH4)**, which acts as a vital cofactor for the enzyme **Phenylalanine Hydroxylase (PAH)**. 1. **Why Phenylketonuria (PKU) is correct:** Classic PKU is caused by a deficiency in PAH, which converts Phenylalanine to Tyrosine. However, some patients have "BH4-responsive PKU." In these cases, the PAH enzyme has a reduced affinity for its cofactor. Administering Sapropterin (synthetic BH4) can stabilize the mutant PAH enzyme and increase its activity, effectively lowering blood phenylalanine levels. It is also used in cases where PKU is caused by a primary deficiency in BH4 synthesis or regeneration. 2. **Why other options are incorrect:** * **Hereditary Fructose Intolerance:** Caused by a deficiency of **Aldolase B**. Treatment involves strict dietary restriction of fructose and sucrose. * **Galactosemia:** Most commonly caused by **GALT** (Galactose-1-phosphate uridyltransferase) deficiency. Treatment requires a lactose-free (galactose-free) diet. * **Gaucher’s Disease:** A lysosomal storage disorder caused by **Glucocerebrosidase** deficiency. Treatment involves Enzyme Replacement Therapy (ERT) with Alglucerase or Imiglucerase. **High-Yield Clinical Pearls for NEET-PG:** * **Mousy/Musty Odor:** A classic clinical sign of PKU due to phenylacetic acid in sweat and urine. * **Maternal PKU:** If a mother with PKU does not control phenylalanine levels during pregnancy, the fetus may suffer from microcephaly, mental retardation, and congenital heart defects. * **Guthrie Test:** A bacterial inhibition assay used for neonatal screening of PKU. * **BH4 Deficiency:** Also affects the synthesis of neurotransmitters like Dopamine and Serotonin (as BH4 is a cofactor for Tyrosine and Tryptophan hydroxylases).

Question 1

All of the following are tryptophan derivatives except?

Accepted Answer

Melanin

Answer

Serotonin

Answer

Melatonin

Answer

Niacin

Question 2

What amino acid is produced by adding a hydroxyl group to the phenyl side chain of phenylalanine?

Accepted Answer

Tyrosine

Answer

Threonine

Answer

Histidine

Answer

Serine

Question 3

Massive aminoaciduria without a corresponding increase in plasma amino acid level is characteristic of which of the following diseases?

Accepted Answer

Hartnup disease

Answer

Homocystinuria

Answer

Tyrosinemia

Answer

Maple syrup urine disease

Question 4

All of the following regarding the urea cycle are true, except:

Accepted Answer

The rate-limiting enzyme is ornithine transcarbamoylase.

Answer

On consumption of a high amount of proteins, excess urea is formed.

Answer

Synthesis of argininosuccinate consumes energy.

Answer

One nitrogen of urea comes from aspartate.

Question 5

A fair-skinned child may be suffering with which of the following diseases?

Accepted Answer

Albinism

Answer

Von Gierke disease

Answer

Alkaptonuria

Answer

Tyrosinemia

Question 6

HHH syndrome is due to deficiency of which enzyme?

Accepted Answer

Ornithine transcarbamylase

Answer

Arginase

Answer

Arginosuccinase

Answer

None of the above

Question 7

Which of the following is NOT a feature of Phenylketonuria?

Accepted Answer

Reduced tendon reflexes

Answer

Severe mental retardation

Answer

Hypopigmentation of skin

Answer

Hair loss

Question 8

Which branched-chain amino acid is both ketogenic and glucogenic?

Accepted Answer

Isoleucine

Answer

Leucine

Answer

Valine

Answer

Tryptophan

Question 9

Methionine is synthesized in the human body from which precursor?

Accepted Answer

Homocysteine

Answer

Cysteine

Answer

Cystine

Answer

Tryptophan

Question 10

Sapropterin is used in the treatment of which of the following conditions?

Accepted Answer

Phenylketonuria

Answer

Hereditary Fructose Intolerance

Answer

Galactosemia

Answer

Gaucher's disease

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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