Amino Acid Metabolism Practice Questions

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).

Q: Hyperphenylalaninemia is due to a defect in which enzyme?

Phenylalanine hydroxylase. ### Explanation **Correct Option: A. Phenylalanine hydroxylase** Hyperphenylalaninemia, most commonly manifesting as **Phenylketonuria (PKU)**, is primarily caused by a deficiency of the hepatic enzyme **Phenylalanine hydroxylase (PAH)**. This enzyme is responsible for the conversion of the essential amino acid Phenylalanine into Tyrosine. This reaction requires the cofactor **Tetrahydrobiopterin (BH4)**. A defect in PAH leads to the accumulation of Phenylalanine in the blood and tissues, which is then diverted into alternative pathways forming phenylketones (phenylpyruvate, phenyllactate), leading to neurotoxicity and intellectual disability. **Analysis of Incorrect Options:** * **B. Tyrosinase:** A deficiency in this enzyme leads to **Albinism**. Tyrosinase is responsible for converting Tyrosine into Melanin; its defect does not cause Phenylalanine accumulation. * **C. Homogentisic acid oxidase:** A defect here causes **Alkaptonuria**, characterized by the accumulation of homogentisic acid, resulting in ochronosis (darkening of tissues) and dark urine upon standing. * **D. Ornithine transcarbamylase (OTC):** This is an enzyme of the **Urea Cycle**. Its deficiency leads to Hyperammonemia and increased orotic acid levels, not hyperphenylalaninemia. **High-Yield Clinical Pearls for NEET-PG:** * **Mousy/Musty Odor:** A classic clinical sign of PKU due to phenylacetate in sweat and urine. * **Maternal PKU:** If a mother with PKU does not maintain a low-phenylalanine diet during pregnancy, the fetus may suffer from microcephaly and congenital heart defects. * **BH4 Deficiency:** About 2% of hyperphenylalaninemia cases are due to a deficiency in **Dihydropteridine reductase** (which regenerates BH4). These patients require BH4 supplementation in addition to dietary restriction. * **Guthrie Test:** A semi-quantitative bacterial inhibition assay used for neonatal screening of PKU.

Q: Conversion of norepinephrine to epinephrine occurs by which biochemical process?

Methylation. **Explanation:** The conversion of **Norepinephrine to Epinephrine** is the final step in the catecholamine biosynthetic pathway. This reaction is a **Methylation** process catalyzed by the enzyme **Phenylethanolamine N-methyltransferase (PNMT)**. 1. **Why Methylation is Correct:** In this reaction, a methyl group is transferred from **S-adenosylmethionine (SAM)**, which acts as the universal methyl donor, to the nitrogen atom of norepinephrine. This specific step occurs primarily in the **adrenal medulla**. Interestingly, this enzyme is induced by **glucocorticoids** (cortisol) reaching the medulla via the intra-adrenal portal system. 2. **Why other options are incorrect:** * **Decarboxylation:** This process occurs earlier in the pathway when **L-Dopa** is converted to **Dopamine** by the enzyme Dopa decarboxylase (requiring Vitamin B6). * **Oxidation:** While the pathway involves hydroxylation (a form of oxidation), such as Tyrosine to L-Dopa, the specific conversion of norepinephrine to epinephrine does not involve oxygen addition or electron loss in the functional group. * **Sulphation:** This is a phase II detoxification reaction used for the excretion of catecholamines in urine, not for their synthesis. **NEET-PG High-Yield Pearls:** * **Rate-limiting enzyme** of catecholamine synthesis: **Tyrosine Hydroxylase**. * **Sequence of synthesis:** Tyrosine → L-Dopa → Dopamine → Norepinephrine → Epinephrine. * **Cofactor for PNMT:** S-adenosylmethionine (SAM). * **Vitamin C** is a required cofactor for **Dopamine β-hydroxylase** (Dopamine → Norepinephrine). * **VMA (Vanillylmandellic acid)** is the end-stage urinary metabolite of both norepinephrine and epinephrine, used to diagnose Pheochromocytoma.

Q: A 2-year-old boy presents with developmental delay, vomiting, irritability, and a skin rash. His mother reports a peculiar 'mousy' odor in his urine. Physical examination reveals an eczema-like rash, hyperreflexia, and increased muscle tone. The child has a notably fair complexion compared to his family. What is the most likely diagnosis?

Phenylketonuria. **Explanation:** The clinical presentation is classic for **Phenylketonuria (PKU)**, an autosomal recessive disorder caused by a deficiency of the enzyme **phenylalanine hydroxylase (PAH)** or its cofactor **tetrahydrobiopterin (BH4)**. **Why Phenylketonuria is correct:** * **Biochemical Pathogenesis:** The enzyme deficiency leads to the accumulation of phenylalanine and its alternative metabolites (phenylketones like phenylacetate and phenyllactate). * **Mousy Odor:** Phenylacetate excretion in sweat and urine produces the characteristic "mousy" or "musty" odor. * **Fair Complexion:** Phenylalanine is a precursor to tyrosine. Reduced tyrosine levels lead to decreased **melanin** synthesis, resulting in fair skin, blonde hair, and blue eyes. * **Neurological Symptoms:** High phenylalanine levels are neurotoxic, causing developmental delay, seizures, and hyperreflexia. **Why other options are incorrect:** * **Tay-Sachs Disease:** Characterized by a "cherry-red spot" on the macula and progressive neurodegeneration due to Hexosaminidase A deficiency, but lacks the mousy odor and skin hypopigmentation. * **McArdle Disease:** A glycogen storage disease (Type V) affecting muscles. It presents with exercise-induced cramps and myoglobinuria, not developmental delay or skin changes. * **Pyruvate Dehydrogenase Deficiency:** Leads to chronic lactic acidosis and neurological impairment, but does not present with the specific odor or pigmentary changes seen here. **High-Yield Clinical Pearls for NEET-PG:** * **Screening:** Guthrie test (bacterial inhibition assay) is used for neonatal screening. * **Management:** Dietary restriction of phenylalanine and supplementation of tyrosine. * **Maternal PKU:** If a mother with PKU doesn't maintain a strict diet during pregnancy, the fetus may develop microcephaly and congenital heart defects (teratogenic effect).

Question 1

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 2

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 3

HHH syndrome is due to deficiency of which enzyme?

Accepted Answer

Ornithine transcarbamylase

Answer

Arginase

Answer

Arginosuccinase

Answer

None of the above

Question 4

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 5

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

Accepted Answer

Isoleucine

Answer

Leucine

Answer

Valine

Answer

Tryptophan

Question 6

Methionine is synthesized in the human body from which precursor?

Accepted Answer

Homocysteine

Answer

Cysteine

Answer

Cystine

Answer

Tryptophan

Question 7

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

Question 8

Hyperphenylalaninemia is due to a defect in which enzyme?

Accepted Answer

Phenylalanine hydroxylase

Answer

Tyrosinase

Answer

Homogentisic acid oxidase

Answer

Ornithine transcarbamylase

Question 9

Conversion of norepinephrine to epinephrine occurs by which biochemical process?

Accepted Answer

Methylation

Answer

Decarboxylation

Answer

Oxidation

Answer

Sulphation

Question 10

A 2-year-old boy presents with developmental delay, vomiting, irritability, and a skin rash. His mother reports a peculiar 'mousy' odor in his urine. Physical examination reveals an eczema-like rash, hyperreflexia, and increased muscle tone. The child has a notably fair complexion compared to his family. What is the most likely diagnosis?

Accepted Answer

Phenylketonuria

Answer

Tay-Sachs disease

Answer

Mcardle disease

Answer

Pyruvate dehydrogenase deficiency

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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