Amino Acid Metabolism Practice Questions

Q: Which of the following is the precursor of adrenaline and noradrenaline?

Tyrosine. **Explanation:** The synthesis of catecholamines (Dopamine, Noradrenaline, and Adrenaline) occurs primarily in the adrenal medulla and sympathetic neurons. **Tyrosine** is the direct precursor for this pathway. **Why Tyrosine is correct:** The biosynthetic pathway follows this sequence: 1. **Tyrosine** → **L-DOPA** (via *Tyrosine Hydroxylase*, the rate-limiting step). 2. L-DOPA → **Dopamine** (via *DOPA decarboxylase*). 3. Dopamine → **Noradrenaline** (via *Dopamine β-hydroxylase*). 4. Noradrenaline → **Adrenaline** (via *Phenylethanolamine N-methyltransferase* or PNMT). **Why other options are incorrect:** * **Phenylalanine:** While Phenylalanine is the precursor to Tyrosine (via *Phenylalanine hydroxylase*), Tyrosine is considered the immediate precursor in the catecholamine pathway. In the context of NEET-PG, if both are listed, Tyrosine is the more specific answer. * **Tryptophan:** This is the precursor for **Serotonin** (5-HT) and **Melatonin**, as well as Niacin (Vitamin B3). It is not involved in catecholamine synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Tyrosine Hydroxylase (requires Tetrahydrobiopterin/BH4). * **PNMT:** This enzyme, which converts Noradrenaline to Adrenaline, is induced by **Glucocorticoids** (Cortisol). * **VMA (Vanillylmandelic Acid):** The major urinary metabolite of adrenaline and noradrenaline, used to diagnose **Pheochromocytoma**. * **Albinism:** Caused by a deficiency in *Tyrosinase*, which converts Tyrosine to Melanin (a different branch of Tyrosine metabolism).

Q: In urea cycle disorder, which of the following substances can be used to reduce the levels of ammonia?

Phenylbutyrate. **Explanation:** In **Urea Cycle Disorders (UCDs)**, the body lacks the enzymes necessary to convert toxic ammonia into urea for excretion. This leads to hyperammonemia, which is neurotoxic. **Why Phenylbutyrate is correct:** Sodium phenylbutyrate acts as an **ammonia scavenger**. Once ingested, it is rapidly metabolized into phenylacetate. Phenylacetate conjugates with **glutamine** (which contains two nitrogen atoms) to form phenylacetylglutamine, which is then excreted by the kidneys. By removing glutamine, the body is forced to use ammonia to synthesize more glutamine, thereby effectively lowering circulating ammonia levels through an alternative pathway. **Analysis of Incorrect Options:** * **L-carnitine:** While used in organic acidemias to conjugate with organic acids, it does not directly lower ammonia in primary urea cycle defects. * **Isoleucine:** This is a branched-chain amino acid. While protein restriction (including essential amino acids) is managed in UCDs, isoleucine itself does not reduce ammonia levels. * **Glutamate:** Adding glutamate would be counterproductive. In hyperammonemia, ammonia combines with glutamate to form glutamine; adding more substrate does not aid excretion and may contribute to the nitrogen load. **NEET-PG High-Yield Pearls:** * **Drug of Choice:** Sodium Phenylbutyrate and Sodium Benzoate (conjugates with Glycine) are the mainstays for chronic management of UCDs. * **Most Common UCD:** Ornithine Transcarbamylase (OTC) deficiency (the only X-linked UCD). * **Acute Management:** In severe neonatal hyperammonemia, **hemodialysis** is the most effective method to rapidly reduce ammonia levels. * **Dietary Rule:** High-calorie, low-protein diet is essential to prevent catabolism.

Q: Xanthurenic acid, the accumulation of which occurs due to pyridoxine deficiency, is a metabolite of which amino acid?

Tryptophan. **Explanation:** The correct answer is **Tryptophan**. **Why Tryptophan is correct:** Tryptophan is metabolized via the **Kynurenine pathway** to produce Niacin (Vitamin B3). A critical step in this pathway involves the conversion of 3-hydroxykynurenine to 3-hydroxyanthranilic acid, catalyzed by the enzyme **Kynureninase**. This enzyme requires **Pyridoxal Phosphate (PLP)**, the active form of Vitamin B6, as a cofactor. In **Pyridoxine (B6) deficiency**, kynureninase activity is impaired. As a result, 3-hydroxykynurenine cannot be converted further down the niacin pathway and is instead shunted toward an alternative metabolic route, leading to the excessive formation and urinary excretion of **Xanthurenic acid**. **Why other options are incorrect:** * **Tyrosine & Phenylalanine:** These are precursors to catecholamines (Dopamine, Epinephrine), Melanin, and Thyroxine. Their metabolism involves enzymes like Phenylalanine hydroxylase and p-hydroxyphenylpyruvate hydroxylase, but they do not produce xanthurenic acid. * **Alanine:** This is a non-essential amino acid primarily involved in the Glucose-Alanine cycle (Cahill cycle) for transporting nitrogen to the liver. Its transamination requires B6, but it does not lead to xanthurenic acid formation. **High-Yield Clinical Pearls for NEET-PG:** * **Xanthurenic Acid Index:** The measurement of xanthurenic acid in urine after a tryptophan load test is a sensitive indicator of **Vitamin B6 deficiency**. * **Niacin Link:** Since Tryptophan is a precursor to Niacin, a B6 deficiency can secondary lead to **Pellagra-like symptoms** because the kynurenine pathway is blocked. * **Hartnup Disease:** A defect in the transport of neutral amino acids (like Tryptophan) can also lead to niacin deficiency and pellagra symptoms.

Q: Phenylketonuria I is due to deficiency of which enzyme?

Phenylalanine hydroxylase. **Explanation:** **Phenylketonuria (PKU) Type I**, also known as Classical PKU, is an autosomal recessive disorder caused by a deficiency of the enzyme **Phenylalanine hydroxylase (PAH)**. 1. **Why Option A is correct:** Under normal physiological conditions, PAH converts the essential amino acid Phenylalanine into Tyrosine. This reaction requires the cofactor **Tetrahydrobiopterin (BH4)**. A deficiency in PAH leads to the accumulation of Phenylalanine in the blood and tissues, which is then alternatively metabolized into phenylketones (like phenylpyruvate), causing neurotoxicity and intellectual disability. 2. **Why other options are incorrect:** * **Option B (Homogentisate oxidase):** Deficiency of this enzyme leads to **Alkaptonuria**, characterized by ochronosis (darkening of tissues) and urine that turns black upon standing. * **Option C (Tyrosinase):** Deficiency of tyrosinase leads to **Oculocutaneous Albinism**, as this enzyme is critical for converting tyrosine into melanin. * **Note on PKU Type II & III:** These are "Malignant PKU" caused by a deficiency in **Dihydrobiopterin reductase** or enzymes synthesizing BH4, rather than the PAH enzyme itself. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad:** Intellectual disability, "mousy" or musty body odor (due to phenylacetic acid), and hypopigmentation (fair skin/blue eyes) because tyrosine is a precursor to melanin. * **Diagnosis:** Guthrie Test (bacterial inhibition assay) or Tandem Mass Spectrometry. * **Management:** Dietary restriction of Phenylalanine and supplementation of Tyrosine (which becomes an "essential" amino acid in PKU patients). * **Maternal PKU:** If a pregnant woman with PKU doesn't maintain a strict diet, the high phenylalanine levels act as a teratogen, causing microcephaly and congenital heart defects in the fetus.

Q: Hyperammonemia inhibits the TCA cycle by depleting which intermediate?

α-ketoglutarate. **Explanation:** The primary mechanism by which hyperammonemia inhibits the TCA cycle is through the **depletion of α-ketoglutarate**. When blood ammonia ($NH_3$) levels rise, the body attempts to detoxify it, particularly in the brain. This occurs via two sequential reactions: 1. **Reductive Amination:** Ammonia reacts with **α-ketoglutarate** to form Glutamate, catalyzed by *Glutamate Dehydrogenase*. 2. **Glutamine Synthesis:** Glutamate further reacts with another molecule of ammonia to form Glutamine, catalyzed by *Glutamine Synthetase*. Because α-ketoglutarate is a vital intermediate of the TCA cycle, its excessive consumption to "mop up" ammonia leads to its depletion. This halts the TCA cycle, resulting in a failure of aerobic respiration and a subsequent **ATP deficit** in neurons, which manifests as encephalopathy. **Analysis of Incorrect Options:** * **Oxaloacetate:** While it can be converted to Aspartate via transamination, it is not the primary substrate consumed during acute ammonia detoxification. * **Citrate:** This is the product of the first step of the TCA cycle; its levels fall secondary to the lack of α-ketoglutarate and slowed cycle turnover, but it is not the direct target of depletion. * **Succinyl CoA:** This is a downstream intermediate. Its levels decrease as a consequence of the cycle's failure, not as the primary cause. **NEET-PG High-Yield Pearls:** * **Brain's main detoxification route:** Glutamine synthesis (this causes osmotic swelling of astrocytes, leading to cerebral edema). * **Key Enzyme inhibited:** High ammonia also inhibits the **α-ketoglutarate dehydrogenase** complex directly. * **Clinical Presentation:** Asterixis (flapping tremors), altered sensorium, and elevated blood lactate (due to shifted metabolism toward glycolysis).

Question 1

Which of the following statements regarding amino acid metabolism is FALSE?

Accepted Answer

Threonine provides the thioethanol moiety for the biosynthesis of coenzyme A.

Answer

Histamine is a product of decarboxylation of histidine.

Answer

Ornithine serves as a precursor of both spermine and spermidine.

Answer

Serotonin and melatonin are metabolites of tryptophan.

Question 2

A child presents with microcephaly, blue eyes, fair skin, and mental retardation. The ferric chloride test is positive. What is the likely diagnosis?

Accepted Answer

Phenylketonuria

Answer

Homocystinuria

Answer

Tyrosinosis

Answer

Alkaptonuria

Question 3

Which of the following is the precursor of adrenaline and noradrenaline?

Accepted Answer

Tyrosine

Answer

Phenylalanine

Answer

Tryptophan

Answer

None of the above

Question 4

An 8-year-old child presents with tall stature, long limbs, scoliosis, pectus carinatum, ectopia lentis, and a history of recurrent thromboembolic events. He shows improvement with pharmacologic doses of vitamin B6. Which enzyme is most likely deficient in this child?

Accepted Answer

Cystathionine beta-synthase

Answer

S-Adenosylhomocysteine hydrolase

Answer

Methionine synthase

Answer

N5,N10-Methylenetetrahydrofolate reductase

Question 5

In urea cycle disorder, which of the following substances can be used to reduce the levels of ammonia?

Accepted Answer

Phenylbutyrate

Answer

L-carnitine

Answer

Isoleucine

Answer

Glutamate

Question 6

A 40-year-old woman presents with progressive palmoplantar pigmentation. X-ray of the spine shows calcification of intervertebral discs. On adding Benedict's reagent to her urine, a greenish-brown precipitate forms, and the supernatant fluid appears blue-black. What is the diagnosis?

Accepted Answer

Alkaptonuria

Answer

Phenylketonuria

Answer

Tyrosinemia type-2

Answer

Argininosuccinic aciduria

Question 7

Xanthurenic acid, the accumulation of which occurs due to pyridoxine deficiency, is a metabolite of which amino acid?

Accepted Answer

Tryptophan

Answer

Tyrosine

Answer

Phenylalanine

Answer

Alanine

Question 8

Phenylketonuria I is due to deficiency of which enzyme?

Accepted Answer

Phenylalanine hydroxylase

Answer

Homogentisate oxidase

Answer

Tyrosinase

Answer

None of the above

Question 9

Hyperammonemia inhibits the TCA cycle by depleting which intermediate?

Accepted Answer

α-ketoglutarate

Answer

Oxaloacetate

Answer

Citrate

Answer

Succinyl CoA

Question 10

A 6-year-old child presents with mental retardation, failure to walk, failure to grow, seizures, hyperactivity, and tremors. Examination reveals microcephaly, fair hair, light skin color, and blue eyes. Which enzyme is deficient in this child?

Accepted Answer

Phenylalanine hydroxylase

Answer

Homogentisate dioxygenase

Answer

Tyrosinase

Answer

Fumaryl acetoacetate hydroxylase

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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