Amino Acid Metabolism Practice Questions

Q: Nicotinamide is derived from which amino acid?

Tryptophan. **Explanation:** **Correct Option: C (Tryptophan)** Nicotinamide (Vitamin B3/Niacin) is unique because it can be synthesized endogenously from the essential amino acid **Tryptophan**. This occurs via the **Kynurenine pathway** in the liver. * **Conversion Ratio:** Approximately **60 mg of Tryptophan** is required to synthesize **1 mg of Niacin**. * **Co-factors:** This metabolic conversion requires **Vitamin B6 (Pyridoxine)** as a cofactor (specifically for the enzyme kynureninase), Vitamin B2 (Riboflavin), and Iron. **Why Incorrect Options are Wrong:** * **A. Histidine:** This is the precursor for **Histamine** (via decarboxylation) and is involved in the formation of FIGLU (formiminoglutamate). * **B. Methionine:** This is a sulfur-containing amino acid that acts as the primary **methyl donor** (as S-adenosylmethionine or SAM) and is a precursor for Cysteine and Homocysteine. * **D. Phenylalanine:** This is the precursor for **Tyrosine**, which subsequently leads to the synthesis of Catecholamines (Dopamine, Epinephrine, Norepinephrine), Melanin, and Thyroid hormones. **High-Yield Clinical Pearls for NEET-PG:** 1. **Pellagra:** A deficiency of Niacin characterized by the **4 Ds**: Dermatitis (Casal’s necklace), Diarrhea, Dementia, and Death. 2. **Hartnup Disease:** A genetic defect in the absorption of neutral amino acids (Tryptophan) in the gut and kidneys, leading to Pellagra-like symptoms. 3. **Carcinoid Syndrome:** Can lead to Niacin deficiency because up to 60% of Tryptophan is diverted toward the synthesis of **Serotonin**, leaving insufficient amounts for Niacin production. 4. **Vitamin B6 Deficiency:** Can manifest as Niacin deficiency because B6 is a mandatory cofactor in the Tryptophan-Niacin pathway.

Q: A 6-month-old boy presented with failure to thrive. Laboratory investigations revealed hypoglycemia and elevated urinary glutamine and uracil levels. Gastric tube feeding was poorly tolerated. The child became comatose, and after parenteral dextrose administration, recovered within 24 hours. Which of the following enzymes is defective?

Ornithine transcarbamoylase. ### Explanation The clinical presentation of hyperammonemia (coma), failure to thrive, and specific biochemical markers points toward a **Urea Cycle Disorder (UCD)**. **1. Why Ornithine Transcarbamoylase (OTC) is correct:** OTC deficiency is the most common urea cycle defect. When OTC is defective, **Carbamoyl Phosphate (CP)** cannot combine with ornithine to form citrulline. Consequently, excess CP leaks out of the mitochondria into the cytosol, where it enters the pyrimidine synthesis pathway. This leads to an overproduction of **Orotic acid and Uracil**, which are excreted in the urine. The elevation of **Glutamine** is a hallmark of hyperammonemia, as ammonia is detoxified into glutamine in the brain and blood. **2. Why the other options are incorrect:** * **CPS-I Deficiency (Option B):** This also causes severe hyperammonemia, but since the defect occurs *before* the formation of Carbamoyl Phosphate, there is **no elevation** of urinary orotic acid or uracil. * **Argininosuccinate Synthase (Option A) & Lyase (Option C):** These are "downstream" defects (Citrullinemia and Argininosuccinic aciduria). While they cause hyperammonemia, they typically present with significantly elevated levels of citrulline or argininosuccinate, rather than the specific uracil/orotic acid pattern seen in OTC deficiency. **3. NEET-PG High-Yield Pearls:** * **Inheritance:** OTC deficiency is the **only** Urea Cycle Disorder that is **X-linked Recessive**; all others are Autosomal Recessive. * **Differential Diagnosis:** To distinguish between CPS-I and OTC deficiency, look at **Orotic Acid/Uracil**: High = OTC; Low/Normal = CPS-I. * **Management:** Acute treatment involves stopping protein intake, giving IV glucose (to prevent catabolism), and using ammonia scavengers like Sodium Benzoate.

Q: The formation of glycine takes place by the transamination of which of the following?

Glyoxylate. **Explanation:** The correct answer is **Glyoxylate**. Glycine is a non-essential amino acid that can be synthesized through multiple pathways. One of the primary routes is the **transamination of glyoxylate**. In this reaction, the enzyme **glycine aminotransferase** transfers an amino group (usually from glutamate or alanine) to glyoxylate, converting it into glycine. **Why the other options are incorrect:** * **Alanine:** Alanine undergoes transamination to form **Pyruvate**. While alanine can act as an amino group donor in the synthesis of glycine, it is not the direct carbon skeleton precursor. * **Aspartate:** Transamination of aspartate (via AST/GOT) yields **Oxaloacetate**, a key intermediate in the TCA cycle and gluconeogenesis. * **Glutamate:** Transamination of glutamate (via ALT or AST) yields **$\alpha$-ketoglutarate**. Glutamate typically serves as the universal amino group donor rather than the precursor skeleton for glycine. **High-Yield NEET-PG Clinical Pearls:** 1. **Primary Hyperoxaluria Type I:** This condition is caused by a deficiency of the liver-specific peroxisomal enzyme **alanine-glyoxylate aminotransferase**. When glyoxylate cannot be converted to glycine, it is instead oxidized to **oxalate**, leading to renal stones (nephrolithiasis) and kidney failure. 2. **Other Glycine Sources:** Glycine is also synthesized from **Serine** (via *Serine hydroxymethyltransferase* in a folate-dependent reaction) and from **CO₂ and NH₄⁺** (via the *Glycine Cleavage System*). 3. **Specialized Products:** Glycine is a precursor for **Heme, Purines, Creatine, Glutathione, and Conjugated Bile Salts**.

Q: Major metabolite of noradrenaline in urine is:

VMA. **Explanation:** The metabolism of catecholamines involves two primary enzymes: **Monoamine Oxidase (MAO)** and **Catechol-O-methyltransferase (COMT)**. **Why VMA is correct:** **Vanillylmandelic Acid (VMA)** is the end-stage major metabolic product of both **Norepinephrine (Noradrenaline)** and **Epinephrine (Adrenaline)**. In the metabolic pathway, noradrenaline is converted into normetanephrine and subsequently oxidized into VMA. Because VMA is the final, stable metabolite excreted in the urine, it serves as the primary diagnostic marker for catecholamine-secreting tumors. **Why the other options are incorrect:** * **HVA (Homovanillic Acid):** This is the major terminal metabolite of **Dopamine**. It is clinically significant in diagnosing Neuroblastoma. * **Normetanephrine:** This is an intermediate metabolite of Noradrenaline. While it is measured in urine/plasma, it is not the "major" or final metabolite. * **Metanephrine:** This is an intermediate metabolite of Epinephrine. **High-Yield Clinical Pearls for NEET-PG:** * **Pheochromocytoma:** A tumor of the adrenal medulla where 24-hour urinary VMA levels are significantly elevated. * **Diagnostic Gold Standard:** While VMA is the major metabolite, **Plasma Free Metanephrines** are currently considered the most sensitive screening test for Pheochromocytoma. * **Dietary Restrictions:** When testing for VMA, patients must avoid vanilla, caffeine, chocolate, and bananas, as these can cause false-positive results. * **Neuroblastoma:** Characterized by high levels of both **VMA and HVA** in the urine.

Q: All of the following amino acids are one-carbon donors, EXCEPT?

Threonine. **Explanation:** The one-carbon pool refers to the collection of single-carbon units (e.g., methyl, methylene, formyl) attached to the carrier **Tetrahydrofolate (THF)**. These units are essential for the synthesis of purines and thymidine. **Why Threonine is the Correct Answer:** Threonine is primarily metabolized via threonine dehydratase into alpha-ketobutyrate or via threonine aldolase into glycine and acetaldehyde. Unlike the other options, **Threonine does not directly contribute a carbon unit to the THF pool.** While it can be converted into glycine, it is not classified as a primary one-carbon donor in standard biochemical pathways. **Analysis of Incorrect Options:** * **Serine:** The **most important** source of one-carbon units. The enzyme *Serine hydroxymethyltransferase* transfers a hydroxymethyl group to THF, forming N5,N10-methylene-THF and glycine. * **Glycine:** It contributes a carbon unit through the **Glycine Cleavage System**, releasing CO2, NH4+, and a methylene group to THF. * **Histidine:** During its catabolism to glutamate, histidine forms **FIGLU** (formiminoglutamate). The formimino group is then transferred to THF, forming N5-formimino-THF. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for 1-C Donors:** **"H-S-G-T"** (Histidine, Serine, Glycine, Tryptophan). Note that Tryptophan contributes via the formate produced during its breakdown. * **FIGLU Excretion Test:** In **Vitamin B12 or Folate deficiency**, FIGLU cannot transfer its carbon unit to THF and is excreted in the urine. This is a diagnostic marker for folate deficiency. * **Primary Carrier:** Tetrahydrofolate (THF) is the active form of Folic Acid (Vitamin B9).

Q: The hair abnormality shown in the image is a distinguishing clinical feature of which of the following urea cycle disorders?

Argininosuccinic aciduria. ***Argininosuccinic aciduria*** - **Trichorrhexis nodosa** (transverse fractures and fraying of the hair shaft) is a **pathognomonic** clinical feature specific to argininosuccinic aciduria (ASL deficiency). - This hair abnormality results from **argininosuccinic acid accumulation** affecting hair structure, making it a distinguishing diagnostic feature from other urea cycle disorders. *Hyperammonemia type 1* - Caused by **carbamoyl phosphate synthetase I (CPS1) deficiency** and does not present with characteristic hair abnormalities. - Clinical features include **severe hyperammonemia** and neurological symptoms but lack the distinctive hair findings seen in argininosuccinic aciduria. *Hyperammonemia type 2* - Results from **ornithine transcarbamylase (OTC) deficiency** and is **X-linked**, affecting males more severely. - Does not cause **trichorrhexis nodosa** or other specific hair abnormalities, presenting primarily with hyperammonemic episodes. *Argininemia* - Caused by **arginase deficiency** leading to arginine accumulation and presents with **spastic diplegia** and intellectual disability. - Hair abnormalities like **trichorrhexis nodosa** are not associated with this condition, distinguishing it from argininosuccinic aciduria.

Q: What is the allosteric activator of NAG synthase?

Arginine. **Explanation:** The correct answer is **Arginine**. **1. Why Arginine is Correct:** The Urea Cycle is the primary pathway for disposing of toxic ammonia. The rate-limiting step of this cycle is catalyzed by **Carbamoyl Phosphate Synthetase I (CPS-I)**. However, CPS-I is inactive without its obligatory allosteric activator, **N-acetylglutamate (NAG)**. NAG is synthesized from Acetyl-CoA and Glutamate by the enzyme **NAG Synthase**. **Arginine** acts as a potent allosteric activator of NAG Synthase. Therefore, when arginine levels are high (signaling an abundance of amino acids), it stimulates the production of NAG, which in turn activates CPS-I, accelerating the urea cycle to process the nitrogen load. **2. Why Other Options are Incorrect:** * **Aspartate:** This is a substrate in the urea cycle (combining with citrulline to form argininosuccinate), not an activator of NAG synthase. * **CPS-I:** This is the enzyme activated *by* NAG, not an activator of NAG synthase itself. * **CPS-II:** This is the rate-limiting enzyme of **Pyrimidine synthesis** located in the cytosol. It is inhibited by UTP and activated by PRPP; it has no role in the urea cycle or NAG regulation. **Clinical Pearls & High-Yield Facts:** * **Obligatory Activator:** Remember that CPS-I has an absolute requirement for NAG. Without NAG, urea synthesis ceases, leading to hyperammonemia. * **NAGS Deficiency:** A rare genetic defect in NAG Synthase presents clinically identical to CPS-I deficiency. It is treated with **Carglumic acid** (a synthetic analog of NAG). * **Mnemonic:** **A**rginine **A**ctivates the **A**ctivator (NAG).

Q: All of the following can be used in the treatment of hyperammonemia EXCEPT?

L-carnitine. **Explanation:** Hyperammonemia is a medical emergency where the body cannot effectively clear ammonia, typically due to urea cycle enzyme deficiencies or liver failure. The goal of treatment is to provide alternative pathways for nitrogen excretion or replenish cycle intermediates. **Why L-carnitine is the correct answer:** L-carnitine is primarily involved in the transport of long-chain fatty acids into the mitochondria for beta-oxidation. While it is used in specific organic acidemias (to conjugate with toxic organic acids) or Valproate toxicity, it is **not** a standard treatment for primary hyperammonemia or urea cycle disorders. It does not directly facilitate the removal of ammonia or provide an alternative nitrogen disposal pathway. **Analysis of Incorrect Options:** * **Sodium benzoate:** Acts as a "nitrogen scavenger." It conjugates with **Glycine** to form Hippuric acid, which is excreted in the urine. Since glycine contains nitrogen, this bypasses the urea cycle. * **Phenylacetate (or Phenylbutyrate):** Another nitrogen scavenger. It conjugates with **Glutamine** to form Phenylacetylglutamine. Because glutamine contains two nitrogen atoms, this is a highly efficient way to remove excess nitrogen. * **L-arginine:** In many urea cycle defects (except Arginase deficiency), arginine becomes an essential amino acid. Supplementation helps provide intermediates to keep the cycle functioning and promotes the excretion of nitrogen through citrulline or argininosuccinate. **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice for acute hyperammonemia:** Hemodialysis (fastest way to reduce levels). * **Nitrogen Scavenger Mechanism:** Benzoate → Glycine; Phenylacetate → Glutamine. * **Lactulose:** Used in hepatic encephalopathy to "trap" ammonia ($NH_3$) as ammonium ($NH_4^+$) in the gut by lowering luminal pH. * **N-acetylglutamate (NAG) analog:** Carglumic acid is used specifically for NAGS deficiency.

Q: A vitamin B6 deficiency reduces the effectiveness of transaminase enzymes. Which amino acid is formed from the transamination of alpha-ketoglutarate?

Glutamate. ### Explanation **Correct Answer: D. Glutamate** **The Concept of Transamination** Transamination is the process where an amino group ($\text{-NH}_2$) is transferred from an amino acid to a keto acid, resulting in the formation of a new amino acid and a new keto acid. This reaction is catalyzed by **aminotransferases (transaminases)**, which require **Pyridoxal Phosphate (PLP)**, the active form of **Vitamin B6**, as an essential cofactor. In this specific reaction, **$\alpha$-ketoglutarate** (a 5-carbon dicarboxylic keto acid) acts as the universal amino group acceptor. When it accepts an amino group, it is converted into its corresponding amino acid, **Glutamate**. **Analysis of Incorrect Options:** * **A. Glycine:** This is the simplest amino acid. Its corresponding keto acid is glyoxylate. It is not derived from $\alpha$-ketoglutarate. * **B. Glutamine:** While related to glutamate, glutamine is formed via the action of **glutamine synthetase**, which adds a second amino group (as free ammonia) to glutamate. This is an ATP-dependent amidation, not a transamination. * **C. Asparagine:** This is the amide form of aspartate. It is synthesized from aspartate and glutamine via asparagine synthetase. The keto acid precursor for aspartate (via transamination) is **oxaloacetate**, not $\alpha$-ketoglutarate. **NEET-PG High-Yield Pearls:** * **Cofactor:** All transaminases require **Vitamin B6 (PLP)**. Deficiency leads to impaired amino acid metabolism and neurological symptoms (due to decreased GABA synthesis). * **Key Pairs to Remember:** * Pyruvate $\leftrightarrow$ Alanine (via ALT) * Oxaloacetate $\leftrightarrow$ Aspartate (via AST) * $\alpha$-ketoglutarate $\leftrightarrow$ Glutamate * **Exception:** Lysine, Threonine, Proline, and Hydroxyproline **do not** undergo transamination. * **Clinical Marker:** AST and ALT are sensitive markers for liver injury; AST is also found in cardiac and skeletal muscle.

Q: Metabolites of tryptophan can give rise to all of the following except?

Phenylketonuria. **Explanation:** Tryptophan is an essential amino acid that serves as a precursor for several vital molecules, including **Serotonin (5-HT)**, **Melatonin**, and **Niacin (Vitamin B3)**. **Why Phenylketonuria (PKU) is the correct answer:** Phenylketonuria 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**, not Tryptophan. Therefore, Tryptophan metabolites are not involved in the pathogenesis of PKU. **Analysis of Incorrect Options:** * **Vasoconstriction & Flushing:** Tryptophan is converted into Serotonin. In **Carcinoid Syndrome** (a tumor of enterochromaffin cells), there is massive overproduction of Serotonin. Serotonin acts as a potent **vasoconstrictor** in certain vascular beds, while its metabolites and associated kinins cause episodic **flushing**. * **Diarrhoea:** Serotonin increases gastrointestinal motility. In Carcinoid Syndrome, the excess production of Serotonin leads to secretory diarrhoea. **High-Yield Clinical Pearls for NEET-PG:** * **Carcinoid Triad:** Flushing, Diarrhoea, and Right-sided heart failure (Valvular lesions). * **Diagnostic Marker:** Elevated urinary **5-HIAA** (5-Hydroxyindoleacetic acid), which is the end-metabolite of Serotonin. * **Hartnup Disease:** A defect in the transport of neutral amino acids (Tryptophan) in the gut and kidneys, leading to **Pellagra-like symptoms** (Dermatitis, Diarrhoea, Dementia) due to Niacin deficiency. * **Conversion Ratio:** Approximately 60 mg of Tryptophan is required to synthesize 1 mg of Niacin.

Question 1

Nicotinamide is derived from which amino acid?

Accepted Answer

Tryptophan

Answer

Histidine

Answer

Methionine

Answer

Phenylalanine

Question 2

A 6-month-old boy presented with failure to thrive. Laboratory investigations revealed hypoglycemia and elevated urinary glutamine and uracil levels. Gastric tube feeding was poorly tolerated. The child became comatose, and after parenteral dextrose administration, recovered within 24 hours. Which of the following enzymes is defective?

Accepted Answer

Ornithine transcarbamoylase

Answer

Argininosuccinate synthase

Answer

Carbamoyl phosphate synthase I

Answer

Argininosuccinate lyase

Question 3

The formation of glycine takes place by the transamination of which of the following?

Accepted Answer

Glyoxylate

Answer

Alanine

Answer

Aspartate

Answer

Glutamate

Question 4

Major metabolite of noradrenaline in urine is:

Accepted Answer

VMA

Answer

HVA

Answer

Normetanephrine

Answer

Metanephrine

Question 5

All of the following amino acids are one-carbon donors, EXCEPT?

Accepted Answer

Threonine

Answer

Glycine

Answer

Serine

Answer

Histidine

Question 6

The hair abnormality shown in the image is a distinguishing clinical feature of which of the following urea cycle disorders?

Accepted Answer

Argininosuccinic aciduria

Answer

Hyperammonemia type 1

Answer

Hyperammonemia type 2

Answer

Argininemia

Question 7

What is the allosteric activator of NAG synthase?

Accepted Answer

Arginine

Answer

Aspartate

Answer

CPS-I

Answer

CPS-II

Question 8

All of the following can be used in the treatment of hyperammonemia EXCEPT?

Accepted Answer

L-carnitine

Answer

Sodium benzoate

Answer

L-arginine

Answer

Phenylacetate

Question 9

A vitamin B6 deficiency reduces the effectiveness of transaminase enzymes. Which amino acid is formed from the transamination of alpha-ketoglutarate?

Accepted Answer

Glutamate

Answer

Glycine

Answer

Glutamine

Answer

Asparagine

Question 10

Metabolites of tryptophan can give rise to all of the following except?

Accepted Answer

Phenylketonuria

Answer

Diarrhoea

Answer

Vasoconstriction

Answer

Flushing

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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