Amino Acid Metabolism Practice Questions

Q: Cabbage-like odor is found in the body fluids in which of the following metabolic disorders?

Tyrosinemia. ### Explanation The correct answer is **Tyrosinemia (specifically Type I)**. **1. Why Tyrosinemia is correct:** Tyrosinemia Type I (Hepatorenal Tyrosinemia) is caused by a deficiency of the enzyme **Fumarylacetoacetate hydrolase (FAH)**. This leads to the accumulation of fumarylacetoacetate and its conversion into **succinylacetone**. The characteristic **cabbage-like or boiled vegetable odor** in body fluids (urine and sweat) is attributed to the accumulation of methionine metabolites and sulfur-containing compounds that occur secondary to liver damage in these patients. **2. Why the other options are incorrect:** * **Phenylketonuria (PKU):** Caused by Phenylalanine hydroxylase deficiency. It is characterized by a **mousy or musty odor** due to the accumulation of phenylacetic acid. * **Maple Syrup Urine Disease (MSUD):** Caused by a deficiency in the Branched-chain alpha-keto acid dehydrogenase complex. It results in a **burnt sugar or maple syrup odor** due to the accumulation of isoleucine. * **Isovaleric Acidemia:** Caused by Isovaleryl-CoA dehydrogenase deficiency. It presents with a characteristic **"sweaty feet" or "cheesy" odor** due to the accumulation of isovaleric acid. **3. Clinical Pearls for NEET-PG:** * **Tyrosinemia Type I:** High risk of early-onset **Hepatocellular Carcinoma (HCC)** and renal rickets. * **Diagnostic Marker:** Succinylacetone in urine is pathognomonic. * **Treatment:** Nitisinone (NTBC), which inhibits 4-hydroxyphenylpyruvate dioxygenase to prevent the formation of toxic metabolites. **Summary of Odors in Metabolic Disorders:** * **Mousy:** PKU * **Cabbage-like:** Tyrosinemia * **Maple Syrup:** MSUD * **Sweaty Feet:** Isovaleric acidemia / Glutaric acidemia Type II * **Fishy:** Trimethylaminuria * **Swimming Pool:** Hawkinsinuria

Q: Which of the following amino acids enters the TCA cycle at the succinyl CoA step?

Methionine. ### Explanation The entry of amino acids into the TCA cycle depends on their carbon skeleton degradation pathway. **Methionine** is a sulfur-containing amino acid that follows a specific catabolic route to enter the TCA cycle at the **Succinyl CoA** step. **Why Methionine is Correct:** Methionine is converted into **S-adenosylmethionine (SAM)**, which then forms **Homocysteine**. Homocysteine combines with serine to form cystathionine, eventually yielding **Propionyl CoA**. Propionyl CoA undergoes carboxylation (Biotin-dependent) and isomerization (Vitamin B12-dependent) to become **Succinyl CoA**. Other amino acids following this "VOMIT" mnemonic (Valine, Odd-chain fatty acids, Methionine, Isoleucine, Threonine) also enter at this step. **Analysis of Incorrect Options:** * **A. Tryptophan:** This is both glucogenic and ketogenic. It enters the TCA cycle via **Pyruvate** and **Acetoacetyl CoA**. * **B. Tyrosine:** This is also both glucogenic and ketogenic. Its carbon skeleton is degraded to **Fumarate** and **Acetoacetate**. * **C. Glycine:** Glycine is primarily converted to **Pyruvate** (via serine) or undergoes the glycine cleavage system to CO₂ and NH₄⁺. **High-Yield Clinical Pearls for NEET-PG:** * **Vitamin B12 Deficiency:** Leads to the accumulation of **Methylmalonic acid (MMA)** because the conversion of Methylmalonyl CoA to Succinyl CoA is blocked. This is a key diagnostic marker to differentiate B12 deficiency from Folate deficiency. * **Homocystinuria:** Most commonly due to a deficiency in **Cystathionine β-synthase**, an enzyme in the methionine catabolic pathway. * **Ketogenic vs. Glucogenic:** Leucine and Lysine are the only *purely* ketogenic amino acids. Methionine is *purely* glucogenic.

Q: Which of the following amino acids is NOT both glucogenic and ketogenic?

Leucine. ### Explanation Amino acids are classified based on the metabolic intermediates they produce during catabolism. They can be **glucogenic** (converted into glucose via gluconeogenesis), **ketogenic** (converted into ketone bodies or acetyl-CoA), or **both**. **1. Why Leucine is the Correct Answer:** Leucine is one of only two **purely ketogenic** amino acids (the other being Lysine). Its catabolism yields only Acetyl-CoA and Acetoacetate, which cannot be used for the net synthesis of glucose in humans. Therefore, it is NOT both glucogenic and ketogenic. **2. Analysis of Incorrect Options:** The amino acids that are **both glucogenic and ketogenic** can be remembered by the mnemonic **"PITTT"** (Phenylalanine, Isoleucine, Tyrosine, Threonine, Tryptophan). * **A. Phenylalanine:** It is both; it yields Fumarate (glucogenic) and Acetoacetate (ketogenic). * **B. Tyrosine:** It is both; as a derivative of phenylalanine, it also yields Fumarate and Acetoacetate. * **C. Tryptophan:** It is both; it yields Pyruvate (glucogenic) and Acetoacetate (ketogenic). **3. High-Yield Facts for NEET-PG:** * **Purely Ketogenic:** Leucine and Lysine (The "L" amino acids). * **Both Glucogenic & Ketogenic:** Phenylalanine, Isoleucine, Tyrosine, Threonine, Tryptophan. * **Purely Glucogenic:** All other 13 amino acids. * **Clinical Pearl:** In patients with **Maple Syrup Urine Disease (MSUD)**, there is a defect in the branched-chain alpha-keto acid dehydrogenase complex, leading to an accumulation of Leucine, Isoleucine, and Valine. Leucine is the most toxic of these to the CNS.

Q: Ingestion of l-tryptophan increases the levels of which neurotransmitter in the brain?

Serotonin. ### Explanation **Correct Option: A. Serotonin** Tryptophan is an essential amino acid that serves as the direct precursor for **Serotonin (5-hydroxytryptamine)**. The synthesis occurs in a two-step pathway: 1. **Tryptophan hydroxylase** (the rate-limiting enzyme) converts L-tryptophan to 5-hydroxytryptophan (5-HTP). This enzyme requires **Tetrahydrobiopterin (BH4)** as a cofactor. 2. **Aromatic L-amino acid decarboxylase** converts 5-HTP to Serotonin (requires Vitamin B6/Pyridoxine). Since the transport of tryptophan across the blood-brain barrier is a saturable process, increasing dietary intake of L-tryptophan directly raises brain serotonin levels. **Why Other Options are Incorrect:** * **B & C (Norepinephrine and Dopamine):** These are catecholamines derived from the amino acid **Tyrosine** (via the L-Dopa pathway). Tryptophan does not contribute to their synthesis. * **D (Acetylcholine):** This neurotransmitter is synthesized from **Choline and Acetyl-CoA** by the enzyme Choline acetyltransferase. It is not derived from an amino acid precursor like tryptophan. **High-Yield Clinical Pearls for NEET-PG:** * **Melatonin Connection:** Serotonin is further converted to Melatonin in the pineal gland; thus, Tryptophan is also the precursor for Melatonin. * **Niacin Synthesis:** About 60 mg of Tryptophan can produce 1 mg of **Niacin (Vitamin B3)**. * **Hartnup Disease:** A defect in the transport of neutral amino acids (like Tryptophan) leads to pellagra-like symptoms due to niacin deficiency. * **Carcinoid Syndrome:** In this condition, tumor cells divert up to 60% of tryptophan metabolism toward serotonin production, leading to secondary niacin deficiency (Pellagra).

Q: Which of the following best describes Tryptophan?

Non polar, essential, both glucogenic & ketogenic. ### Explanation **Tryptophan** is a unique and complex amino acid. To arrive at the correct answer, we evaluate it based on three criteria: 1. **Polarity:** Tryptophan contains an **indole ring** (a bicyclic structure). Despite having a nitrogen atom in the ring, the large hydrocarbon structure makes it hydrophobic and **non-polar**. 2. **Nutritional Status:** It is one of the 10 **essential amino acids** (PVT TIM HALL), meaning it cannot be synthesized by the body and must be obtained through diet. 3. **Metabolic Fate:** Tryptophan is **both glucogenic and ketogenic**. Its breakdown yields **Alanine** (which enters gluconeogenesis via pyruvate) and **Acetoacetyl-CoA** (which is ketogenic). #### Why other options are incorrect: * **Option A:** Incorrect because Tryptophan is not purely glucogenic; it is also ketogenic. * **Option B:** Incorrect because Tryptophan is classified as non-polar due to its bulky hydrophobic side chain. * **Option C:** Incorrect because Tryptophan is an essential amino acid, not non-essential. --- ### High-Yield Clinical Pearls for NEET-PG * **Precursor Role:** Tryptophan is the precursor for **Serotonin** (neurotransmitter), **Melatonin** (sleep hormone), and **Niacin** (Vitamin B3). * **Hartnup Disease:** A defect in the transport of neutral amino acids (including Tryptophan) in the gut and kidneys, leading to pellagra-like symptoms. * **Carcinoid Syndrome:** Tumors that divert Tryptophan metabolism toward Serotonin, potentially leading to a secondary Niacin deficiency. * **Largest Amino Acid:** Tryptophan has the highest molecular weight among the 20 standard amino acids. * **Absorbance:** It is responsible for the maximum UV light absorbance of proteins at **280 nm**.

Q: Which of the following is not an enzyme involved in urea biosynthesis?

Carbamoyl phosphate synthetase II. ### Explanation The **Urea Cycle (Ornithine Cycle)** is the primary mechanism for detoxifying ammonia into urea in the liver. To answer this question correctly, one must distinguish between the enzymes of the urea cycle and those of pyrimidine synthesis. **1. Why Carbamoyl Phosphate Synthetase II (CPS-II) is the correct answer:** CPS-II is **not** involved in the urea cycle. It is the rate-limiting enzyme for **De Novo Pyrimidine Synthesis**. It is located in the **cytosol** of all nucleated cells and uses glutamine as a nitrogen source. In contrast, **CPS-I** is the rate-limiting enzyme for the urea cycle, located in the **mitochondria**, and requires N-acetylglutamate (NAG) as an activator. **2. Analysis of Incorrect Options:** * **Ornithine Transcarbamylase (OTC):** This is the second enzyme of the urea cycle (mitochondrial). It combines carbamoyl phosphate with ornithine to form citrulline. Deficiency of OTC is the most common urea cycle disorder (X-linked). * **Argininosuccinase (Argininosuccinate Lyase):** This cytosolic enzyme cleaves argininosuccinate into arginine and fumarate. Fumarate provides a link to the TCA cycle (Krebs' Bi-cycle). * **Arginase:** This is the final enzyme of the cycle. It hydrolyzes arginine into urea and regenerates ornithine. **3. NEET-PG High-Yield Pearls:** * **Mnemonic for Urea Cycle Enzymes:** **C**an **O**ur **A**unt **A**lways **A**ssist? (**C**PS-I, **O**TC, **A**rgininosuccinate synthetase, **A**rgininosuccinate lyase, **A**rginase). * **Compartmentalization:** The first two steps occur in the **mitochondria**, while the remaining steps occur in the **cytosol**. * **CPS-I vs. CPS-II:** Remember **"I"** for **Urea** (one "u" in urea) and **"II"** for **Pyrimidines** (two "i"s in pyrimidine). * **Rate-limiting step:** CPS-I (requires N-acetylglutamate as an absolute allosteric activator).

Q: In one-carbon metabolism, when serine is converted to glycine, which carbon atom is transferred to tetrahydrofolate (THFA)?

Beta carbon. The conversion of **Serine to Glycine** is a pivotal reaction in one-carbon metabolism, catalyzed by the enzyme **Serine Hydroxymethyltransferase (SHMT)**. This reaction requires **Pyridoxal Phosphate (PLP)** as a cofactor and **Tetrahydrofolate (THF)** as the one-carbon acceptor. ### Why the Beta Carbon is Correct Serine is a three-carbon amino acid, while glycine is a two-carbon amino acid. During this reaction, the **side chain carbon (the Beta-carbon)** of serine is cleaved and transferred to THF. This results in the formation of **N5, N10-methylene THF**, which is the most important source of one-carbon units in the body. This one-carbon unit is subsequently used for the synthesis of thymidine (DNA) and purines. ### Why Other Options are Incorrect * **Alpha Carbon:** The alpha carbon (C2) remains part of the amino acid structure, becoming the alpha carbon of the resulting glycine molecule. * **Gamma and Delta Carbons:** These options are structurally impossible for serine. Serine is a small amino acid with only three carbons (Carboxyl C1, Alpha C2, and Beta C3). Gamma and Delta carbons are found in longer-chain amino acids like glutamate or lysine. ### High-Yield Clinical Pearls for NEET-PG * **Cofactor Requirement:** SHMT is a **PLP-dependent** enzyme. Do not confuse this with the THF requirement; both are essential. * **The "One-Carbon Pool":** Serine is considered the **major physiological source** of one-carbon units in human metabolism. * **Clinical Link:** Methotrexate (an anticancer drug) inhibits dihydrofolate reductase, depleting the THF pool and indirectly halting the serine-to-glycine conversion, thereby inhibiting DNA synthesis. * **Reversibility:** This reaction is reversible and also serves as a pathway for the de novo synthesis of serine from glycine.

Q: Transfer of an amino group from an amino acid to an alpha keto acid is catalyzed by which class of enzymes?

Transaminases. **Explanation:** **1. Why Transaminases is Correct:** Transamination is the first step in the catabolism of most amino acids. This process involves the transfer of an $\alpha$-amino group from an amino acid to an $\alpha$-keto acid (usually $\alpha$-ketoglutarate), resulting in the formation of a new amino acid (glutamate) and a new $\alpha$-keto acid. This reaction is catalyzed by **Transaminases** (also known as Aminotransferases). These enzymes require **Pyridoxal Phosphate (PLP)**, a derivative of Vitamin B6, as an essential cofactor. **2. Why Other Options are Incorrect:** * **Aminases:** These enzymes catalyze the addition of an amine group (amination), but the term is not used for the specific transfer reaction between amino acids and keto acids. * **Transketolase:** This enzyme is part of the Pentose Phosphate Pathway (HMP Shunt). It transfers two-carbon units between sugars and requires Thiamine Pyrophosphate (TPP) as a cofactor. * **Decarboxylase:** These enzymes catalyze the removal of a carboxyl group (as $CO_2$) from substrates. In amino acid metabolism, decarboxylation converts amino acids into biogenic amines (e.g., Histidine to Histamine). **3. NEET-PG High-Yield Clinical Pearls:** * **Cofactor:** All transaminases require **Vitamin B6 (PLP)**. * **Diagnostic Markers:** AST (SGOT) and ALT (SGPT) are transaminases used as sensitive markers for liver injury. AST is also found in the heart and muscle. * **Exceptions:** Two amino acids, **Lysine and Threonine**, do not undergo transamination. * **The "Funnel":** Most amino acids transfer their amino group to $\alpha$-ketoglutarate to form **Glutamate**, which then undergoes oxidative deamination in the liver to release free ammonia.

Question 1

Which of the following statements is true regarding catecholamines?

Accepted Answer

Production terminates with dopamine in the brain but epinephrine in the adrenal gland

Answer

Production begins with the action of tyrosine hydroxylase on tyrosine

Answer

Are metabolized to both glucogenic and ketogenic fragments

Answer

All contain a methyl group donated by S-adenosylmethionine

Question 2

A 2-week-old neonate presents with complete hypotonia, convulsions, failure to thrive, and metabolic acidosis. The urine has a 'burnt sugar' odor, and a positive DNPH test. What is the enzyme deficiency in this metabolic disorder?

Accepted Answer

Branched chain keto acid dehydrogenase

Answer

Isovaleryl CoA dehydrogenase

Answer

Dihydrolipoamide dehydrogenase

Answer

Transacylase

Question 3

Cabbage-like odor is found in the body fluids in which of the following metabolic disorders?

Accepted Answer

Tyrosinemia

Answer

Phenylketonuria

Answer

Maple syrup urine disease

Answer

Isovaleric academia

Question 4

Which of the following amino acids enters the TCA cycle at the succinyl CoA step?

Accepted Answer

Methionine

Answer

Tryptophan

Answer

Tyrosine

Answer

Glycine

Question 5

Which of the following amino acids is NOT both glucogenic and ketogenic?

Accepted Answer

Leucine

Answer

Phenylalanine

Answer

Tyrosine

Answer

Tryptophan

Question 6

Ingestion of l-tryptophan increases the levels of which neurotransmitter in the brain?

Accepted Answer

Serotonin

Answer

Norepinephrine

Answer

Dopamine

Answer

Acetylcholine

Question 7

Which of the following best describes Tryptophan?

Accepted Answer

Non polar, essential, both glucogenic & ketogenic

Answer

Non polar, essential, and pure glucogenic

Answer

Polar, essential, and both glucogenic & ketogenic

Answer

Polar, non-essential, both glucogenic & ketogenic

Question 8

Which of the following is not an enzyme involved in urea biosynthesis?

Accepted Answer

Carbamoyl phosphate synthetase II

Answer

Ornithine transcarbamylase

Answer

Argininosuccinase

Answer

Arginase

Question 9

In one-carbon metabolism, when serine is converted to glycine, which carbon atom is transferred to tetrahydrofolate (THFA)?

Accepted Answer

Beta carbon

Answer

Alpha carbon

Answer

Delta carbon

Answer

Gamma carbon

Question 10

Transfer of an amino group from an amino acid to an alpha keto acid is catalyzed by which class of enzymes?

Accepted Answer

Transaminases

Answer

Aminases

Answer

Transketolase

Answer

Decarboxylase

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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