Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 161: Which of the following compounds does not contain glycine?

A. Glutathione
B. Creatine
C. Glutamine (Correct Answer)
D. Purine nucleotide

Explanation: **Explanation:** The correct answer is **Glutamine**. This question tests your knowledge of the metabolic precursors and derivatives of glycine, a high-yield topic in NEET-PG biochemistry. **1. Why Glutamine is the correct answer:** Glutamine is a **primary amino acid** itself, not a derivative of glycine. It is synthesized from glutamate and ammonia by the enzyme *glutamine synthetase*. While both glycine and glutamine are involved in nitrogen metabolism and the urea cycle, they are structurally distinct and one does not contain the other. **2. Why the other options are incorrect:** * **Glutathione (GSH):** This is a tripeptide composed of **Glutamate, Cysteine, and Glycine** (Glu-Cys-Gly). It is a vital antioxidant. * **Creatine:** Synthesized from three amino acids: **Arginine, Glycine, and Methionine** (specifically the methyl group from S-adenosylmethionine). * **Purine Nucleotides:** Glycine contributes the **entire C2-C7-N7** skeleton to the purine ring (Adenine and Guanine). **High-Yield Clinical Pearls for NEET-PG:** * **Glycine Derivatives (Mnemonic: "G-CHAMP"):** **G**lutathione, **C**reatine, **H**eme, **A**mmonia (via glycine cleavage), **M**ethyl groups, and **P**urines. * **Heme Synthesis:** Glycine + Succinyl CoA are the starting substrates for heme synthesis (catalyzed by ALA synthase). * **Conjugation:** Glycine is used to conjugate bile acids (e.g., Glycocholic acid) and detoxify benzoic acid into **Hippuric acid**. * **Inhibitory Neurotransmitter:** Glycine acts as an inhibitory neurotransmitter in the spinal cord (strychnine is its antagonist).

Question 162: A 7-day-old infant presents with lethargy, decreased feeding, emesis, poor weight gain, hypotonia, a high-pitched cry, seizures, and the characteristic maple syrup smell of the urine. These clinical features are due to a defect in which biochemical process?

A. Oxidation
B. Deamination
C. Carboxylation
D. Decarboxylation (Correct Answer)

Explanation: ### Explanation **Correct Option: D. Decarboxylation** The clinical presentation (lethargy, seizures, and maple syrup-scented urine) is diagnostic of **Maple Syrup Urine Disease (MSUD)**. This autosomal recessive disorder is caused by a deficiency in the **Branched-Chain Alpha-Keto Acid Dehydrogenase (BCKAD) complex**. The BCKAD complex is responsible for the **oxidative decarboxylation** of alpha-keto acids derived from the branched-chain amino acids (BCAAs): **Leucine, Isoleucine, and Valine**. When this enzyme is defective, alpha-keto acids accumulate in the blood and spill into the urine, giving it the characteristic burnt sugar smell. Specifically, the decarboxylation step is the rate-limiting step in the catabolism of these amino acids. **Analysis of Incorrect Options:** * **A. Oxidation:** While the overall process is "oxidative decarboxylation," the specific enzymatic failure in the BCKAD complex that leads to the pathology is the inability to remove the carboxyl group (decarboxylation). * **B. Deamination:** This is the first step in BCAA metabolism, where an amino group is removed by *branched-chain amino acid aminotransferase* to form alpha-keto acids. This step is functional in MSUD; the problem lies in the subsequent processing of those keto acids. * **C. Carboxylation:** This involves the addition of CO₂ (e.g., Pyruvate to Oxaloacetate via Pyruvate Carboxylase). It is not involved in the primary defect of BCAA catabolism. **Clinical Pearls for NEET-PG:** * **Mnemonic (I Love Vermont):** **I**soleucine, **L**eucine, **V**aline are the BCAAs involved. * **Cofactors:** The BCKAD complex requires five cofactors: **T**hiamine (B1), **R**iboflavin (B2), **N**iacin (B3), **L**ipoic acid, and **P**antothenate (B5). (*Mnemonic: **T**ender **R**oving **N**ights **L**ove **P**arties*). * **Thiamine-responsive MSUD:** Some patients improve with high doses of Vitamin B1. * **Diagnosis:** Elevated levels of BCAAs in plasma and **Allo-isoleucine** (pathognomonic marker).

Question 163: Nitrogen atoms in Urea are derived from which source?

A. Aspartate
B. Ammonia
C. Both Aspartate and Ammonia (Correct Answer)
D. None of the above

Explanation: ### Explanation The Urea Cycle (Krebs-Henseleit cycle) is the primary mechanism for detoxifying ammonia into urea in the liver. Urea ($NH_2-CO-NH_2$) contains two nitrogen atoms, each originating from a distinct source: 1. **First Nitrogen:** Derived from **Free Ammonia ($NH_3$)**. This ammonia combines with $CO_2$ (as bicarbonate) and ATP to form Carbamoyl Phosphate via the enzyme *Carbamoyl Phosphate Synthetase I (CPS-I)*. 2. **Second Nitrogen:** Derived from the amino group of **Aspartate**. This occurs later in the cycle when Aspartate condenses with Citrulline to form Argininosuccinate, catalyzed by *Argininosuccinate Synthetase*. **Analysis of Options:** * **Option A (Aspartate):** While true, it only accounts for one of the two nitrogen atoms. * **Option B (Ammonia):** While true, it only accounts for the first nitrogen atom entering the cycle. * **Option C (Correct):** This is the most accurate answer as urea synthesis requires one nitrogen from ammonia and one from aspartate. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-Limiting Step:** The reaction catalyzed by **CPS-I** is the rate-limiting step of the urea cycle. * **Obligatory Activator:** CPS-I requires **N-acetylglutamate (NAG)** as an essential allosteric activator. * **Carbon Source:** The single carbon atom in urea is derived from **Bicarbonate ($HCO_3^-$)**. * **Link to TCA Cycle:** The "Aspartate-Argininosuccinate Shunt" (Krebs Bicycle) connects the urea cycle to the TCA cycle via the release of **Fumarate**. * **Hyperammonemia:** Defects in any urea cycle enzyme lead to ammonia toxicity, presenting clinically with flapping tremors (asterixis), vomiting, and cerebral edema.

Question 164: What is the most important amino acid for the formation of Neutrophilic extracellular traps (NETs)?

A. Leucine
B. Methionine
C. Citrulline (Correct Answer)
D. Valine

Explanation: **Explanation:** **The Correct Answer is C: Citrulline.** The formation of **Neutrophil Extracellular Traps (NETs)**, a process known as **NETosis**, is a unique form of programmed cell death where neutrophils release a web-like mesh of chromatin and antimicrobial proteins to trap and kill pathogens. The critical step in NETosis is the **decondensation of chromatin**. This is mediated by the enzyme **Peptidylarginine Deiminase 4 (PAD4)**. PAD4 converts **Arginine** residues on histones into **Citrulline** (a process called citrullination). Because citrulline is neutral compared to the positively charged arginine, this conversion reduces the electrostatic attraction between histones and DNA, allowing the chromatin to unravel and be expelled from the cell. Therefore, the presence of citrulline is a hallmark and a functional requirement for NET formation. **Why other options are incorrect:** * **A. Leucine & D. Valine:** These are Branched-Chain Amino Acids (BCAAs). While essential for protein synthesis and muscle metabolism, they do not play a specific role in the epigenetic modifications required for NETosis. * **B. Methionine:** This is a sulfur-containing amino acid essential for initiation of translation and acting as a methyl donor (via SAM). It is not involved in the histone decondensation process. **High-Yield Clinical Pearls for NEET-PG:** * **PAD4 Enzyme:** The key enzyme for NETosis; it is calcium-dependent. * **Rheumatoid Arthritis (RA):** Anti-Cyclic Citrullinated Peptide (**anti-CCP**) antibodies are highly specific for RA. These antibodies target citrullinated proteins, many of which are generated during NETosis in the joints. * **NET Components:** NETs consist of DNA, histones, and granular enzymes like **Myeloperoxidase (MPO)** and **Neutrophil Elastase**.

Question 165: Tyrosine deficiency causes which of the following?

A. Depression (Correct Answer)
B. Hyperthyroidism
C. Hyperpigmentation
D. Phenylketonuria

Explanation: **Explanation:** **Tyrosine** is a non-essential amino acid synthesized from Phenylalanine. It serves as a critical precursor for several physiologically active compounds, including **Catecholamines** (Dopamine, Norepinephrine, and Epinephrine), **Thyroid hormones** (T3, T4), and **Melanin**. **1. Why Depression is Correct:** Tyrosine is converted into L-DOPA by *tyrosine hydroxylase*, which is then converted into **Dopamine**. Dopamine is further metabolized into **Norepinephrine**. Both neurotransmitters are vital for mood regulation, focus, and alertness. A deficiency in Tyrosine leads to decreased synthesis of these catecholamines in the brain, which is clinically associated with **Depression**, lethargy, and cognitive dysfunction. **2. Analysis of Incorrect Options:** * **Hyperthyroidism:** Tyrosine is required for thyroid hormone synthesis. Therefore, a deficiency would lead to *Hypothyroidism*, not hyperthyroidism. * **Hyperpigmentation:** Tyrosine is the precursor for Melanin (via the enzyme Tyrosinase). Deficiency results in *Hypopigmentation* or Albinism, as seen in Oculocutaneous Albinism. * **Phenylketonuria (PKU):** PKU is caused by a deficiency of the enzyme *Phenylalanine Hydroxylase*, leading to an accumulation of Phenylalanine. While Tyrosine becomes an "essential" amino acid in PKU patients, the deficiency of Tyrosine is a *consequence* of PKU, not the cause of the disease itself. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** Tyrosine hydroxylase is the rate-limiting enzyme in catecholamine synthesis. * **PKU Connection:** In Phenylketonuria, Tyrosine becomes a **conditionally essential amino acid**. * **Alkaptonuria:** Caused by a deficiency of *Homogentisate oxidase* in the Tyrosine catabolic pathway, leading to dark urine and ochronosis.

Question 166: Ochronosis is associated with which of the following conditions?

A. Alkaptonuria (Correct Answer)
B. Isovaleric aciduria
C. Phenylketonuria
D. Tyrosinemia

Explanation: **Explanation:** **Alkaptonuria** is an autosomal recessive disorder caused by a deficiency of the enzyme **Homogentisate Oxidase**. This enzyme is essential in the catabolic pathway of phenylalanine and tyrosine. Its absence leads to the accumulation of **Homogentisic Acid (HGA)**. When HGA is excreted in urine, it oxidizes upon exposure to air, turning the urine black. **Ochronosis** refers to the clinical manifestation where HGA polymerizes into a brownish-black pigment that deposits in connective tissues, such as the sclera, ear cartilage, and large joints (leading to ochronotic arthritis). **Why other options are incorrect:** * **Isovaleric aciduria:** A branched-chain amino acid disorder (leucine metabolism) characterized by a "sweaty feet" odor, not pigment deposition. * **Phenylketonuria (PKU):** Caused by Phenylalanine Hydroxylase deficiency. It presents with intellectual disability and a "mousy odor," but results in hypopigmentation (fair skin/blue eyes) due to decreased melanin, rather than ochronosis. * **Tyrosinemia:** Involves defects in various enzymes of the tyrosine pathway (e.g., Fumarylacetoacetate hydrolase in Type I). It typically presents with liver failure, renal tubular defects (Fanconi syndrome), or cabbage-like odor, but not ochronosis. **High-Yield Clinical Pearls for NEET-PG:** * **Triad of Alkaptonuria:** 1. Homogentisic aciduria (black urine), 2. Ochronosis (pigmentation), 3. Arthritis (spine and large joints). * **Diagnostic Test:** Ferric chloride test (turns transient deep blue). * **Dietary Management:** Restriction of Phenylalanine and Tyrosine; high doses of Vitamin C (ascorbic acid) may reduce pigment formation. * **Newer Drug:** **Nitisinone** (inhibits 4-hydroxyphenylpyruvate dioxygenase) reduces HGA production.

Question 167: Succinyl CoA is formed from which of the following amino acids?

A. Histidine
B. Leucine
C. Valine (Correct Answer)
D. Lysine

Explanation: **Explanation:** The conversion of amino acids into TCA cycle intermediates is a high-yield concept in biochemistry. Amino acids are classified as glucogenic, ketogenic, or both, based on their catabolic end products. **1. Why Valine is Correct:** Valine is a **branched-chain amino acid (BCAA)** that is purely glucogenic. Its catabolism follows the pathway: *Valine → α-ketoisovalerate → Isobutyryl-CoA → Propionyl-CoA → Methylmalonyl-CoA → **Succinyl-CoA**.* Succinyl-CoA then enters the TCA cycle to be used for gluconeogenesis. Other amino acids entering at Succinyl-CoA include **Isoleucine, Threonine, and Methionine** (Mnemonic: **VOMIT** – Valine, Odd-chain fatty acids, Methionine, Isoleucine, Threonine). **2. Why the Other Options are Incorrect:** * **Histidine (A):** This is a glucogenic amino acid that is converted to **α-ketoglutarate** via Glutamate. * **Leucine (B):** This is a **purely ketogenic** amino acid. It is catabolized into Acetyl-CoA and Acetoacetate; it cannot form Succinyl-CoA or glucose. * **Lysine (D):** Along with Leucine, Lysine is **purely ketogenic**. It is converted into Acetoacetyl-CoA. **Clinical Pearls for NEET-PG:** * **Maple Syrup Urine Disease (MSUD):** Caused by a deficiency in the *Branched-chain α-keto acid dehydrogenase* complex, leading to the buildup of Valine, Leucine, and Isoleucine. * **Vitamin B12 Connection:** The conversion of Methylmalonyl-CoA to Succinyl-CoA requires **Vitamin B12**. Deficiency leads to Methylmalonic aciduria. * **Purely Ketogenic Amino Acids:** Only two—Leucine and Lysine.

Question 168: Which class of amino acids contains only non-essential amino acids?

A. Acidic (Correct Answer)
B. Basic
C. Aromatic
D. Branched chain

Explanation: ### Explanation **Correct Option: A (Acidic)** In human biochemistry, amino acids are classified based on their side-chain properties and nutritional requirements. The **acidic amino acids** are **Aspartic acid (Aspartate)** and **Glutamic acid (Glutamate)**. Both are synthesized within the body via transamination of TCA cycle intermediates (Oxaloacetate to Aspartate; $\alpha$-Ketoglutarate to Glutamate). Since the body can produce them endogenously, they are strictly **non-essential**. **Analysis of Incorrect Options:** * **B. Basic:** This group includes Lysine, Arginine, and Histidine. **Lysine** is strictly essential. **Arginine and Histidine** are semi-essential (required during periods of rapid growth or positive nitrogen balance). * **C. Aromatic:** This group includes Phenylalanine, Tyrosine, and Tryptophan. **Phenylalanine and Tryptophan** are essential. Only Tyrosine is non-essential (synthesized from Phenylalanine). * **D. Branched-chain (BCAA):** This group includes **Leucine, Isoleucine, and Valine**. All three are strictly essential and must be obtained from the diet. **High-Yield NEET-PG Pearls:** * **Mnemonic for Essential Amino Acids:** "PVT TIM HALL" (Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Arginine, Leucine, Lysine). * **Purely Ketogenic Amino Acids:** Leucine and Lysine (the only two that cannot be converted to glucose). * **Clinical Correlation:** Defective metabolism of Branched-chain amino acids (BCAAs) leads to **Maple Syrup Urine Disease (MSUD)** due to a deficiency in the branched-chain $\alpha$-keto acid dehydrogenase complex. * **Amphoteric Nature:** At physiological pH, acidic amino acids carry a net negative charge, while basic amino acids carry a net positive charge.

Question 169: Oxaloacetate is formed from which amino acid?

A. Proline
B. Glutamate
C. Aspartate (Correct Answer)
D. Lysine

Explanation: ### Explanation **Correct Option: C (Aspartate)** The conversion of **Aspartate** to **Oxaloacetate (OAA)** is a classic example of a **transamination** reaction. In this process, the enzyme **Aspartate Aminotransferase (AST/SGOT)** transfers the amino group from aspartate to α-ketoglutarate, directly yielding oxaloacetate and glutamate. This reaction requires **Pyridoxal Phosphate (Vitamin B6)** as a cofactor. Since oxaloacetate is a key intermediate of the TCA cycle and a precursor for gluconeogenesis, aspartate is classified as a **glucogenic amino acid**. **Analysis of Incorrect Options:** * **A. Proline:** Proline is converted to glutamate-5-semialdehyde, which then enters the TCA cycle as **α-ketoglutarate**, not oxaloacetate. * **B. Glutamate:** Glutamate undergoes oxidative deamination (via Glutamate Dehydrogenase) or transamination to form **α-ketoglutarate**. * **D. Lysine:** Lysine is one of the two purely **ketogenic** amino acids (along with Leucine). It is metabolized into Acetoacetyl-CoA or Acetyl-CoA, never contributing to the net synthesis of glucose or oxaloacetate. **High-Yield Clinical Pearls for NEET-PG:** * **AST (SGOT) Localization:** Unlike ALT (which is purely cytosolic), AST is found in both the **mitochondria and cytosol**. It is a sensitive marker for hepatocellular injury and myocardial infarction. * **Malate-Aspartate Shuttle:** Aspartate and Oxaloacetate are crucial components of this shuttle, which transports reducing equivalents (NADH) from the cytosol into the mitochondria for the Electron Transport Chain. * **Asparaginase Therapy:** The enzyme Asparaginase converts Asparagine to Aspartate; it is used as a chemotherapeutic agent in **Acute Lymphoblastic Leukemia (ALL)** to deprive tumor cells of asparagine.

Question 170: What enzyme is deficient in maple syrup urine disease?

A. a-ketoacid decarboxylase (Correct Answer)
B. Transaminase
C. Isomerase
D. Mutase

Explanation: **Explanation:** Maple Syrup Urine Disease (MSUD) is an autosomal recessive metabolic disorder caused by a deficiency in the **Branched-Chain α-Keto Acid Dehydrogenase (BCKDH) complex**. This multi-enzyme complex is responsible for the oxidative decarboxylation of the α-ketoacid derivatives of the three branched-chain amino acids (BCAAs): **Leucine, Isoleucine, and Valine**. 1. **Why A is correct:** The BCKDH complex functions as an **α-ketoacid decarboxylase**. When this enzyme is deficient, α-ketoacids (such as α-ketoisovalerate, α-ketoisocaproate, and α-keto-β-methylvalerate) accumulate in the blood and spill into the urine, giving it a characteristic sweet, maple syrup-like odor. 2. **Why B is incorrect:** Transaminases (specifically Branched-Chain Amino Acid Aminotransferase) catalyze the *first* step of BCAA metabolism, converting amino acids to α-ketoacids. This step is functional in MSUD; the block occurs at the subsequent decarboxylation step. 3. **Why C & D are incorrect:** Isomerases and Mutases (like Methylmalonyl-CoA mutase) are involved in later stages of the metabolic pathways of Valine and Isoleucine (propionate pathway), but they are not the primary defect in MSUD. **Clinical Pearls for NEET-PG:** * **Mnemonic:** "I Love Vermont maple syrup" (**I**soleucine, **L**eucine, **V**aline). * **Odor:** The sweet smell is specifically due to the accumulation of **α-keto-β-methylvalerate** (derived from Isoleucine). * **Cofactors:** The BCKDH complex requires five cofactors: **T**hiamine (B1), **R**iboflavin (B2), **N**iacin (B3), **L**ipoic acid, and **P**antothenate (B5) [Mnemonic: **T**ender **R**oving **N**ights **L**ove **P**laces]. * **Treatment:** Dietary restriction of BCAAs and high-dose **Thiamine** supplementation (in thiamine-responsive variants).

Practice by Chapter

Protein Digestion and Absorption

Practice Questions

Transamination and Deamination

Practice Questions

Urea Cycle

Practice Questions

Disorders of Urea Cycle

Practice Questions

Metabolism of Individual Amino Acids

Practice Questions

Inborn Errors of Amino Acid Metabolism

Practice Questions

Phenylketonuria and Alkaptonuria

Practice Questions

Homocystinuria and Methionine Metabolism

Practice Questions

Synthesis of Biologically Important Compounds from Amino Acids

Practice Questions

Nitrogen Balance

Practice Questions

Ammonia Metabolism and Toxicity

Practice Questions

One-Carbon Transfer Reactions

Practice Questions

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