Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 111: All of the following amino acids undergo transamination except?

A. Arginine
B. Alanine
C. Proline (Correct Answer)
D. Glycine

Explanation: ### Explanation **Core Concept: Transamination** Transamination is the process where an $\alpha$-amino group is transferred from an amino acid to an $\alpha$-keto acid (usually $\alpha$-ketoglutarate), catalyzed by **aminotransferases** (transaminases) using **Pyridoxal Phosphate (PLP)** as a cofactor. For this reaction to occur, the amino acid must possess a free primary $\alpha$-amino group. **Why Proline is the Correct Answer:** **Proline** is technically an **imino acid**, not an amino acid. It contains a secondary amino group within a pyrrolidine ring structure. Because it lacks a free primary $\alpha$-amino group, it cannot participate in classical transamination reactions. Other amino acids that do not undergo transamination include **Lysine, Threonine, and Hydroxyproline.** **Analysis of Incorrect Options:** * **Alanine:** Readily undergoes transamination via **ALT (Alanine Aminotransferase)** to form pyruvate. This is a central reaction in the glucose-alanine cycle. * **Arginine:** Can undergo transamination, particularly in the kidney, though its primary metabolic routes involve the urea cycle. * **Glycine:** While it has a unique metabolic pathway (Glycine Cleavage System), it can undergo transamination to form glyoxylate via glycine aminotransferase. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Non-transaminating Amino Acids:** " **L**ost **T**hree **P**ros" (**L**ysine, **T**hreonine, **P**roline/Hydroxyproline). * **Cofactor:** All transaminases require **Vitamin B6 (Pyridoxal Phosphate)**. * **Diagnostic Marker:** ALT and AST are key biomarkers for liver injury; AST is also found in cardiac and skeletal muscle. * **Final Acceptor:** Most transamination reactions funnel amino groups toward **$\alpha$-ketoglutarate** to form **Glutamate**, which then undergoes oxidative deamination.

Question 112: What is the primary endogenous substrate for nitric oxide synthase (NOS)?

A. Citrulline
B. Arginine (Correct Answer)
C. Heme
D. Methionine

Explanation: **Explanation:** The synthesis of **Nitric Oxide (NO)**, a potent vasodilator and signaling molecule, is catalyzed by the enzyme **Nitric Oxide Synthase (NOS)**. **1. Why Arginine is Correct:** The primary endogenous substrate for all isoforms of NOS (eNOS, nNOS, and iNOS) is the semi-essential amino acid **L-Arginine**. In a five-electron oxidation reaction requiring oxygen and several cofactors, NOS converts L-Arginine into **L-Citrulline** and **Nitric Oxide**. This reaction occurs in the vascular endothelium, neurons, and macrophages. **2. Analysis of Incorrect Options:** * **Citrulline:** This is the **by-product** (end-product) of the reaction, not the substrate. However, it can be recycled back into arginine via the argininosuccinate pathway (the "Arginine-Citrulline cycle"). * **Heme:** While NOS is a heme-containing metalloenzyme, heme acts as a **prosthetic group** (cofactor) essential for electron transfer, not the primary substrate consumed in the reaction. * **Methionine:** This is an essential sulfur-containing amino acid involved in methylation reactions (SAMe) and the synthesis of cysteine, but it plays no direct role in NO production. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Cofactors for NOS:** Remember the mnemonic **"BH4, FAD, FMN, NADPH"**. A deficiency in **Tetrahydrobiopterin (BH4)** leads to "NOS uncoupling," producing superoxide instead of NO. * **Inhibitor:** **ADMA** (Asymmetric dimethylarginine) is an endogenous competitive inhibitor of NOS; elevated levels are linked to cardiovascular disease. * **Isoforms:** * **Type I (nNOS):** Neuronal (Calcium-dependent). * **Type II (iNOS):** Inducible/Macrophages (Calcium-independent). * **Type III (eNOS):** Endothelial (Calcium-dependent).

Question 113: Which of the following is the precursor of adrenaline and noradrenaline?

A. Phenylalanine
B. Tyrosine (Correct Answer)
C. Tryptophan
D. None of the above

Explanation: **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).

Question 114: Hyperlysinemia may be associated with which of the following findings?

A. Subluxation of the lens
B. Spherophakia
C. Strabismus
D. All of the above (Correct Answer)

Explanation: **Explanation:** **Hyperlysinemia** is an autosomal recessive metabolic disorder caused by a deficiency in the bifunctional enzyme **α-aminoadipic semialdehyde synthase**. This enzyme possesses both lysine-ketoglutarate reductase and saccharopine dehydrogenase activities. When deficient, it leads to an accumulation of lysine in the blood and urine. **Why "All of the above" is correct:** While many individuals with hyperlysinemia are asymptomatic (benign), clinical presentations often involve connective tissue and neurological abnormalities. The excess lysine interferes with the cross-linking of collagen and elastin. * **Subluxation of the lens (Ectopia lentis):** Similar to Homocystinuria, the disruption of collagenous zonular fibers leads to lens displacement. * **Spherophakia:** The weakening of the zonules can result in a small, spherical lens. * **Strabismus:** Weakness in the extraocular muscles or connective tissue support often manifests as "squint" or malalignment of the eyes. Because all three ocular findings are documented clinical manifestations of the systemic connective tissue weakness seen in symptomatic hyperlysinemia, **Option D** is the correct choice. **Clinical Pearls for NEET-PG:** * **Enzyme Defect:** α-aminoadipic semialdehyde synthase (Lysine $\rightarrow$ Saccharopine $\rightarrow$ α-aminoadipic semialdehyde). * **Key Association:** It is often a "differential diagnosis" for Homocystinuria due to the shared finding of ectopia lentis. * **Other Features:** Muscle hypotonia, mental retardation, and joint hypermobility may also be present. * **High-Yield Fact:** Lysine is a **purely ketogenic** amino acid; its metabolism bypasses pyruvate and enters the TCA cycle via Acetyl-CoA.

Question 115: Which is the most non-polar amino acid?

A. Leucine (Correct Answer)
B. Glycine
C. Arginine
D. Lysine

Explanation: ### Explanation **Correct Option: A (Leucine)** The polarity of an amino acid is determined by the chemical nature of its side chain (R-group). **Leucine** is a branched-chain amino acid (BCAA) with a purely hydrocarbon aliphatic side chain (isobutyl group). Because it lacks any electronegative atoms (like O, N, or S) or formal charges in its side chain, it is highly hydrophobic and non-polar. In a protein's tertiary structure, Leucine is typically buried within the hydrophobic core, away from the aqueous environment. **Analysis of Incorrect Options:** * **B. Glycine:** While classified as non-polar, Glycine has only a single Hydrogen atom as its side chain. It is considered the "least non-polar" of the hydrophobic group because its small size makes it relatively neutral and less effective at driving hydrophobic interactions compared to the bulky hydrocarbon chain of Leucine. * **C. Arginine:** This is a **positively charged (basic)** amino acid. Its guanidino group is highly polar and hydrophilic, making it one of the most polar amino acids. * **D. Lysine:** Similar to Arginine, Lysine is a **positively charged (basic)** amino acid with an ε-amino group. It is highly soluble in water and polar. **High-Yield NEET-PG Pearls:** * **Hydrophobicity Scale:** Among the common amino acids, **Isoleucine** and **Leucine** are consistently ranked as the most hydrophobic/non-polar. * **BCAA Clinical Correlation:** Defective metabolism of Leucine, Isoleucine, and Valine (due to deficiency of Branched-chain α-keto acid dehydrogenase) leads to **Maple Syrup Urine Disease (MSUD)**. * **Ketogenic Status:** Leucine is one of the two **purely ketogenic** amino acids (the other is Lysine). * **Membrane Proteins:** Non-polar amino acids like Leucine are predominantly found in the **transmembrane domains** of integral membrane proteins.

Question 116: Which enzyme is used to convert phenylalanine to tyrosine?

A. Tyrosine synthase
B. Tyrosine hydroxylase
C. Phenylalanine hydroxylase (Correct Answer)
D. Phenyl ethanolamine methyltransferase

Explanation: **Explanation:** The conversion of **Phenylalanine** (an essential amino acid) to **Tyrosine** (a non-essential amino acid) is a critical step in amino acid metabolism. This reaction is catalyzed by the enzyme **Phenylalanine hydroxylase (PAH)**. 1. **Why Option C is correct:** Phenylalanine hydroxylase adds a hydroxyl (-OH) group to the para-position of the phenylalanine ring to form tyrosine. This reaction requires **Molecular Oxygen** and the cofactor **Tetrahydrobiopterin ($BH_4$)**. $BH_4$ is regenerated by the enzyme Dihydrobiopterin reductase. 2. **Why other options are incorrect:** * **Tyrosine synthase:** This is not a human enzyme; tyrosine is synthesized via the hydroxylation of phenylalanine. * **Tyrosine hydroxylase:** This enzyme converts Tyrosine to **L-DOPA**, which is the rate-limiting step in catecholamine (Dopamine, Epinephrine, Norepinephrine) synthesis. * **Phenyl ethanolamine methyltransferase (PNMT):** This enzyme converts Norepinephrine to **Epinephrine** in the adrenal medulla. **Clinical Pearls for NEET-PG:** * **Phenylketonuria (PKU):** Deficiency of Phenylalanine hydroxylase (Classic PKU) or its cofactor $BH_4$ leads to the accumulation of phenylalanine, resulting in intellectual disability, "mousy" body odor, and hypopigmentation. * **Essentiality:** Because tyrosine is derived from phenylalanine, it becomes a **conditionally essential amino acid** in patients with PKU. * **Screening:** PKU is a part of newborn screening programs, typically detected using the **Guthrie Test** or Tandem Mass Spectrometry.

Question 117: Urea is the end product of which metabolic pathway?

A. Amino acid-nitrogen metabolism (Correct Answer)
B. HMP Pathway
C. Fatty acid oxidation
D. Glycogenolysis

Explanation: **Explanation:** **1. Why Option A is Correct:** Urea is the primary vehicle for the excretion of excess nitrogen from the body. When amino acids are metabolized, their amino groups (-NH₂) are removed via **transamination** and **oxidative deamination**, releasing ammonia (NH₃). Since ammonia is highly toxic, especially to the central nervous system, the liver converts it into **urea**—a water-soluble, non-toxic compound—through the **Urea Cycle (Ornithine Cycle)**. Thus, urea is the definitive end product of amino acid-nitrogen catabolism. **2. Why Other Options are Incorrect:** * **B. HMP Pathway:** The primary products are **NADPH** (for reductive biosynthesis) and **Ribose-5-phosphate** (for nucleotide synthesis). * **C. Fatty Acid Oxidation:** The end product of β-oxidation is **Acetyl-CoA**, which enters the TCA cycle or forms ketone bodies. * **D. Glycogenolysis:** The end product is **Glucose-1-phosphate** (converted to Glucose-6-phosphate), which provides free glucose in the liver or enters glycolysis in muscles. **3. High-Yield Clinical Pearls for NEET-PG:** * **Site of Urea Cycle:** It occurs exclusively in the **Liver**. The first two steps occur in the **mitochondria**, and the remaining steps occur in the **cytosol**. * **Rate-Limiting Enzyme:** Carbamoyl Phosphate Synthetase I (CPS-I), which requires **N-acetylglutamate (NAG)** as an essential activator. * **BUN (Blood Urea Nitrogen):** Elevated levels (Azotemia) can indicate renal failure or increased protein catabolism. * **Link to TCA Cycle:** Fumarate, produced in the urea cycle, provides a link to the TCA cycle (the "Krebs Bicycle").

Question 118: Xanthurenic acid, the accumulation of which occurs due to pyridoxine deficiency, is a metabolite of which amino acid?

A. Tyrosine
B. Phenylalanine
C. Tryptophan (Correct Answer)
D. Alanine

Explanation: **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.

Question 119: Which amino acid can protonate and deprotonate at neutral pH?

A. Histidine (Correct Answer)
B. Leucine
C. Homocysteine
D. Arginine

Explanation: **Explanation:** The ability of an amino acid to protonate and deprotonate at a specific pH depends on the **pKa of its side chain (R-group)**. An amino acid acts as an effective buffer when the environmental pH is close to its pKa. **Why Histidine is Correct:** Histidine contains an **imidazole ring** with a pKa of approximately **6.0**. Since this value is close to the physiological pH (~7.4), the imidazole group can exist in both protonated (charged) and deprotonated (neutral) forms in the body. This unique property allows Histidine to function as a versatile catalyst in enzyme active sites (e.g., hemoglobin and chymotrypsin) by shuttling protons back and forth. **Why Other Options are Incorrect:** * **Leucine:** It is a branched-chain aliphatic amino acid with a non-polar side chain that does not ionize; thus, it cannot protonate/deprotonate at neutral pH. * **Homocysteine:** An intermediate in methionine metabolism. While it has a thiol group, its pKa is significantly higher (~8.9), meaning it remains protonated at neutral pH. * **Arginine:** A basic amino acid with a guanidinium group. Its pKa is very high (~12.5), meaning it remains almost entirely protonated (positively charged) at physiological pH. **High-Yield Clinical Pearls for NEET-PG:** * **Hemoglobin Buffering:** Histidine residues are the primary reason hemoglobin is an excellent buffer in the blood. * **FIGLU Test:** Formiminoglutamic acid (FIGLU) excretion in urine is a clinical marker for **Folic acid deficiency**, as Histidine metabolism requires THF. * **Precursor:** Histidine undergoes decarboxylation (via Vitamin B6) to form **Histamine**, a key mediator of allergic responses and gastric acid secretion.

Question 120: Phenylketonuria I is due to deficiency of which enzyme?

A. Phenylalanine hydroxylase (Correct Answer)
B. Homogentisate oxidase
C. Tyrosinase
D. None of the above

Explanation: **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.

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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