Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 151: Hydrolysis of arginine forms what?

A. Lysine
B. Arginine
C. Ornithine (Correct Answer)
D. Urocanic acid

Explanation: ### Explanation The correct answer is **Ornithine**. **1. Why Ornithine is correct:** The hydrolysis of Arginine is the final step of the **Urea Cycle** (Krebs-Henseleit cycle). This reaction is catalyzed by the enzyme **Arginase**. In this step, a molecule of water is added to Arginine, resulting in the cleavage of its guanidino group to form **Urea** and regenerating **Ornithine**. Ornithine then re-enters the mitochondria to react with Carbamoyl Phosphate, allowing the cycle to continue. **2. Why the other options are incorrect:** * **Lysine:** This is an essential basic amino acid. It is not a product of arginine metabolism; rather, it is structurally similar but metabolically distinct. * **Arginine:** This is the substrate, not the product of its own hydrolysis. * **Urocanic acid:** This is an intermediate in the catabolism of **Histidine** (catalyzed by histidase), not Arginine. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Arginase Deficiency:** A rare urea cycle disorder characterized by hyperargininemia. Unlike other urea cycle defects, it typically presents with **progressive spastic diplegia** rather than early-onset severe neonatal hyperammonemia. * **Site of Reaction:** The hydrolysis of arginine occurs primarily in the **cytosol of hepatocytes** (Liver), where Arginase is most abundant. * **Semi-essential nature:** Arginine is considered a semi-essential amino acid because while the urea cycle produces it, the amount is often insufficient during periods of rapid growth or illness. * **Nitric Oxide Precursor:** Apart from the urea cycle, Arginine is the direct precursor for **Nitric Oxide (NO)** via the enzyme Nitric Oxide Synthase (NOS).

Question 152: What is the typical plasma tyrosine level in Richner-Hanhart syndrome?

A. 1-2 mg/dL
B. 2-3 mg/dL
C. 4-5 mg/dL (Correct Answer)
D. 8-10 mg/dL

Explanation: **Explanation:** **Richner-Hanhart Syndrome**, also known as **Tyrosinemia Type II**, is an autosomal recessive disorder caused by a deficiency of the enzyme **Tyrosine Aminotransferase (TAT)**. This enzyme is responsible for the first step of tyrosine catabolism (converting tyrosine to p-hydroxyphenylpyruvate). 1. **Why Option C is Correct:** In Tyrosinemia Type II, the metabolic block leads to significant hypertyrosinemia. While normal plasma tyrosine levels are typically <1 mg/dL, patients with Richner-Hanhart syndrome characteristically exhibit levels in the range of **4–5 mg/dL (or higher, often >10 mg/dL in SI units depending on the lab reference)**. In the context of standard medical examinations like NEET-PG, 4-5 mg/dL represents the clinically significant elevation threshold that distinguishes it from transient neonatal tyrosinemia or normal physiological states. 2. **Why Other Options are Incorrect:** * **Options A & B (1-3 mg/dL):** These levels are either near-normal or only mildly elevated. Such levels do not correlate with the severe metabolic block seen in TAT deficiency. * **Option D (8-10 mg/dL):** While tyrosine levels can occasionally reach very high peaks, 4-5 mg/dL is the classic diagnostic benchmark cited in standard biochemistry textbooks for the sustained baseline elevation in this syndrome. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad:** 1. **Ocular:** Photophobia and painful dendritic corneal ulcers (pseudo-herpetic keratitis). 2. **Dermatological:** Painful palmoplantar hyperkeratosis. 3. **Neurological:** Variable mental retardation. * **Enzyme Defect:** Tyrosine Aminotransferase (Cytosolic). * **Diagnosis:** Elevated plasma tyrosine and presence of urinary tyrosine metabolites (tyrosyllactate, tyrosylacetate). * **Treatment:** Dietary restriction of **Phenylalanine and Tyrosine**.

Question 153: What is isoelectric pH?

A. The point at which all amino acids exist in their zwitterionic form.
B. The pH at which the electrical charge of ions is equal.
C. The pH at which the charge of ions does not alter.
D. The pH at which the positive and negative charges on an amino acid are equal. (Correct Answer)

Explanation: ### Explanation **Concept Overview:** The **Isoelectric Point (pI)** is the specific pH at which an amino acid exists as a **Zwitterion** (a dipolar ion). At this pH, the molecule carries both a positive charge (on the amino group) and a negative charge (on the carboxyl group) in equal magnitude. Consequently, the **net electrical charge is zero**. **Why Option D is Correct:** At the pI, the number of protonated (positive) groups equals the number of deprotonated (negative) groups. Because the net charge is zero, the amino acid becomes **electrically neutral** and will not migrate toward either the anode or the cathode when placed in an electric field (electrophoresis). **Analysis of Incorrect Options:** * **Option A:** Not all amino acids share the same pI. Each amino acid has a unique pI based on its specific pKa values. Therefore, there is no single pH where *all* amino acids are zwitterions simultaneously. * **Option B:** This is a vague description. It is not the "charge of ions" being equal, but the balance of opposite charges within a single molecule. * **Option C:** The charge of an amino acid is highly dynamic and *does* alter if the pH shifts away from the pI. **High-Yield NEET-PG Pearls:** 1. **Solubility:** At the isoelectric pH, proteins have **minimum solubility** and minimum buffering capacity. They tend to precipitate because there is no electrostatic repulsion between molecules. 2. **Calculation:** For simple amino acids, $pI = \frac{pK_1 + pK_2}{2}$. For acidic amino acids (Asp, Glu), use the two lowest pKa values; for basic amino acids (His, Arg, Lys), use the two highest. 3. **Electrophoresis:** If $pH > pI$, the amino acid is negatively charged (anion) and moves to the **Anode**. If $pH < pI$, it is positively charged (cation) and moves to the **Cathode**.

Question 154: Which of the following amino acids has two amino groups in its side chain?

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

Explanation: ### Explanation The correct answer is **Arginine**. **1. Why Arginine is Correct:** Arginine is a basic amino acid characterized by a unique **guanidino group** in its side chain. This group contains **two amino groups** (specifically, two nitrogen atoms that are part of the functional group, though the entire guanidino group contains three nitrogens). In the context of biochemistry exams like NEET-PG, Arginine is recognized for having the most nitrogen-rich side chain among the standard amino acids, making it the most basic. **2. Why the Other Options are Incorrect:** * **Glycine (A):** The simplest amino acid; its side chain is a single hydrogen atom. It has no amino groups in its side chain. * **Lysine (C):** While Lysine is a basic amino acid, it contains only **one** amino group (the $\epsilon$-amino group) in its side chain. * **Asparagine (D):** This is the amide derivative of aspartic acid. Its side chain contains an **amide group** ($-CONH_2$), not a free amino group. **3. High-Yield Clinical Pearls for NEET-PG:** * **Urea Cycle:** Arginine is a key intermediate in the urea cycle and is the immediate precursor of **Urea** and **Ornithine** via the enzyme Arginase. * **Nitric Oxide (NO):** Arginine is the sole precursor for the synthesis of Nitric Oxide, a potent vasodilator, catalyzed by NO synthase. * **Semi-essential:** Arginine is considered a semi-essential (conditionally essential) amino acid because while adults can synthesize it, the rate is insufficient during periods of rapid growth or severe stress (e.g., sepsis, burns). * **Charge:** At physiological pH (7.4), both Arginine and Lysine are positively charged.

Question 155: Which of the following hormones is synthesized from the amino acid tyrosine?

A. Calcitriol
B. Calcitonin
C. Thyroxin (Correct Answer)
D. Cortisol

Explanation: ### Explanation **Correct Answer: C. Thyroxin** **Medical Concept:** Tyrosine is a non-essential amino acid (synthesized from phenylalanine) that serves as a precursor for several biologically active molecules. In the thyroid gland, tyrosine residues within the protein **thyroglobulin** undergo iodination and coupling to form the thyroid hormones **Thyroxin (T4)** and **Triiodothyronine (T3)**. Beyond thyroid hormones, Tyrosine is also the precursor for: 1. **Catecholamines:** Dopamine, Norepinephrine, and Epinephrine. 2. **Melanin:** The pigment produced by melanocytes. --- **Analysis of Incorrect Options:** * **A. Calcitriol:** This is the active form of **Vitamin D** (1,25-dihydroxycholecalciferol). It is a steroid-like hormone derived from **cholesterol**, not amino acids. * **B. Calcitonin:** This is a **peptide hormone** secreted by the parafollicular (C-cells) of the thyroid gland. While it consists of amino acids, it is a gene product synthesized via translation, not a direct derivative of a single tyrosine molecule. * **D. Cortisol:** This is a glucocorticoid produced by the adrenal cortex. Like all adrenal steroids, it is synthesized from **cholesterol**. --- **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** The conversion of Tyrosine to L-DOPA by *tyrosine hydroxylase* is the rate-limiting step in catecholamine synthesis. * **Albinism:** Caused by a deficiency in the enzyme *tyrosinase*, preventing the conversion of tyrosine to melanin. * **PKU (Phenylketonuria):** Results from a deficiency of *phenylalanine hydroxylase*; in these patients, tyrosine becomes an **essential amino acid** because it can no longer be synthesized from phenylalanine.

Question 156: Deficiency of enzyme $\alpha$-keto acid decarboxylase leads to a block in the metabolism of branched-chain amino acids. What is the condition observed?

A. Maple syrup urine disease (Correct Answer)
B. Hanup's disease
C. Alkaptonuria
D. Phenylketonuria

Explanation: ### Explanation **Correct Option: A. Maple Syrup Urine Disease (MSUD)** Maple Syrup Urine Disease is caused by a deficiency in the **Branched-Chain $\alpha$-Keto Acid Dehydrogenase (BCKAD)** complex. This enzyme is responsible for the oxidative decarboxylation of $\alpha$-keto acids derived from the three branched-chain amino acids (BCAAs): **Leucine, Isoleucine, and Valine**. * **Pathophysiology:** A block in this step leads to the accumulation of these amino acids and their corresponding $\alpha$-keto acids in the blood and urine. * **Clinical Feature:** The characteristic "maple syrup" or "burnt sugar" odor of the urine is due to the accumulation of **$\alpha$-keto-isovaleric acid** (specifically the derivative of Isoleucine). **Analysis of Incorrect Options:** * **B. Hartnup’s Disease:** This is a transport defect involving **neutral amino acids** (specifically Tryptophan) in the renal tubules and intestine, leading to pellagra-like symptoms. It is not an enzyme deficiency of BCAA metabolism. * **C. Alkaptonuria:** Caused by a deficiency of **Homogentisate oxidase** in the Tyrosine catabolic pathway. It is characterized by urine that turns black upon standing and ochronosis. * **D. Phenylketonuria (PKU):** Caused by a deficiency of **Phenylalanine hydroxylase**, leading to the accumulation of Phenylalanine. It presents with intellectual disability and a "mousy" body odor. **High-Yield NEET-PG Pearls:** * **Mnemonic:** Remember the BCAAs as **LIV** (Leucine, Isoleucine, Valine). * **Cofactors:** The BCKAD complex requires five cofactors: **T**hiamine (B1), **R**iboflavin (B2), **N**iacin (B3), **L**ipoic acid, and **C**oenzyme A (**Tender Loving Care For No-one**). * **Treatment:** Thiamine supplementation is effective in the "Thiamine-responsive" variant of MSUD. * **Diagnosis:** Elevated levels of **Alloisoleucine** in the plasma is pathognomonic for MSUD.

Question 157: A 5-year-old child presents with mental retardation, seizures, and a lighter skin complexion. What is the most likely diagnosis?

A. Maple syrup urine disease
B. Albinism
C. Phenylketonuria (Correct Answer)
D. Alkaptonuria

Explanation: **Explanation:** The clinical triad of **mental retardation (intellectual disability)**, **seizures**, and **hypopigmentation (lighter skin/hair)** is classic for **Phenylketonuria (PKU)**. **Why Phenylketonuria is correct:** PKU is most commonly caused by a deficiency of the enzyme **Phenylalanine Hydroxylase (PAH)**, which converts Phenylalanine to Tyrosine. 1. **Mental Retardation/Seizures:** Accumulation of phenylalanine and its metabolites (phenylpyruvate, phenyllactate) in the brain is neurotoxic and interferes with neurotransmitter synthesis. 2. **Lighter Complexion:** Tyrosine is a precursor for **melanin**. In PKU, tyrosine levels are low, and high phenylalanine levels competitively inhibit the enzyme **Tyrosinase**, leading to decreased melanin production and "fair skin/blonde hair." **Why other options are incorrect:** * **Maple Syrup Urine Disease (MSUD):** Caused by a deficiency in Branched-Chain Alpha-Keto Acid Dehydrogenase. It presents with poor feeding, vomiting, and a characteristic "burnt sugar" odor in urine, but not typically hypopigmentation. * **Albinism:** Caused by a primary defect in Tyrosinase. While it features hypopigmentation, it does **not** cause mental retardation or seizures. * **Alkaptonuria:** Caused by Homogentisate oxidase deficiency. It presents with dark urine (on standing), ochronosis (dark pigmentation of connective tissue), and arthritis in later life; it does not cause intellectual disability. **High-Yield Clinical Pearls for NEET-PG:** * **Mousy/Musty Odor:** A hallmark sign of PKU due to phenylacetic acid in sweat and urine. * **Guthrie Test:** A bacterial inhibition assay used for neonatal screening of PKU. * **Dietary Management:** Restriction of Phenylalanine and supplementation of **Tyrosine** (which becomes an essential amino acid in PKU patients). * **Maternal PKU:** If a mother with PKU doesn't control her diet during pregnancy, the fetus may develop microcephaly and congenital heart defects.

Question 158: Carbamoyl phosphate synthetase I activity is regulated by the presence of:

A. N-Acetyl glutamate (Correct Answer)
B. N-Acetyl Aspartate
C. Neuraminic acid
D. Oxalate

Explanation: **Explanation:** **Carbamoyl Phosphate Synthetase I (CPS-I)** is the rate-limiting and first committed enzyme of the **Urea Cycle**, occurring in the mitochondria. Its primary function is to convert ammonia, CO₂, and ATP into carbamoyl phosphate. 1. **Why N-Acetylglutamate (NAG) is correct:** CPS-I is an allosteric enzyme that is **obligatorily activated** by N-Acetylglutamate. Without NAG, CPS-I is inactive. NAG is synthesized from glutamate and acetyl-CoA by the enzyme *N-acetylglutamate synthase (NAGS)*. High levels of arginine (indicating high protein intake) further stimulate NAGS, thereby increasing NAG levels and accelerating the urea cycle to dispose of excess nitrogen. 2. **Why the other options are incorrect:** * **N-Acetyl Aspartate (NAA):** This is a derivative of aspartate found in high concentrations in the brain. It serves as a marker for neuronal integrity but has no role in the urea cycle. * **Neuraminic acid:** This is a 9-carbon sugar (e.g., Sialic acid) involved in the structure of glycoproteins and glycolipids, not metabolic regulation of nitrogen. * **Oxalate:** This is a metabolic end-product often associated with calcium oxalate renal stones; it does not regulate CPS-I. **Clinical Pearls & High-Yield Facts:** * **CPS-I vs. CPS-II:** Do not confuse them. CPS-I is mitochondrial and involved in the **Urea Cycle** (activated by NAG). CPS-II is cytosolic and involved in **Pyrimidine Synthesis** (inhibited by UTP). * **Hyperammonemia Type I:** Caused by a deficiency of CPS-I. It is the most severe urea cycle disorder. * **NAGS Deficiency:** Mimics CPS-I deficiency clinically. It can be treated with **Carglumic acid**, a functional analog of NAG that can activate CPS-I.

Question 159: Isovaleric academia is caused by deficiency of which enzyme?

A. Branched chain ketoacid dehydrogenase
B. Isovaleryl CoA decarboxylase
C. Isovaleryl CoA dehydrogenase (Correct Answer)
D. Isovaleryl CoA synthase

Explanation: **Explanation:** **Isovaleric Acidemia (IVA)** is an autosomal recessive organic acidemia caused by a deficiency of **Isovaleryl-CoA dehydrogenase (IVD)**. This enzyme is a mitochondrial flavoprotein that catalyzes the conversion of isovaleryl-CoA to β-methylcrotonyl-CoA during the catabolism of the branched-chain amino acid **Leucine**. 1. **Why Option C is correct:** When IVD is deficient, isovaleryl-CoA and its metabolites (like isovaleric acid) accumulate in the blood and tissues. This leads to metabolic acidosis and the characteristic clinical presentation. 2. **Why other options are incorrect:** * **Option A:** Deficiency of **Branched-chain ketoacid dehydrogenase (BCKDH)** leads to **Maple Syrup Urine Disease (MSUD)**, affecting the breakdown of all three branched-chain amino acids (Leucine, Isoleucine, and Valine). * **Option B & D:** These enzymes are not part of the physiological metabolic pathway for leucine degradation. The reaction is a dehydrogenation (oxidation), not a decarboxylation or synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **Characteristic Odor:** Patients classically present with a pungent **"sweaty feet"** or **"cheese-like"** body odor due to the accumulation of isovaleric acid. * **Clinical Presentation:** Typically presents in the neonatal period with poor feeding, vomiting, metabolic acidosis, and "lethargy progressing to coma." * **Diagnosis:** Elevated **isovalerylglycine** in urine and **isovalerylcarnitine** in blood. * **Management:** Protein-restricted diet (specifically **low Leucine**) and supplementation with **L-carnitine** and **Glycine** to help conjugate and excrete toxic metabolites.

Question 160: Putrescine, a polyamine, is formed by the decarboxylation of which amino acid?

A. Arginine
B. Ornithine (Correct Answer)
C. Citrulline
D. Adenine

Explanation: **Explanation:** **Ornithine** is the correct answer because it serves as the direct precursor for the synthesis of **Putrescine**. This reaction is catalyzed by the enzyme **Ornithine Decarboxylase (ODC)**, which requires Pyridoxal Phosphate (Vitamin B6) as a cofactor. Putrescine is the simplest polyamine and serves as the starting point for the synthesis of higher polyamines like Spermidine and Spermine, which are essential for cell growth, DNA stabilization, and proliferation. **Analysis of Incorrect Options:** * **Arginine (Option A):** While Arginine is a precursor to Ornithine (via the enzyme Arginase in the Urea cycle), it does not directly decarboxylate to form Putrescine in humans. In some bacteria, Arginine can be decarboxylated to form Agmatine. * **Citrulline (Option C):** Citrulline is an intermediate in the Urea cycle formed from Ornithine and Carbamoyl Phosphate. It is not a substrate for polyamine synthesis. * **Adenine (Option D):** Adenine is a purine nitrogenous base involved in nucleic acid synthesis (DNA/RNA) and energy metabolism (ATP), unrelated to the polyamine pathway. **Clinical Pearls & High-Yield Facts for NEET-PG:** 1. **Rate-Limiting Step:** Ornithine Decarboxylase (ODC) is the rate-limiting enzyme in polyamine biosynthesis. 2. **Cancer Marker:** ODC activity is significantly elevated in rapidly proliferating cells and various cancers, making it a target for chemotherapy research (e.g., Eflornithine). 3. **S-Adenosylmethionine (SAM):** While Ornithine provides the carbon skeleton, SAM acts as the methyl donor for the conversion of Putrescine to Spermidine and Spermine. 4. **Biological Function:** Polyamines are highly polycationic; they bind to negatively charged DNA to stabilize its structure during cell division.

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

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