Question 1

A 5-day-old neonate presents with poor feeding, progressive encephalopathy, and a sweet caramel-like odour to the urine. Plasma amino acid analysis shows markedly elevated leucine, isoleucine, and valine. The newborn screening card is flagged. The relevant catabolic pathway is shown (Image 1), depicting transamination of branched-chain amino acids (leucine, isoleucine, valine) to their corresponding branched-chain α-keto acids, followed by the shared oxidative decarboxylation step (marked with a bold red X) catalysed by the branched-chain α-keto acid dehydrogenase (BCKDH) complex, which converts the α-keto acids to their respective acyl-CoA products. Which coenzyme is directly required by the enzymatic complex blocked at the step indicated in the image?

Accepted Answer

A. Thiamine pyrophosphate (TPP), lipoic acid, CoA, FAD, and NAD+. ***A. Thiamine pyrophosphate (TPP), lipoic acid, CoA, FAD, and NAD+***
- The **branched-chain α-keto acid dehydrogenase (BCKDH)** complex belongs to a family of α-keto acid dehydrogenases that require five essential cofactors: **Thiamine (B1)**, **Lipoate**, **CoA (B5)**, **FAD (B2)**, and **NAD+ (B3)**.
- A deficiency in this complex leading to **Maple Syrup Urine Disease (MSUD)** causes the accumulation of **leucine, isoleucine, and valine**, resulting in a characteristic caramel-like odor and neurotoxicity.

*B. Pyridoxal phosphate (PLP), FAD, and NAD+*
- **Pyridoxal phosphate (PLP)** is the essential coenzyme for the **transamination** step, which occurs before the metabolic block mentioned in the question.
- While FAD and NAD+ are used in the BCKDH complex, PLP is specifically involved in transferring amino groups, not in **oxidative decarboxylation**.

*C. Biotin, ATP, and Mn2+*
- **Biotin** is the required cofactor for **carboxylation** reactions, such as those catalyzed by pyruvate carboxylase or propionyl-CoA carboxylase.
- These components are not involved in the **decarboxylation** of α-keto acids derived from branched-chain amino acids.

*D. Tetrahydrofolate (THF), NADPH, and pyridoxal phosphate (PLP)*
- **Tetrahydrofolate (THF)** is primarily involved in **one-carbon metabolism** and the synthesis of nucleic acids and certain amino acids.
- **NADPH** is typically used in **reductive biosynthesis** and antioxidant pathways (like the pentose phosphate pathway), not in the catabolic breakdown of BCAAs via BCKDH. [Practice more Biochemistry questions on OnCourse]

Question 2

Which of the following amino acids is not coded by a standard triplet codon?

Accepted Answer

Hydroxyproline. ### Explanation

The correct answer is **B. Hydroxyproline**.

**1. Why Hydroxyproline is the correct answer:**
In protein synthesis, amino acids are typically incorporated into polypeptide chains via **translation**, guided by the genetic code. Hydroxyproline is a **non-standard amino acid** that is not coded by a specific triplet codon. Instead, it is formed through the **post-translational modification** of Proline residues already present in a protein chain (most notably collagen). This hydroxylation is catalyzed by the enzyme *prolyl hydroxylase*, which requires Vitamin C (ascorbic acid), ferrous iron, and oxygen as cofactors.

**2. Analysis of Incorrect Options:**
*   **A. Lysine:** This is one of the 20 standard amino acids coded by the codons AAA and AAG.
*   **C. Selenocysteine:** Known as the **21st amino acid**, it is incorporated during translation via the **UGA** codon (normally a stop codon) when a specific "SECIS" element is present in the mRNA.
*   **D. Pyrrolysine:** Known as the **22nd amino acid**, it is coded by the **UAG** codon (amber stop codon) in certain methanogenic archaea and bacteria.

**3. Clinical Pearls & High-Yield Facts for NEET-PG:**
*   **Scurvy Connection:** Deficiency of Vitamin C leads to defective hydroxylation of proline and lysine, resulting in unstable collagen triple helices. This manifests as bleeding gums, poor wound healing, and petechiae.
*   **21st vs. 22nd Amino Acid:** Selenocysteine is found in human enzymes like *Glutathione peroxidase* and *Thioredoxin reductase*. Pyrrolysine is NOT found in humans.
*   **Hydroxylysine:** Like hydroxyproline, it is also a post-translational modification and is not coded by a triplet codon. [Practice more Biochemistry questions on OnCourse]

Question 3

Cathelicidins are rich in which of the following amino acids?

Accepted Answer

Arginine. **Explanation:**

**Cathelicidins** are a vital family of antimicrobial peptides (AMPs) that serve as a primary component of the innate immune system. They are characterized by a highly conserved N-terminal "cathelin" domain and a structurally diverse C-terminal domain that exhibits potent antimicrobial activity.

**Why Arginine is the correct answer:**
The antimicrobial efficacy of cathelicidins depends on their **cationic (positively charged)** nature. **Arginine** is a basic amino acid that carries a positive charge at physiological pH. This positive charge allows the peptide to electrostatically attract and bind to the negatively charged phospholipids (such as lipopolysaccharides) on the surface of bacterial membranes. Once bound, the peptide disrupts the membrane integrity, leading to cell lysis and death. In humans, the only cathelicidin-derived peptide is **LL-37**, which is notably rich in Arginine and Leucine.

**Why other options are incorrect:**
*   **Cysteine/Cystine:** While some other antimicrobial peptides, like **Defensins**, are rich in Cysteine (forming disulfide bridges/Cystine), Cathelicidins are primarily defined by their cationic charge and amphipathic alpha-helical structures rather than disulfide stability.
*   **Methionine:** This is a sulfur-containing start codon amino acid but does not contribute to the cationic charge required for the specific function of cathelicidins.

**Clinical Pearls & High-Yield Facts for NEET-PG:**
*   **LL-37:** The only human cathelicidin, produced by neutrophils and epithelial cells.
*   **Vitamin D Link:** The expression of the cathelicidin gene is regulated by Vitamin D; hence, Vitamin D deficiency is linked to increased susceptibility to infections like Tuberculosis.
*   **Disease Association:** Abnormal levels of LL-37 are seen in **Rosacea** (excessive) and **Atopic Dermatitis** (deficiency, leading to frequent skin infections).
*   **Comparison:** Remember: **Defensins = Cysteine-rich**; **Cathelicidins = Arginine-rich.** [Practice more Biochemistry questions on OnCourse]

Question 4

The conversion of an optically pure isomer (enantiomer) into a mixture of equal amounts of both dextro and levo forms is called as?

Accepted Answer

Racemization. ### Explanation

**Correct Option: C. Racemization**
Racemization is the chemical process by which an optically active substance (a pure enantiomer) is converted into an optically inactive mixture containing equal amounts of both dextrorotatory (+) and levorotatory (–) forms. This mixture is known as a **racemic mixture**. Because the two enantiomers rotate plane-polarized light in opposite directions by equal amounts, the net optical rotation of a racemic mixture is zero.

**Why other options are incorrect:**
*   **A. Polymerization:** This is a process where small molecules (monomers) combine chemically to produce a very large chain-like or network molecule (polymer). It is unrelated to optical activity.
*   **B. Stereoisomerization:** This is a broad umbrella term for the interconversion of any stereoisomers (including diastereomers or geometric isomers). While racemization is a *type* of stereoisomerization, "Racemization" is the specific and most accurate term for creating a 50:50 mixture of enantiomers.
*   **D. Fractionation:** This is a separation process (like fractional distillation) used to divide a mixture into its individual components or "fractions" based on physical properties like boiling points.

**High-Yield Clinical Pearls for NEET-PG:**
*   **Thalidomide Tragedy:** A classic example of enantiomer differences. The (R)-enantiomer is a sedative, while the (S)-enantiomer is a potent teratogen causing phocomelia. In the body, these enantiomers can undergo **spontaneous in-vivo racemization**, which is why giving only the "safe" isomer did not prevent toxicity.
*   **Esmomeprazole vs. Omeprazole:** Omeprazole is a racemic mixture, whereas Esomeprazole is the pure (S)-enantiomer, which offers better bioavailability and more consistent acid suppression.
*   **Levocetirizine:** The (R)-enantiomer of Cetirizine; it has a higher affinity for H1 receptors and fewer side effects than the racemic mixture. [Practice more Biochemistry questions on OnCourse]

Question 5

What are amphipathic helices?

Accepted Answer

Alpha helices with predominantly hydrophobic R-groups projecting from one side and predominantly hydrophilic R-groups projecting from the other side of the helix axis.. **Explanation:**

**Why Option A is Correct:**
An **amphipathic (or amphiphilic) helix** is a specialized alpha-helix where amino acid residues are arranged such that **hydrophobic (non-polar)** R-groups concentrate on one face of the helix, while **hydrophilic (polar/charged)** R-groups concentrate on the opposite face. This spatial arrangement is achieved because the alpha-helix has 3.6 residues per turn; if polar and non-polar residues alternate every 3–4 positions, they align on separate sides of the cylindrical axis. This duality allows the helix to interface simultaneously with lipid environments (membranes) and aqueous environments (cytosol).

**Why Other Options are Incorrect:**
*   **Option B:** This describes **Beta-barrels** or complex tertiary folds, not the specific property of an amphipathic helix.
*   **Option C:** This defines **Loops or Bends (Turns)**, which are non-repetitive secondary structures that connect alpha-helices and beta-sheets.
*   **Option D:** **Supersecondary structures** (or motifs) like the "Helix-turn-helix" or "Zinc finger" refer to specific combinations of secondary structures, whereas amphipathic helices are a specific *type* of secondary structure.

**High-Yield Clinical Pearls for NEET-PG:**
*   **Apolipoproteins:** Amphipathic helices are hallmark features of proteins like **Apo A-I**, allowing them to bind to the hydrophobic lipid core of lipoproteins while remaining soluble in plasma.
*   **Membrane Proteins:** Many transmembrane channels use amphipathic helices to form a pore; the hydrophobic side faces the fatty acid tails of the bilayer, while the hydrophilic side forms the water-filled channel.
*   **Peptide Hormones:** Glucagon and certain antimicrobial peptides often adopt amphipathic helical structures to interact with target cell membranes. [Practice more Biochemistry questions on OnCourse]

Biochemistry Questions

Topics

Protein Structure and Function

Enzymes

Carbohydrate Metabolism

Lipid Metabolism

Amino Acid Metabolism

Nucleic Acid Biochemistry

Energy Production and Metabolism

Signal Transduction

Vitamins and Coenzymes

Metabolic Integration

Genetic Disorders and Biochemical Pathology

Biochemical Techniques

Acid-Base and Electrolyte Balance

Hemoglobin and Iron Metabolism

Detoxification Pathways

Membrane Biochemistry

Nutrition and Metabolism

Clinical Biochemistry

Molecular Biology and Genomics

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