Protein Structure and Function — MCQs

Protein Structure and Function Indian Medical PG Practice Questions and MCQs

Question 301: Which portion of an antibody binds to an antigen?

A. Hinge region
B. Constant region
C. Variable region
D. Hypervariable region (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Hypervariable region** The **Hypervariable regions** (also known as **Complementarity Determining Regions or CDRs**) are the specific loops located within the variable domains of both heavy and light chains. While the variable region as a whole provides the structural framework, it is these three specific hypervariable loops (CDR1, CDR2, and CDR3) that form the actual **paratope** (antigen-binding site). The amino acid sequences in these loops are highly diverse, allowing antibodies to recognize a vast array of specific epitopes with high affinity. **Why other options are incorrect:** * **A. Hinge region:** This is a flexible amino acid stretch (rich in proline and cysteine) between the Fab and Fc portions. It allows the two Fab arms to move independently but does not participate in antigen binding. * **B. Constant region:** This region (C-terminal) determines the biological effector functions of the antibody (e.g., opsonization, complement activation, placental transfer) and defines the isotype (IgG, IgM, etc.). * **C. Variable region:** While the hypervariable regions are *contained within* the variable region, the variable region also includes "framework regions" that provide structural stability. The hypervariable region is the most precise and correct answer for the actual binding interface. **High-Yield Clinical Pearls for NEET-PG:** * **Papain vs. Pepsin:** Papain cleaves *above* the hinge (2 Fab + 1 Fc); Pepsin cleaves *below* the hinge (1 F(ab')₂ + degraded Fc). * **CDR3:** Among the three hypervariable regions, **CDR3** is the most variable and contributes most significantly to antigen specificity. * **Isotype Switching:** This involves changes in the **Constant region** of the heavy chain, while the Variable region (and thus antigen specificity) remains the same. * **Idiotype:** The unique antigenic determinant of the hypervariable region itself is called an idiotype.

Question 302: Nucleosomes are primarily composed of which type of protein?

A. Histone (Correct Answer)
B. Asparagine
C. Aspartate
D. Glutamate

Explanation: **Explanation:** **1. Why Histone is Correct:** Nucleosomes are the fundamental repeating units of eukaryotic chromatin. They consist of a segment of DNA (approximately 146 base pairs) wrapped around an octamer of **Histone proteins**. This octamer contains two copies each of histones **H2A, H2B, H3, and H4**. Histones are highly basic proteins, rich in the amino acids **Arginine and Lysine**. This positive charge is crucial because it allows them to bind tightly to the negatively charged phosphate backbone of DNA, facilitating efficient DNA packaging. **2. Why the Other Options are Incorrect:** * **Asparagine (B):** This is a neutral, polar amino acid. While it is a component of many proteins, it does not form the structural core of the nucleosome. * **Aspartate (C) and Glutamate (D):** These are **acidic amino acids** that carry a negative charge at physiological pH. If nucleosomes were composed of these, they would electrostatically repel the negatively charged DNA, making DNA packaging impossible. **3. High-Yield Clinical Pearls for NEET-PG:** * **Linker Histone:** **H1** is known as the "linker histone." It resides outside the nucleosome core and helps stabilize the 30nm chromatin fiber. * **Epigenetics:** Histone tails undergo post-translational modifications (Acetylation, Methylation, Phosphorylation) which regulate gene expression. * **Acetylation:** Usually associated with "Euchromatin" (transcriptionally active) because it neutralizes the positive charge on histones, loosening the DNA-histone bond. * **Drug Link:** **Sodium Valproate** (anti-epileptic) acts as a Histone Deacetylase (HDAC) inhibitor.

Question 303: In the synthesis of collagen, which enzyme requires copper?

A. Lysyl hydroxylase
B. Lysyl oxidase (Correct Answer)
C. Prolyl hydroxylase
D. Prolyl oxidase

Explanation: ### Explanation The correct answer is **Lysyl oxidase**. **1. Why Lysyl Oxidase is Correct:** Lysyl oxidase is an extracellular enzyme responsible for the final step in collagen synthesis: the **cross-linking** of collagen fibrils. It oxidatively deaminates specific lysine and hydroxylysine residues into reactive aldehydes (allysine and hydroxyallysine). These aldehydes then form covalent bonds with neighboring chains, providing collagen with its structural integrity and tensile strength. This enzyme requires **Copper ($Cu^{2+}$)** as a vital cofactor. **2. Why the Other Options are Incorrect:** * **Lysyl hydroxylase (Option A) & Prolyl hydroxylase (Option C):** These enzymes are involved in the post-translational modification of collagen within the Rough Endoplasmic Reticulum (RER). They require **Vitamin C (Ascorbic acid)**, **Ferrous iron ($Fe^{2+}$)**, and $\alpha$-ketoglutarate as cofactors. Deficiency of Vitamin C leads to Scurvy due to impaired hydroxylation. * **Prolyl oxidase (Option D):** This enzyme is involved in the metabolic degradation of proline, not the synthesis or structural maturation of collagen. **3. High-Yield Clinical Pearls for NEET-PG:** * **Menkes Disease:** A defect in the ATP7A protein leads to copper deficiency. This results in decreased activity of Lysyl oxidase, leading to "kinky" hair and connective tissue defects. * **Lathyrism:** Consumption of *Lathyrus sativus* (sweet pea) contains $\beta$-aminopropionitrile, which inhibits Lysyl oxidase, causing skeletal deformities. * **Location:** Remember that hydroxylation (Vitamin C dependent) occurs **intracellularly**, while cross-linking (Copper dependent) occurs **extracellularly**.

Question 304: Which type of collagen produces basement membranes?

A. Type I
B. Type II
C. Type IV (Correct Answer)
D. Type VI

Explanation: **Explanation:** The correct answer is **Type IV Collagen**. **1. Why Type IV is Correct:** Collagen Type IV is a **network-forming collagen** that serves as the primary structural component of the **basement membrane** (basal lamina). Unlike fibrillar collagens, Type IV molecules associate at their terminals to form a flexible, sheet-like meshwork. This meshwork provides a scaffold for other components like laminin and heparan sulfate proteoglycans, facilitating filtration and structural support. **2. Why Other Options are Incorrect:** * **Type I:** This is the most abundant collagen in the body. It is a **fibrillar collagen** found in high-tensile strength structures like bone, skin, tendons, and late-stage scar tissue. * **Type II:** This is primarily found in **cartilage** (hyaline and elastic) and the vitreous body of the eye. (Mnemonic: Type **Two** for Car-**two**-lage). * **Type VI:** This is a microfibrillar collagen that forms "anchoring plaques" and is found in the extracellular matrix of various tissues, but it is not the primary constituent of the basement membrane. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Alport Syndrome:** Caused by mutations in Type IV collagen (α3, α4, or α5 chains), leading to hereditary nephritis, sensorineural deafness, and ocular defects. * **Goodpasture Syndrome:** An autoimmune disease where antibodies are directed against the **NC1 domain of the α3 chain** of Type IV collagen, affecting the glomerular and alveolar basement membranes (Hematuria + Hemoptysis). * **Type III Collagen:** Found in reticular fibers (skin, blood vessels, fetal tissue) and is the first collagen deposited during wound healing (granulation tissue). * **Vitamin C:** Essential for the **hydroxylation of proline and lysine** residues during collagen synthesis; deficiency leads to Scurvy.

Question 305: Which amino acid is abundant in collagen?

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

Explanation: **Explanation:** **1. Why Glycine is Correct:** Collagen is the most abundant protein in the human body, characterized by a unique **triple-helical structure**. This helix consists of three polypeptide chains (alpha chains) wound tightly around each other. To maintain this tight packing, every third amino acid in the chain must be **Glycine**, resulting in a repeating sequence of **(Gly-X-Y)**, where X is usually Proline and Y is usually Hydroxyproline or Hydroxylysine. Glycine is the smallest amino acid (having only a hydrogen atom as its side chain), which allows it to fit into the restricted space at the center of the triple helix where the three chains come together. **2. Why Other Options are Incorrect:** * **Lysine (B):** While Lysine is present in collagen and undergoes post-translational hydroxylation (to hydroxylysine) for cross-linking, it is not the most abundant. * **Leucine (C) & Isoleucine (D):** These are branched-chain amino acids (BCAAs). They have bulky side chains that would sterically hinder the tight winding of the collagen triple helix; therefore, they are not found in high concentrations within the core of collagen. **3. NEET-PG Clinical Pearls:** * **Vitamin C Requirement:** Prolyl and lysyl hydroxylase enzymes require Vitamin C as a cofactor. Deficiency leads to **Scurvy** (defective collagen cross-linking). * **Osteogenesis Imperfecta:** Often caused by mutations where Glycine is substituted by a bulkier amino acid, preventing proper triple helix formation ("Brittle Bone Disease"). * **Ehlers-Danlos Syndrome:** A group of disorders caused by defects in the synthesis or processing of Type I, III, or V collagen. * **High-Yield Fact:** Glycine makes up approximately **1/3rd** of the total amino acid residues in collagen.

Question 306: Which of the following statements regarding covalent bonds is true?

A. Electrons involved in the bond have the same spin.
B. Electrons involved in the bond have opposite spins. (Correct Answer)
C. Covalent bonds are weak bonds.
D. None of the above.

Explanation: ### Explanation **1. Why Option B is Correct:** A covalent bond is formed by the **sharing of a pair of electrons** between two atoms. According to the **Pauli Exclusion Principle**, two electrons occupying the same orbital must have **opposite (antiparallel) spins** ($+1/2$ and $-1/2$). This pairing minimizes electronic repulsion and allows the electrons to occupy the same spatial region, creating a stable, high-energy bond that holds biological molecules like proteins and DNA together. **2. Why Other Options are Incorrect:** * **Option A:** If electrons had the same spin, they would repel each other due to the Pauli Exclusion Principle, preventing the formation of a stable molecular orbital. * **Option C:** Covalent bonds are **strong bonds** (bond energy typically 50–100 kcal/mol). In biochemistry, they form the "backbone" of macromolecules (e.g., peptide bonds in proteins, phosphodiester bonds in DNA). In contrast, weak bonds include hydrogen bonds, van der Waals forces, and hydrophobic interactions (typically <5 kcal/mol), which are responsible for the reversible folding and stabilization of protein structures. **3. NEET-PG High-Yield Clinical Pearls:** * **The Peptide Bond:** The most important covalent bond in protein biochemistry. It has **partial double-bond character**, is planar, and usually exists in the **trans configuration** to minimize steric hindrance. * **Disulfide Bridges:** These are the only covalent bonds that stabilize the **tertiary structure** of proteins (formed between two Cysteine residues). * **Bond Strength Hierarchy:** Covalent > Ionic > Hydrogen > Van der Waals. * **Clinical Correlation:** Many irreversible enzyme inhibitors (e.g., Aspirin inhibiting COX, Organophosphates inhibiting Acetylcholinesterase) work by forming stable **covalent bonds** with the enzyme's active site.

Question 307: Which of the following interactions contributes most to protein folding?

A. Covalent Bond
B. Ionic interactions
C. Hydrophobic (Correct Answer)
D. Vander waal's interaction

Explanation: **Explanation:** Protein folding is primarily driven by the **Hydrophobic Effect**. In an aqueous environment (the cytosol), non-polar side chains of amino acids are "pushed" away from water molecules and cluster together in the interior of the protein. This process increases the entropy of the surrounding water molecules, making it the most significant thermodynamic force driving the transition from an unfolded polypeptide to a compact, globular 3D structure. **Analysis of Options:** * **A. Covalent Bonds:** While peptide bonds form the primary structure and disulfide bridges stabilize the tertiary structure, they do not initiate the folding process. They are strong but occur only at specific sites. * **B. Ionic Interactions (Salt Bridges):** These occur between oppositely charged R-groups (e.g., Lysine and Aspartate). While they stabilize the structure, they are relatively few in number compared to hydrophobic interactions. * **D. Van der Waals Interactions:** These are weak, short-range forces between neutral atoms. They contribute to the tight packing of the protein core but are much weaker than the hydrophobic effect. **High-Yield NEET-PG Pearls:** * **Hydrophobic Collapse:** This is the term used for the rapid sequestration of non-polar residues into the protein core during folding. * **Chaperones (HSP70):** These are specialized proteins that prevent misfolding by binding to exposed hydrophobic regions of nascent polypeptides. * **Clinical Correlation:** Misfolding of proteins due to disrupted interactions leads to **Amyloidosis** (e.g., Alzheimer’s disease, Prion diseases), where proteins form insoluble β-pleated sheets. * **Hierarchy of Stability:** Hydrophobic effect (Primary driver) > Hydrogen bonding > Ionic bonds > Van der Waals.

Question 308: Glutathione is a tripeptide. How many amino acids does it contain?

A. 2
B. 3 (Correct Answer)
C. 4
D. 5

Explanation: **Explanation:** The correct answer is **3**. Glutathione (GSH) is a vital antioxidant found in high concentrations in almost all mammalian cells. The term "tripeptide" directly indicates that the molecule is composed of **three amino acids** linked by peptide bonds. **Why Option B is Correct:** Glutathione is specifically composed of three amino acids: **Glutamate (Glutamic acid), Cysteine, and Glycine.** Its chemical name is **$\gamma$-glutamyl-cysteinyl-glycine**. A unique structural feature of glutathione is that the linkage between Glutamate and Cysteine occurs via the **$\gamma$-carboxyl group** of Glutamate rather than the standard $\alpha$-carboxyl group. This makes it resistant to degradation by most intracellular peptidases. **Why Other Options are Incorrect:** * **Option A (2):** A molecule with two amino acids is a dipeptide (e.g., Aspartame). * **Option C (4):** A molecule with four amino acids is a tetrapeptide (e.g., Tuftsin). * **Option D (5):** A molecule with five amino acids is a pentapeptide (e.g., Enkephalins). **High-Yield Clinical Pearls for NEET-PG:** * **Function:** It acts as a major intracellular reducing agent. It protects cells from oxidative stress by scavenging free radicals and reducing hydrogen peroxide ($\text{H}_2\text{O}_2$) to water, catalyzed by **Glutathione Peroxidase** (which requires **Selenium** as a cofactor). * **Redox State:** It exists in two forms: Reduced (**GSH**) and Oxidized (**GSSG**). The enzyme **Glutathione Reductase** restores GSH using **NADPH** (primarily from the HMP Shunt). * **Clinical Relevance:** Glutathione is essential for maintaining hemoglobin in the reduced state and preserving RBC membrane integrity. A deficiency is linked to hemolysis, particularly in G6PD deficiency where NADPH levels are low.

Question 309: Which among the following are the essential amino acids?

A. Phenylalanine, Tryptophan, Lysine (Correct Answer)
B. Phenylalanine, Arginine, Methionine
C. Phenylalanine, Valine, Glycine
D. Histidine, Glutamine, Valine

Explanation: **Explanation:** The classification of amino acids into **Essential** and **Non-essential** is a high-yield topic for NEET-PG. Essential amino acids are those that the human body cannot synthesize *de novo* at a rate sufficient to meet metabolic demands; therefore, they must be obtained through the diet. **1. Why Option A is Correct:** Phenylalanine, Tryptophan, and Lysine are all strictly essential amino acids. The complete list of the 10 essential amino acids can be easily remembered using the mnemonic **"PVT TIM HALL"**: * **P**henylalanine, **V**aline, **T**hreonine * **T**ryptophan, **I**soleucine, **M**ethionine * **H**istidine, **A**rginine, **L**eucine, **L**ysine **2. Analysis of Incorrect Options:** * **Option B:** While Phenylalanine and Methionine are essential, **Arginine** is considered "semi-essential" (conditionally essential) because it is synthesized in the urea cycle but in insufficient quantities during periods of rapid growth or illness. * **Option C:** **Glycine** is a non-essential amino acid (the simplest amino acid, synthesized from serine). * **Option D:** **Glutamine** is a non-essential amino acid (synthesized from glutamate). **3. NEET-PG High-Yield Clinical Pearls:** * **Semi-essential Amino Acids:** Histidine and Arginine (required during growth, pregnancy, and lactation). * **Purely Ketogenic Amino Acids:** Leucine and Lysine (cannot be converted to glucose). * **Both Glucogenic and Ketogenic:** Phenylalanine, Tyrosine, Tryptophan, and Isoleucine (Mnemonic: **PITTT**). * **Limiting Amino Acids:** Pulses are usually deficient in Methionine, while Cereals are deficient in Lysine. This is why a combination diet (Dal-Chawal) provides a complete protein profile.

Question 310: Chemically, what characterises oestrogens?

A. Absence of O2 at D-17
B. Aromatic character at D-ring
C. Absence of methyl group at C-10 (Correct Answer)
D. A 21-carbon steroid skeleton

Explanation: **Explanation:** The chemical structure of estrogens (such as estradiol, estrone, and estriol) is unique among steroid hormones due to the specific modifications of the steroid nucleus. **1. Why Option C is Correct:** Estrogens are **C-18 steroids**, meaning they contain 18 carbon atoms. During the biosynthesis of estrogens from androgens (like testosterone, which is a C-19 steroid), the enzyme **Aromatase** catalyzes the aromatization of the A-ring. This process involves the **loss of the methyl group at the C-10 position** (the angular methyl group). The absence of this C-10 methyl group allows the A-ring to become phenolic (aromatic), which is the hallmark of estrogenic compounds. **2. Analysis of Incorrect Options:** * **Option A:** Estrogens typically possess a hydroxyl (-OH) or ketone (=O) group at **C-17** (e.g., 17β-estradiol). Oxygen is present at this position, not absent. * **Option B:** The **A-ring** is aromatic in estrogens, not the D-ring. The D-ring remains a saturated five-membered ring. * **Option C:** A 21-carbon skeleton (C-21) characterizes **Progestogens** (e.g., progesterone) and **Corticosteroids** (e.g., cortisol, aldosterone), not estrogens. **High-Yield Clinical Pearls for NEET-PG:** * **Precursor:** Cholesterol (C-27) → Pregnenolone (C-21) → Progesterone (C-21) → Testosterone (C-19) → Estradiol (C-18). * **Rate-limiting step in Steroidogenesis:** Conversion of Cholesterol to Pregnenolone by the enzyme **Desmolase** (CYP11A1) in the mitochondria. * **Aromatase Inhibitors:** Drugs like Anastrozole and Letrozole inhibit the conversion of C-19 androgens to C-18 estrogens, used clinically in ER-positive breast cancer. * **Potency:** 17β-estradiol is the most potent naturally occurring estrogen in premenopausal women.

Practice by Chapter

Amino Acids: Structure and Properties

Practice Questions

Peptide Bond Formation

Practice Questions

Primary Structure of Proteins

Practice Questions

Secondary Structure of Proteins

Practice Questions

Tertiary and Quaternary Structures

Practice Questions

Protein Folding and Chaperones

Practice Questions

Protein Domains and Motifs

Practice Questions

Structure-Function Relationships

Practice Questions

Hemoglobin and Myoglobin

Practice Questions

Collagen and Elastin

Practice Questions

Albumin and Plasma Proteins

Practice Questions

Post-Translational Modifications

Practice Questions

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