Protein Structure and Function — MCQs

Protein Structure and Function Indian Medical PG Practice Questions and MCQs

Question 91: Elongation arrest occurs due to?

A. SRP-R
B. SRP (Correct Answer)
C. Signal peptide
D. Docking protein

Explanation: **Explanation:** The correct answer is **SRP (Signal Recognition Particle)**. This process is a critical step in the targeting of secretory and membrane proteins to the Endoplasmic Reticulum (ER). 1. **Mechanism of Elongation Arrest:** When a ribosome begins translating an mRNA for a protein destined for the ER, the first sequence produced is the **Signal Peptide**. As this peptide emerges from the ribosome, it is recognized and bound by the **SRP**. Upon binding, the SRP induces a conformational change that halts further translation. This phenomenon is known as **Elongation Arrest**. Its purpose is to prevent the protein from folding prematurely in the cytosol and to ensure it is synthesized directly into the ER lumen (co-translational translocation). 2. **Analysis of Incorrect Options:** * **SRP-R (SRP Receptor):** Also known as the **Docking Protein**, it is located on the ER membrane. It binds the SRP-ribosome complex. Binding to the SRP-R triggers the release of the SRP and the *resumption* of translation, not the arrest. * **Signal Peptide:** This is the "address tag" (usually 15-30 amino acids at the N-terminus). While it triggers the process by binding to the SRP, it is the SRP itself that physically causes the arrest. * **Docking Protein:** Another name for the SRP-R (see above). **High-Yield NEET-PG Pearls:** * **SRP Composition:** It is a ribonucleoprotein complex consisting of **7S RNA** and six different proteins. * **GTPase Activity:** Both the SRP and the SRP-receptor have GTPase activity; GTP hydrolysis provides the energy for the ribosome to be transferred to the translocon. * **Zellweger Syndrome:** While primarily a peroxisomal disorder, understanding protein trafficking (like the SRP pathway) is essential for distinguishing between ER, mitochondrial, and peroxisomal targeting defects.

Question 92: Collagen ------ forms the main part of anchoring fibrils in epithelial tissues?

A. VII (Correct Answer)
B. IV
C. VIII
D. I

Explanation: **Explanation:** The correct answer is **Type VII Collagen**. **1. Why Type VII is Correct:** Type VII collagen is the primary component of **anchoring fibrils**. These fibrils are specialized structures that extend from the basal lamina (specifically the lamina densa) into the underlying papillary dermis. They "anchor" the epidermis to the dermis by looping around Type I and Type III collagen fibers, ensuring dermo-epidermal stability. **2. Analysis of Incorrect Options:** * **Type IV (Option B):** This is a **network-forming collagen** and the major structural component of the **basal lamina** itself. It forms a "mesh-like" scaffold rather than anchoring fibrils. * **Type VIII (Option C):** This is a short-chain collagen found primarily in the **Descemet’s membrane** of the corneal endothelium. * **Type I (Option D):** This is the most abundant collagen in the body, found in **bone, skin, and tendons**. It provides tensile strength but does not form anchoring fibrils. **3. High-Yield Clinical Pearls for NEET-PG:** * **Dystrophic Epidermolysis Bullosa (DEB):** This condition is caused by mutations in the *COL7A1* gene (encoding Type VII collagen). It leads to fragile skin and subepidermal blistering because the epidermis is not properly anchored to the dermis. * **Alport Syndrome:** Associated with mutations in **Type IV collagen**, leading to nephritis and sensorineural deafness. * **Goodpasture Syndrome:** An autoimmune condition where antibodies attack the alpha-3 chain of **Type IV collagen** in glomerular and alveolar basement membranes. * **Mnemonic for Collagen Types:** * Type **I**: **B**one (One/Bone) * Type **II**: **C**artilage (Two/Car-two-lage) * Type **III**: **R**eticular fibers (Three/Reticul-three-um) * Type **IV**: **F**loor (Basement membrane)

Question 93: How many amino acids does insulin contain?

A. 25
B. 30
C. 51 (Correct Answer)
D. 71

Explanation: **Explanation:** Insulin is a peptide hormone synthesized by the beta cells of the pancreatic islets. It is a classic example of a protein with a quaternary-like structure consisting of two polypeptide chains linked by disulfide bridges. 1. **Why 51 is correct:** Mature insulin consists of a total of **51 amino acids**. These are distributed across two chains: * **A-chain:** 21 amino acids. * **B-chain:** 30 amino acids. The chains are held together by two interchain disulfide bonds (A7-B7 and A20-B19) and one intrachain disulfide bond (A6-A11). 2. **Analysis of Incorrect Options:** * **25 & 30:** These numbers are often confused with the individual chain lengths. While the B-chain has 30 amino acids, 25 does not correspond to any specific chain in mature insulin. * **71:** This number is incorrect for mature insulin. However, students often confuse the total count with **Proinsulin**, which contains approximately **86 amino acids** (A-chain + B-chain + C-peptide). **High-Yield Clinical Pearls for NEET-PG:** * **Biosynthesis Pathway:** Preproinsulin (110 AA) → Proinsulin (86 AA) → Insulin (51 AA) + C-peptide (31 AA). * **C-peptide:** It is secreted in 1:1 equimolar amounts with insulin. It is a key clinical marker used to distinguish between Type 1 Diabetes (low C-peptide) and Type 2 Diabetes or Insulinoma (high C-peptide). * **Zinc Coordination:** In the pancreas, insulin is stored as a **hexamer** coordinated around a zinc ion. * **Post-translational modification:** The conversion of proinsulin to insulin occurs in the Golgi apparatus/secretory granules via proteolytic cleavage.

Question 94: Which of the following is NOT a protein misfolding disease?

A. Prion disease
B. Alzheimer's disease
C. Beta thalassemia
D. Ehlers-Danlos syndrome (Correct Answer)

Explanation: **Explanation:** The core concept tested here is the distinction between **conformational diseases (protein misfolding)** and **genetic defects in protein synthesis or post-translational modification.** **Why Ehlers-Danlos Syndrome (EDS) is the correct answer:** EDS is not caused by protein misfolding. Instead, it is a heterogeneous group of disorders caused by **genetic mutations** affecting the synthesis or processing of collagen. For example, Classical EDS often involves mutations in *COL5A1* or *COL5A2*, while Vascular EDS involves *COL3A1*. The pathology arises from quantitative or qualitative deficiencies in collagen fibers, or defects in enzymes like **lysyl hydroxylase**, rather than the accumulation of misfolded protein aggregates. **Analysis of Incorrect Options:** * **Prion Disease:** The hallmark of protein misfolding. Normal cellular prion protein ($PrP^C$) undergoes a conformational change into the scrapie isoform ($PrP^{Sc}$), which is rich in $\beta$-sheets and resistant to proteolysis. * **Alzheimer’s Disease:** Characterized by the misfolding and extracellular aggregation of **Amyloid-$\beta$** plaques and intracellular neurofibrillary tangles of **Tau protein**. * **Beta Thalassemia:** While primarily a quantitative globin synthesis defect, the excess unpaired $\alpha$-globin chains are unstable; they misfold and precipitate as **inclusion bodies** (Heinz-like bodies), leading to oxidative stress and hemolysis. **High-Yield Clinical Pearls for NEET-PG:** * **Chaperones:** These are specialized proteins (e.g., Heat Shock Proteins) that prevent misfolding by assisting in the correct folding of nascent polypeptides. * **Amyloidosis:** A systemic misfolding disease where proteins like Transthyretin or Light chains form insoluble $\beta$-pleated sheets. * **Cystic Fibrosis:** Often caused by the $\Delta$F508 mutation, which leads to the misfolding and premature degradation of the CFTR protein in the endoplasmic reticulum.

Question 95: What is the predominant secondary structure of prion particles?

A. Alpha helix
B. Beta bends
C. Beta sheets (Correct Answer)
D. Beta turns

Explanation: **Explanation:** The conversion of the normal prion protein (**PrPc**) into the infectious, disease-causing isoform (**PrPsc**) is a classic example of a conformational change in protein secondary structure. 1. **Why Beta Sheets are correct:** In its normal state (PrPc), the protein is primarily composed of **alpha-helices** (~42%) and contains very few beta-sheets (~3%). However, during the pathogenesis of Prion diseases (like Creutzfeldt-Jakob Disease), the protein undergoes a post-translational conformational shift where the alpha-helical content decreases and the **beta-sheet content increases significantly (~43%)**. These enriched beta-sheets make the protein insoluble, resistant to proteolysis (Protease K), and prone to forming neurotoxic amyloid aggregates. 2. **Analysis of Incorrect Options:** * **Alpha helix:** This is the predominant structure of the *normal* cellular prion protein (PrPc), not the infectious particle. * **Beta bends & Beta turns:** These are types of non-repetitive secondary structures that involve a change in direction of the polypeptide chain (often involving Proline or Glycine). While present in most proteins, they do not constitute the "predominant" structural feature of the infectious prion. **High-Yield Clinical Pearls for NEET-PG:** * **Prion Diseases:** Characterized by "spongiform" degeneration of the brain (e.g., Kuru, CJD, Mad Cow Disease). * **Resistance:** Prions are notoriously resistant to standard sterilization methods like boiling or UV; they require **autoclaving at 134°C** or concentrated Sodium Hydroxide (NaOH). * **Genetics:** Encoded by the *PRNP* gene on **Chromosome 20**. * **Pathology:** The accumulation of PrPsc leads to amyloid plaques, but unlike Alzheimer’s, there is **no inflammatory response**.

Question 96: Ubiquitin is the key protein involved in what cellular process?

A. Translation
B. Protein degradation (Correct Answer)
C. Protein activation
D. Glycosylation

Explanation: **Explanation:** **Ubiquitin** is a small, highly conserved regulatory protein found in almost all eukaryotic tissues. Its primary function is to mark unwanted or damaged proteins for destruction via the **Ubiquitin-Proteasome Pathway (UPP)**. 1. **Why Protein Degradation is Correct:** The process involves the covalent attachment of multiple ubiquitin molecules (polyubiquitination) to a target protein. This acts as a "molecular tag" or "kiss of death." These tagged proteins are then recognized and shuttled to the **26S Proteasome**, a barrel-shaped multi-protein complex that acts as the cell's "garbage disposal," breaking the protein down into small peptides. This process is ATP-dependent. 2. **Why Other Options are Incorrect:** * **Translation:** This is the synthesis of proteins from mRNA, primarily involving ribosomes and tRNA, not ubiquitin. * **Protein Activation:** While some post-translational modifications (like phosphorylation) activate proteins, polyubiquitination specifically targets them for destruction. * **Glycosylation:** This is the addition of carbohydrate chains to proteins (occurring in the ER and Golgi), which is essential for protein folding and cell signaling, not degradation. **High-Yield Clinical Pearls for NEET-PG:** * **E1, E2, E3 Enzymes:** Ubiquitination requires three enzymes: E1 (Activating), E2 (Conjugating), and **E3 (Ligase)**. E3 is the most specific, as it chooses the target substrate. * **Clinical Correlation:** Defects in the ubiquitin system are linked to neurodegenerative diseases like **Parkinson’s** (accumulation of Lewy bodies) and **Alzheimer’s**. * **Bortezomib:** A proteasome inhibitor used clinically to treat **Multiple Myeloma** by preventing the degradation of pro-apoptotic proteins.

Question 97: Which amino acid does not participate in alpha-helix formation?

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

Explanation: **Explanation** **Proline** is known as an **"alpha-helix breaker"** due to its unique cyclic structure. Unlike other amino acids, Proline is a secondary amino acid (imino acid) where the side chain is covalently bonded to the nitrogen atom of the peptide backbone. This creates two major structural hurdles: 1. **Steric Hindrance:** The rigid five-membered pyrrolidine ring causes a "kink" or bend in the polypeptide chain, disrupting the smooth spiral of the helix. 2. **Lack of Hydrogen Bonding:** Because the nitrogen is part of a ring, it lacks the hydrogen atom necessary to participate in the intrachain hydrogen bonding (between the $C=O$ of one residue and the $N-H$ of the fourth residue ahead) that stabilizes the alpha-helix. **Analysis of Incorrect Options:** * **Leucine (A):** This is a strong helix former. It has a high "helix-forming potential" due to its unbranched side chain at the beta-carbon. * **Glycine (B):** While Glycine is technically a "helix breaker" in some contexts because its tiny side chain (H) provides too much conformational flexibility (entropy), it *can* participate in helices. However, **Proline** is the classic, absolute answer for NEET-PG as it is structurally incapable of maintaining the helix. * **Lysine (D):** This is a charged amino acid that can participate in helices, though its stability depends on the pH and the proximity of other charged residues. **High-Yield Clinical Pearls for NEET-PG:** * **Proline's Role:** While it breaks alpha-helices, it is essential for the **collagen triple helix** and is frequently found in **beta-turns** (hairpin bends). * **Alpha-Helix Basics:** It is a right-handed spiral with **3.6 residues per turn** and a pitch of **0.54 nm**. * **Other Breakers:** Large branches at the beta-carbon (Valine, Threonine, Isoleucine) can also destabilize helices due to steric clashes.

Question 98: Carnosine is a major constituent of which of the following?

A. Kidney
B. Muscle (Correct Answer)
C. Brain
D. Adipose tissue

Explanation: **Explanation:** **Carnosine** (beta-alanyl-L-histidine) is a dipeptide composed of the amino acids **beta-alanine** and **histidine**. It is found in high concentrations in skeletal muscle and the brain, but it is considered a major constituent specifically of **skeletal muscle**. **Why Muscle is Correct:** In skeletal muscle, carnosine acts as an essential **intracellular buffer**. During high-intensity exercise, muscles produce lactic acid, leading to a drop in pH (acidosis). Carnosine helps maintain the acid-base balance by buffering hydrogen ions, thereby delaying muscle fatigue and improving performance. It also possesses antioxidant properties, protecting muscle cells from oxidative stress. **Why Other Options are Incorrect:** * **Kidney:** While the kidneys are involved in the metabolism of various amino acids, carnosine is not a major structural or functional constituent of renal tissue. * **Brain:** Although carnosine and its methylated derivative, **Anserine**, are present in the brain (acting as neuroprotective agents), the concentration is significantly lower compared to the bulk found in skeletal muscle. * **Adipose Tissue:** This tissue primarily stores triglycerides and does not require the specific buffering capacity provided by carnosine. **High-Yield Clinical Pearls for NEET-PG:** * **Composition:** Carnosine = $\beta$-alanine + L-histidine (Note: $\beta$-alanine is the rate-limiting precursor). * **Anserine:** A related dipeptide found in muscle (especially in birds) is $N$-methyl carnosine. * **Enzyme:** It is synthesized by *carnosine synthetase* and degraded by *carnosinase*. * **Homocarnosine:** A dipeptide found exclusively in the **brain**, composed of GABA and Histidine. * **Function:** Primary role is pH buffering and acting as a free radical scavenger.

Question 99: What is the basic structural unit of antibodies?

A. A single peptide chain
B. Two peptide chains
C. Non-sulfur amino acids
D. Two heavy and two light peptide chains (Correct Answer)

Explanation: **Explanation:** Antibodies, or **Immunoglobulins (Ig)**, are Y-shaped glycoproteins produced by plasma cells. The basic structural unit consists of a **heterotetramer** composed of four polypeptide chains: **two identical heavy (H) chains** and **two identical light (L) chains**. These chains are held together by covalent **disulfide bonds** and non-covalent interactions. * **Heavy Chains:** Determine the class (isotype) of the antibody (IgG, IgM, IgA, IgD, IgE). * **Light Chains:** Exist in two types, Kappa (κ) or Lambda (λ). * **Structure:** Each light chain is bound to a heavy chain, and the two heavy chains are bound to each other, forming a "Y" shape. The tips of the "Y" contain the **variable regions** (antigen-binding sites), while the stem consists of **constant regions**. **Analysis of Incorrect Options:** * **Option A & B:** A single or double chain structure is insufficient to form the functional "Y" unit required for bivalent antigen binding and effector functions. * **Option C:** This is incorrect because **disulfide bridges** (formed by the sulfur-containing amino acid **Cysteine**) are essential for stabilizing the quaternary structure of antibodies. **High-Yield Clinical Pearls for NEET-PG:** * **Bence-Jones Proteins:** These are free monoclonal light chains (κ or λ) found in the urine of patients with **Multiple Myeloma**. * **Papain Digestion:** Cleaves the antibody into **two Fab fragments** (antigen-binding) and **one Fc fragment** (crystallizable/effector function). * **Pepsin Digestion:** Cleaves below the hinge region to yield one **F(ab')₂ fragment** and degraded Fc fragments. * **Isotype Switching:** Occurs in the constant region of the heavy chain; the variable region (antigen specificity) remains the same.

Question 100: Which enzyme cleaves a glycopeptide bond?

A. Endopeptidase
B. Exopeptidase
C. Glycosidase (Correct Answer)
D. Trypsin

Explanation: **Explanation:** The core of this question lies in identifying the specific type of chemical bond being cleaved. A **glycopeptide bond** is a covalent linkage between a carbohydrate (glycan) moiety and an amino acid residue of a protein (typically via Nitrogen in Asparagine or Oxygen in Serine/Threonine). **Why Glycosidase is correct:** **Glycosidases** (also known as glycoside hydrolases) are enzymes specifically designed to catalyze the hydrolysis of glycosidic bonds. In the context of glycoproteins, these enzymes remove the sugar chains from the peptide backbone. Because the bond connecting the sugar to the protein is a glycosidic linkage, a glycosidase is required to cleave it. **Why the other options are incorrect:** * **Endopeptidase (A):** These enzymes cleave internal **peptide bonds** within a polypeptide chain (e.g., Pepsin). They do not act on the carbohydrate-protein linkage. * **Exopeptidase (B):** These enzymes cleave **peptide bonds** at the terminal ends (N-terminus or C-terminus) of a protein (e.g., Carboxypeptidase). Like endopeptidases, they target the protein backbone, not the glycan bond. * **Trypsin (D):** This is a specific type of serine protease (endopeptidase) that cleaves peptide bonds specifically at the carboxyl side of Lysine and Arginine. It has no activity against glycosidic bonds. **High-Yield Clinical Pearls for NEET-PG:** * **Lysosomal Storage Diseases:** Deficiencies in specific glycosidases lead to the accumulation of partially degraded glycoproteins or glycosaminoglycans (e.g., **α-mannosidosis** or **Hurler Syndrome**). * **N-linked vs. O-linked:** N-glycosylation occurs on **Asparagine** (in the ER), while O-glycosylation occurs on **Serine/Threonine** (in the Golgi). * **PNGase F:** A common laboratory glycosidase used to "deglycosylate" proteins for biochemical analysis.

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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