Protein Structure and Function — MCQs

Protein Structure and Function Indian Medical PG Practice Questions and MCQs

Question 131: Which of the following is a denaturing substance?

A. Guanosine
B. Guanidine (Correct Answer)
C. Glutamate
D. Glycine

Explanation: **Explanation:** **Guanidine** (specifically Guanidine hydrochloride) is a potent **chaotropic agent** used to denature proteins. It works by disrupting the non-covalent interactions that stabilize a protein's native structure, such as hydrogen bonds, hydrophobic interactions, and van der Waals forces. By weakening the hydrophobic effect, guanidine causes the protein to unfold into a random coil, leading to a loss of biological activity. **Analysis of Options:** * **Guanidine (Correct):** Along with **Urea**, it is the classic denaturant used in biochemical studies. It increases the solubility of non-polar side chains in water, effectively "unzipping" the protein. * **Guanosine (Incorrect):** This is a purine nucleoside comprising guanine attached to a ribose sugar ring. It is a building block for RNA, not a denaturant. * **Glutamate (Incorrect):** An acidic amino acid that acts as the primary excitatory neurotransmitter in the central nervous system. * **Glycine (Incorrect):** The simplest amino acid (achiral). It is often found in collagen and acts as an inhibitory neurotransmitter in the spinal cord. **High-Yield Clinical Pearls for NEET-PG:** * **Denaturation** involves the loss of secondary, tertiary, and quaternary structures, but the **primary structure (peptide bonds) remains intact.** * Other common denaturants include **Urea**, heat, extreme pH, organic solvents (alcohol), and detergents (SDS). * **Beta-mercaptoethanol** is specifically used to break **disulfide bonds** (covalent bonds), which is often a necessary step alongside guanidine for complete protein unfolding. * **Anfinsen’s Experiment:** Demonstrated that the information for protein folding is contained in the primary sequence; removing urea/guanidine can sometimes allow a protein to "renature."

Question 132: Defects in protein folding result in which of the following clinical diseases?

A. Kuru (Correct Answer)
B. Migraine
C. Hypothyroidism
D. Myopia

Explanation: **Explanation:** The correct answer is **Kuru**. This question tests the concept of **Protein Misfolding Diseases (Proteopathies)**. **1. Why Kuru is Correct:** Kuru is a human prion disease caused by the misfolding of the normal cellular prion protein (**PrPc**, which is alpha-helical) into a pathological isoform (**PrPsc**, which is rich in beta-pleated sheets). These misfolded proteins are resistant to proteolysis, aggregate into amyloid plaques, and induce further misfolding of healthy proteins, leading to neurodegeneration. This "template-directed" misfolding is the hallmark of Transmissible Spongiform Encephalopathies (TSEs). **2. Why Incorrect Options are Wrong:** * **Migraine:** A neurovascular disorder primarily involving trigeminal nerve activation and release of neuropeptides (like CGRP); it is not a protein folding defect. * **Hypothyroidism:** Usually results from iodine deficiency, autoimmune destruction (Hashimoto’s), or pituitary dysfunction, rather than systemic protein misfolding. * **Myopia:** A refractive error caused by the axial length of the eye or corneal curvature; it is a structural/optical issue, not a biochemical proteopathy. **3. NEET-PG High-Yield Clinical Pearls:** * **Other Protein Misfolding Diseases:** Alzheimer’s (Amyloid-β), Parkinson’s (α-synuclein), Huntington’s (Huntingtin), and Creutzfeldt-Jakob Disease (Prions). * **Chaperones:** These are specialized proteins (e.g., Heat Shock Proteins) that assist in correct protein folding and prevent aggregation. * **PrPsc Structure:** Characterized by a high content of **β-sheets**, making it insoluble and protease-resistant. * **Kuru History:** Historically associated with ritualistic cannibalism among the Fore people of Papua New Guinea.

Question 133: What is the biochemical classification of gonadotropins?

A. Proteins
B. Glycoproteins (Correct Answer)
C. Phospholipids
D. Polysaccharides

Explanation: **Explanation:** **Gonadotropins** (Follicle-Stimulating Hormone [FSH] and Luteinizing Hormone [LH]), along with Human Chorionic Gonadotropin (hCG) and Thyroid-Stimulating Hormone (TSH), belong to a specific family of **Glycoproteins**. 1. **Why Glycoproteins are correct:** These hormones are complex proteins covalently bonded to carbohydrate side chains (oligosaccharides). Structurally, they are heterodimers consisting of two subunits: * **Alpha (α) subunit:** Identical across all four hormones (FSH, LH, TSH, hCG). * **Beta (β) subunit:** Unique to each hormone, providing biological and receptor specificity. The carbohydrate component is essential for increasing the hormone's biological half-life and ensuring proper folding and receptor activation. 2. **Why other options are incorrect:** * **Proteins:** While they contain polypeptide chains, calling them simple proteins is incomplete as it ignores the essential carbohydrate moiety required for their function. * **Phospholipids:** These are structural components of cell membranes (e.g., lecithin) and are not involved in the structure of gonadotropins. * **Polysaccharides:** These are complex carbohydrates (e.g., glycogen, starch). While gonadotropins contain sugars, they are primarily protein-based signaling molecules. **High-Yield Clinical Pearls for NEET-PG:** * **Commonality:** TSH, FSH, LH, and hCG share the same α-subunit. This is why extremely high levels of hCG (as seen in molar pregnancies) can cross-react with TSH receptors, leading to hyperthyroidism. * **Diagnostic Significance:** Pregnancy tests and tumor marker assays specifically detect the **β-subunit** of hCG to avoid cross-reactivity with LH or FSH. * **Glycosylation:** The degree of sialic acid content in the carbohydrate chain determines the metabolic clearance rate; hCG has the highest sialic acid content and the longest half-life.

Question 134: Which is the amino acid shown below?

A. Histidine
B. Proline (Correct Answer)
C. Phenylalanine
D. Tyrosine

Explanation: ***Proline*** - Contains a unique **cyclic pyrrolidine ring** structure where the side chain bonds back to the backbone **nitrogen atom**, making it the only **imino acid** among the 20 standard amino acids. - The **five-membered ring** creates structural rigidity and introduces **kinks** in protein secondary structure, disrupting regular α-helix and β-sheet formations. *Histidine* - Features an **imidazole ring** in its side chain, which is a **six-membered aromatic ring** containing two nitrogen atoms. - Acts as a **pH buffer** due to its imidazole group having a pKa near physiological pH, unlike the non-ionizable proline ring. *Phenylalanine* - Contains a **benzyl side chain** with a **six-membered aromatic benzene ring** attached to a methylene group. - Classified as a **hydrophobic aromatic amino acid** with no nitrogen atoms in its ring structure, unlike proline's nitrogen-containing ring. *Tyrosine* - Has a **phenolic side chain** consisting of a **benzene ring** with a hydroxyl (-OH) group attached. - The **hydroxyl group** makes it polar and capable of **hydrogen bonding**, contrasting with proline's non-polar cyclic structure.

Question 135: Which of the following protein structures is not affected by denaturation?

A. Primary structure (Correct Answer)
B. Secondary structure
C. Tertiary structure
D. Quaternary structure

Explanation: **Explanation:** Denaturation is the process by which a protein loses its native three-dimensional conformation due to external stress (such as heat, extreme pH, or organic solvents). **Why Primary Structure is the Correct Answer:** The **primary structure** consists of the linear sequence of amino acids held together by strong **covalent peptide bonds**. Denaturation involves the disruption of weak, non-covalent interactions (hydrogen bonds, hydrophobic interactions, and ionic bonds). Since peptide bonds are covalent and highly stable, they are not broken during denaturation. Only proteolytic enzymes or strong acids/bases at high temperatures for prolonged periods can hydrolyze these bonds. **Why Other Options are Incorrect:** * **Secondary Structure:** Maintained by hydrogen bonds between the backbone atoms (alpha-helices and beta-pleated sheets). These bonds are easily disrupted by heat or pH changes. * **Tertiary Structure:** Maintained by disulfide bridges, hydrophobic interactions, and salt bridges. Denaturation unfolds these globular shapes into a random coil. * **Quaternary Structure:** Involves the spatial arrangement of multiple polypeptide subunits. Denaturation causes these subunits to dissociate. **High-Yield Clinical Pearls for NEET-PG:** * **Renaturation:** If the denaturing agent is removed, some proteins can spontaneously refold into their native state (e.g., Ribonuclease), proving that the primary structure contains all the information necessary for folding. * **Chaperones:** These are specialized proteins (Heat Shock Proteins) that assist in the correct folding of proteins and prevent misfolding during cellular stress. * **Prion Diseases:** These occur when a normal alpha-helical protein (PrPc) undergoes a conformational change into a beta-sheet rich form (PrPsc), which is resistant to denaturation and proteolysis.

Question 136: Which of the following is an optically inactive amino acid?

A. Threonine
B. Thyronine
C. Valine
D. Glycine (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Glycine** **1. Why Glycine is the Correct Answer:** Optical activity in amino acids depends on the presence of a **chiral center** (an asymmetric carbon atom bonded to four different groups). In a general amino acid, the $\alpha$-carbon is bonded to an amino group ($-NH_2$), a carboxyl group ($-COOH$), a hydrogen atom ($-H$), and a variable side chain ($-R$). **Glycine** is the simplest amino acid where the side chain ($-R$) is a **hydrogen atom**. Consequently, the $\alpha$-carbon is bonded to two identical hydrogen atoms, making it **achiral** (symmetric). Because it lacks a chiral center, glycine cannot rotate plane-polarized light and is therefore **optically inactive**. **2. Analysis of Incorrect Options:** * **A. Threonine:** Contains two chiral centers (the $\alpha$-carbon and the $\beta$-carbon), making it optically active. * **B. Thyronine:** An iodinated amino acid derivative (precursor to thyroid hormones) that retains the chiral $\alpha$-carbon of tyrosine, making it optically active. * **C. Valine:** Has a branched isopropyl side chain. The $\alpha$-carbon is attached to four different groups, making it optically active. **3. NEET-PG High-Yield Clinical Pearls:** * **Smallest Amino Acid:** Glycine’s small size allows it to fit into tight spaces, such as the interior of the **collagen triple helix** (Gly-X-Y repeats). * **Inhibitory Neurotransmitter:** Glycine acts as a major inhibitory neurotransmitter in the **spinal cord**. * **Heme Synthesis:** Glycine is a key precursor for heme synthesis (combines with Succinyl CoA via ALA synthase). * **Configuration:** All naturally occurring protein-incorporating amino acids (except glycine) are in the **L-configuration**. * **Isoleucine and Threonine:** These are the only two amino acids possessing **two chiral centers**.

Question 137: In determining protein structure, what is the role of mercaptoethanol?

A. Identify C-terminal residue
B. Denature protein
C. Reduce disulfide bonds (Correct Answer)
D. Oxidize disulfide bonds

Explanation: **Explanation:** The correct answer is **C. Reduce disulfide bonds.** **Underlying Concept:** Proteins often maintain their tertiary and quaternary structures through **disulfide bridges** (S-S bonds), which are covalent bonds formed between the sulfhydryl (-SH) groups of two cysteine residues. **$\beta$-mercaptoethanol** is a potent reducing agent. It works by donating hydrogen atoms to these disulfide bridges, converting them back into individual sulfhydryl groups (-SH SH-). This process breaks the cross-links, allowing the protein subunits to separate or the polypeptide chain to unfold. This is a critical step in techniques like SDS-PAGE to ensure proteins are separated based on molecular weight rather than shape. **Analysis of Incorrect Options:** * **A. Identify C-terminal residue:** This is typically done using **Carboxypeptidases** or hydrazinolysis. Mercaptoethanol does not cleave peptide bonds or identify terminal amino acids. * **B. Denature protein:** While breaking disulfide bonds contributes to denaturation, mercaptoethanol specifically targets the chemical bonds. General denaturation is usually achieved using heat or chaotropic agents like **Urea** or **Guanidinium hydrochloride**, which disrupt hydrogen bonds and hydrophobic interactions. * **D. Oxidize disulfide bonds:** This is chemically the opposite of what mercaptoethanol does. Oxidation is the process that *forms* disulfide bonds (e.g., via performic acid in some contexts, though that usually leads to irreversible cysteic acid formation). **High-Yield Clinical Pearls for NEET-PG:** * **Anfinsen’s Experiment:** Used $\beta$-mercaptoethanol and urea to prove that the primary structure of a protein determines its native conformation. * **Cystinuria:** A clinical defect in the transport of cystine (two cysteines linked by a disulfide bond), leading to renal stones. * **Insulin:** Consists of two chains (A and B) held together by two interchain disulfide bonds; treating insulin with mercaptoethanol results in the separation of these chains and loss of biological activity.

Question 138: Which biochemical test requires an intact peptide bond for a positive result?

A. UV diffraction
B. Ninhydrin test (Correct Answer)
C. Diazo reaction
D. All of the above

Explanation: ### Explanation **Correct Option: B (Ninhydrin Test)** The Ninhydrin test is a general chemical test used to detect amino acids and proteins. When Ninhydrin (triketohydrindene hydrate) reacts with a compound containing a **free alpha-amino group**, it undergoes oxidative deamination and decarboxylation to form a deep blue or purple-colored complex known as **Ruhemann's purple**. *Note on the Question:* While the Ninhydrin test primarily detects free amino groups, in the context of protein analysis, it is used to quantify amino acids after the hydrolysis of peptide bonds. However, among the given options, it is the standard biochemical reaction used to identify the presence of the amino group characteristic of the building blocks of peptides. (Note: Proline and hydroxyproline give a yellow color as they contain secondary amino/imino groups). **Why other options are incorrect:** * **A. UV Diffraction:** This is a physical analytical technique (X-ray crystallography/UV spectroscopy) used to determine the 3D structure or concentration of proteins. It is not a chemical "test" for the peptide bond itself. * **C. Diazo Reaction:** Also known as the Pauly’s test, this reaction is specific for the detection of amino acids containing **imidazole** (Histidine) or **phenolic** (Tyrosine) groups. It does not require an intact peptide bond but rather specific R-group side chains. **NEET-PG High-Yield Pearls:** 1. **Biuret Test:** This is the *actual* specific test for **intact peptide bonds**. It requires at least two peptide bonds (a tripeptide) to form a violet-colored coordination complex with copper ions ($Cu^{2+}$) in an alkaline medium. 2. **Xanthoproteic Test:** Detects aromatic amino acids (Phenylalanine, Tyrosine, Tryptophan) by nitration, resulting in a yellow color. 3. **Millon’s Test:** Specific for Tyrosine (phenolic group). 4. **Sakaguchi Test:** Specific for Arginine (guanidino group).

Question 139: Which of the following is NOT a property of the Signal Recognition Particle (SRP)?

A. Contains RNA and several polypeptides
B. Contains a single peptidase activity (Correct Answer)
C. Temporarily arrests translation
D. Binds to the signal sequence of secretory proteins

Explanation: ### Explanation The **Signal Recognition Particle (SRP)** is a cytosolic ribonucleoprotein complex essential for the co-translational targeting of proteins to the Rough Endoplasmic Reticulum (RER). **Why Option B is the correct answer:** The SRP does **not** possess peptidase activity. Its primary role is recognition and transport, not cleavage. The removal of the signal peptide is performed by a separate enzyme called **Signal Peptidase**, which is located on the luminal surface of the ER membrane, not within the SRP itself. **Analysis of Incorrect Options:** * **Option A:** The SRP is a complex consisting of one **7S RNA** molecule and **six different polypeptides** (P9, P14, P54, P68, P72, and P19). * **Option C:** Upon binding to the ribosome-nascent chain complex, the SRP induces a **translation arrest**. This "pause" prevents the protein from being released into the cytoplasm and ensures it doesn't fold prematurely before reaching the ER translocon. * **Option D:** The SRP specifically recognizes and binds to the **N-terminal hydrophobic signal sequence** of nascent secretory or membrane proteins as they emerge from the ribosome. --- ### High-Yield Clinical Pearls for NEET-PG: * **GTPase Activity:** The SRP and its receptor (SR) are both GTP-binding proteins. Hydrolysis of GTP triggers the release of the SRP from the ribosome, allowing translation to resume. * **I-Cell Disease (Mucolipidosis II):** A high-yield related pathology where a defect in the Golgi enzyme (phosphotransferase) prevents the tagging of lysosomal enzymes with **Mannose-6-Phosphate**, leading to their secretion outside the cell rather than targeting to lysosomes. * **Zellweger Syndrome:** Contrast SRP (ER targeting) with **PEX genes/Peroxins**, which are required for protein targeting to **Peroxisomes**.

Question 140: All of the following are required for hydroxylation of proline residues except?

A. Ascorbic acid
B. Glutamate (Correct Answer)
C. Ferrous ions
D. Molecular oxygen

Explanation: **Explanation:** The hydroxylation of proline and lysine residues is a critical post-translational modification in **collagen synthesis**, occurring within the rough endoplasmic reticulum. This process is catalyzed by the enzymes **prolyl hydroxylase** and **lysyl hydroxylase**. **Why Glutamate is the correct answer:** Glutamate is not a cofactor or substrate for this reaction. The reaction requires **$\alpha$-ketoglutarate** (an intermediate of the TCA cycle) as a co-substrate. During the reaction, $\alpha$-ketoglutarate undergoes oxidative decarboxylation to yield **succinate** and $CO_2$. While glutamate is metabolically related to $\alpha$-ketoglutarate, it does not participate directly in the hydroxylation process. **Why the other options are incorrect:** * **Molecular Oxygen ($O_2$):** One atom of $O_2$ is incorporated into the proline residue (forming the hydroxyl group), while the other is incorporated into succinate. * **Ferrous ions ($Fe^{2+}$):** These ions are essential metallic cofactors for the hydroxylase enzymes. * **Ascorbic acid (Vitamin C):** It acts as a reducing agent to maintain the iron in its active **ferrous ($Fe^{2+}$)** state, preventing its oxidation to the inactive ferric ($Fe^{3+}$) state. **High-Yield Clinical Pearls for NEET-PG:** * **Scurvy:** Deficiency of Vitamin C leads to impaired hydroxylation, resulting in unstable collagen triple helices. Clinical signs include bleeding gums, petechiae, and poor wound healing. * **Location:** Hydroxylation occurs **intracellularly** (in the RER). * **Sequence:** Proline is usually hydroxylated at the **Y position** of the Gly-X-Y repeating sequence of collagen.

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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