Protein Structure and Function — MCQs

Protein Structure and Function Indian Medical PG Practice Questions and MCQs

Question 241: The peptide bond has all the following characteristics, EXCEPT?

A. Covalent in nature
B. Planar in nature
C. Partial double bond character
D. Free to rotate (Correct Answer)

Explanation: ### Explanation The peptide bond is the fundamental linkage in protein structure, formed between the $\alpha$-carboxyl group of one amino acid and the $\alpha$-amino group of another. **Why "Free to rotate" is the correct answer:** The peptide bond is **rigid and restricted**, meaning it **cannot rotate** freely. This lack of rotation is due to the **partial double bond character** (resonance) between the Carbon and Nitrogen atoms. While the peptide bond itself is fixed, rotation is permitted only around the single bonds involving the $\alpha$-carbon: the **Phi ($\phi$)** bond (N-C$\alpha$) and the **Psi ($\psi$)** bond (C$\alpha$-C). **Analysis of other options:** * **A. Covalent in nature:** Correct. The peptide bond is a strong covalent amide linkage formed via a dehydration (condensation) reaction. * **B. Planar in nature:** Correct. Due to the double bond character, the six atoms involved in the peptide group (C$\alpha$, C, O, N, H, and the next C$\alpha$) lie in a single plane. * **C. Partial double bond character:** Correct. Resonance allows electrons to be shared between the C-O and C-N bonds. This makes the C-N bond shorter than a typical single bond and prevents rotation. **High-Yield Clinical Pearls for NEET-PG:** * **Configuration:** Most peptide bonds in proteins are in the **trans** configuration to minimize steric hindrance between R-groups. * **Biuret Test:** This clinical chemistry test detects peptide bonds (requires at least two) to quantify protein levels. * **Ramachandran Plot:** Used to visualize the allowed values of $\phi$ and $\psi$ angles, which determine the secondary structure of proteins (e.g., $\alpha$-helix, $\beta$-sheets). * **Cleavage:** Peptide bonds are highly stable and require proteolytic enzymes (proteases) or strong acids/bases at high temperatures to be hydrolyzed.

Question 242: Disulphide bonds are formed between which amino acid residues?

A. Lysine and cysteine
B. Cysteine and cystine
C. Cysteine and cysteine (Correct Answer)
D. Arginine and histidine

Explanation: **Explanation:** **1. Why Cysteine and Cysteine is Correct:** Disulphide bonds (S-S bonds) are strong **covalent bonds** formed by the oxidation of the sulfhydryl (-SH) groups of two **Cysteine** residues. When two cysteine molecules are linked via a disulphide bridge, the resulting dimeric amino acid is called **Cystine**. These bonds are crucial for stabilizing the tertiary and quaternary structures of proteins, especially secreted proteins like insulin and immunoglobulins. **2. Why Other Options are Incorrect:** * **Option A (Lysine and Cysteine):** Lysine is a basic amino acid with an amino group, while Cysteine has a thiol group. They do not form covalent disulphide bridges; they may interact via ionic or hydrogen bonds. * **Option B (Cysteine and Cystine):** Cystine is the *product* of a disulphide bond formation, not a reactant. A bond forms between two monomers (Cysteine) to create the dimer (Cystine). * **Option D (Arginine and Histidine):** Both are basic amino acids. They are involved in ionic interactions (salt bridges) but lack the sulfur atoms necessary for disulphide bond formation. **3. NEET-PG High-Yield Clinical Pearls:** * **Location of Formation:** Disulphide bonds are primarily formed in the **Lumen of the Endoplasmic Reticulum (ER)**. * **Insulin Structure:** Insulin consists of two polypeptide chains (A and B) held together by **two interchain** and **one intrachain** disulphide bonds. * **Keratin:** The high "hardness" of hair and nails is due to extensive disulphide cross-linking in keratin. * **Clinical Correlation:** In **Cystinuria**, there is a defect in the renal transport of Cystine, leading to the formation of hexagonal cystine stones in the kidney (detected by the Cyanide-Nitroprusside test).

Question 243: The stability of a toxoid derived from snake venom is primarily attributed to which type of chemical bond?

A. Disulfide bonds (Correct Answer)
B. Hydrogen bonds
C. Ionic bonds
D. Van der Waals forces

Explanation: **Explanation:** The stability of snake venom toxins (and their derived toxoids) is primarily due to **disulfide bonds** (Option A). Snake venoms, such as α-bungarotoxin, are small, extracellular proteins that must maintain their structural integrity in harsh environments. Disulfide bonds are **covalent linkages** formed between the sulfhydryl (-SH) groups of two cysteine residues. These bonds provide a rigid "scaffold" that locks the protein into its functional tertiary structure, making it highly resistant to heat, pH changes, and proteolytic degradation. When venom is converted into a toxoid for immunization, these covalent cross-links ensure the molecule remains stable enough to retain its antigenic properties. **Why other options are incorrect:** * **B. Hydrogen bonds:** These are weak non-covalent interactions. While essential for secondary structures (α-helices and β-sheets), they are easily disrupted by heat and do not provide the "rugged" stability characteristic of toxins. * **C. Ionic bonds (Salt bridges):** These occur between oppositely charged side chains. They are sensitive to changes in pH and ionic strength, making them less reliable than covalent bonds for extracellular stability. * **D. Van der Waals forces:** These are the weakest intermolecular forces, occurring between non-polar side chains. They contribute to the hydrophobic core but cannot independently stabilize a protein against denaturation. **High-Yield Clinical Pearls for NEET-PG:** * **Cysteine vs. Cystine:** A single amino acid is Cysteine; two cysteines linked by a disulfide bond are referred to as **Cystine**. * **Extracellular Stability:** Disulfide bonds are rarely found in intracellular proteins (due to the reducing environment of the cytosol) but are hallmarks of **extracellular proteins** like Insulin, Immunoglobulins, and Snake Venoms. * **Reducing Agents:** Beta-mercaptoethanol and Dithiothreitol (DTT) are laboratory reagents used to break disulfide bonds, leading to protein denaturation.

Question 244: At isoelectric pH, what is the net charge of proteins?

A. Have net charge '0' (Correct Answer)
B. Are positively charged
C. Are negatively charged
D. Do not migrate

Explanation: ### Explanation **1. Why Option A is Correct:** The **isoelectric point (pI)** is the specific pH at which a protein or amino acid exists as a **zwitterion** (dipolar ion). At this pH, the number of positive charges (from protonated amino groups) exactly equals the number of negative charges (from dissociated carboxyl groups). Consequently, the **net electrical charge is zero**. This is a fundamental concept in protein chemistry as it dictates the protein's physical behavior in a solution. **2. Why Other Options are Incorrect:** * **Option B & C:** Proteins become **positively charged** when the pH is *below* their pI (acidic medium) because they pick up H⁺ ions. Conversely, they become **negatively charged** when the pH is *above* their pI (alkaline medium) as they lose H⁺ ions. * **Option D:** While it is true that proteins do not migrate in an electric field at their pI, the question asks specifically for the **net charge**. Option D describes a *consequence* of the zero charge, whereas Option A describes the *state* itself. In the context of NEET-PG, always prioritize the most direct definition. **3. Clinical Pearls & High-Yield Facts:** * **Solubility:** At isoelectric pH, protein solubility is **minimum**. Because there is no net charge, molecules do not repel each other and tend to aggregate and precipitate. * **Buffering Capacity:** Proteins exhibit minimum buffering capacity at their pI. * **Electrophoresis:** This principle is utilized in **Isoelectric Focusing**, where proteins are separated in a pH gradient until they reach the zone where their net charge is zero and they stop moving. * **Case Study:** The pI of Casein (milk protein) is 4.6. When milk sours, lactic acid lowers the pH toward 4.6, causing casein to lose its charge and precipitate (curdling).

Question 245: Which of the following is not a transport protein?

A. Transferrin
B. Collagen (Correct Answer)
C. Ceruloplasmin
D. Hemoglobin

Explanation: **Explanation:** The correct answer is **Collagen** because it is a **structural protein**, not a transport protein. In biochemistry, proteins are classified by their functional roles. Transport proteins bind and move specific molecules or ions across cell membranes or through the bloodstream, whereas structural proteins provide mechanical support and shape to tissues. * **Collagen (Correct Option):** It is the most abundant protein in the human body. It provides tensile strength to connective tissues such as bone, cartilage, tendons, and skin. It does not possess binding sites for the systemic transport of ligands. * **Transferrin (Incorrect):** This is a classic transport protein (beta-globulin) synthesized in the liver. Its primary function is to bind and transport **iron (Fe³⁺)** in the plasma to the bone marrow and other tissues. * **Ceruloplasmin (Incorrect):** This is an alpha-2 globulin that acts as the primary **copper-transporting** protein in the blood. It also functions as a ferroxidase, converting Fe²⁺ to Fe³⁺ so it can be loaded onto transferrin. * **Hemoglobin (Incorrect):** This is a specialized transport protein found in erythrocytes. It is responsible for the transport of **oxygen** from the lungs to the peripheral tissues and assists in the transport of carbon dioxide and protons. **High-Yield Clinical Pearls for NEET-PG:** * **Wilson’s Disease:** Characterized by a deficiency in **Ceruloplasmin**, leading to copper deposition in the liver and brain (Kayser-Fleischer rings). * **Collagen Structure:** It is a fibrous protein characterized by a **triple helix** (Gly-X-Y sequence), where X and Y are often Proline and Hydroxyproline. * **Albumin:** The most versatile transport protein in the plasma, carrying free fatty acids, bilirubin, calcium, and various drugs.

Question 246: Which of the following amino acids is the most basic?

A. Alanine
B. Arginine (Correct Answer)
C. Histidine
D. Lysine

Explanation: **Explanation:** The basicity of an amino acid is determined by the **pKa of its side chain (R-group)**. Arginine, Lysine, and Histidine are the three positively charged, basic amino acids. **Why Arginine is the correct answer:** Arginine is the **most basic** amino acid because its side chain contains a **guanidino group**. This group has a very high pKa (approximately **12.5**), meaning it remains protonated and positively charged at all physiological pH levels. The resonance stabilization of the protonated guanidino group makes Arginine a much stronger base than Lysine or Histidine. **Analysis of Incorrect Options:** * **Alanine:** This is a non-polar, aliphatic amino acid. Its side chain is a simple methyl group (-CH₃), which is chemically neutral and lacks any acidic or basic properties. * **Histidine:** While basic, it is the **least basic** of the three. Its imidazole side chain has a pKa of approximately **6.0**. At physiological pH (7.4), Histidine is largely uncharged, making it a versatile residue in enzyme active sites (e.g., Hemoglobin). * **Lysine:** Lysine has an ε-amino group with a pKa of approximately **10.5**. While it is strongly basic and positively charged at physiological pH, its pKa is significantly lower than that of Arginine. **High-Yield NEET-PG Pearls:** * **Order of Basicity:** Arginine > Lysine > Histidine. * **Isoelectric Point (pI):** Arginine has the highest pI (~10.76) among all proteinogenic amino acids. * **Histones:** These DNA-binding proteins are rich in Arginine and Lysine, giving them a positive charge to bind the negatively charged phosphate backbone of DNA. * **Urea Cycle:** Arginine is a key intermediate and the immediate precursor of Urea.

Question 247: All of the following are storage proteins, except?

A. Ovalbumin
B. Ricin (Correct Answer)
C. Ferritin
D. Glutelin

Explanation: **Explanation:** The classification of proteins is based on their biological functions. Storage proteins serve as a reservoir of essential nutrients (like amino acids or metal ions) for the development of embryos or for metabolic needs. **Why Ricin is the Correct Answer:** **Ricin** is not a storage protein; it is a potent **Phytotoxin** (toxic plant protein) derived from the castor bean (*Ricinus communis*). It functions as a **Type II Ribosome-Inactivating Protein (RIP)**. Mechanistically, it inhibits protein synthesis by removing an adenine residue from the 28S rRNA of the 60S ribosomal subunit, leading to cell death. This makes it a biological toxin rather than a nutritional reserve. **Analysis of Incorrect Options:** * **Ovalbumin:** The primary protein found in egg white. It serves as a source of amino acids for the developing bird embryo. * **Ferritin:** A critical intracellular protein that **stores Iron** in a non-toxic form and releases it in a controlled manner. It is the primary diagnostic marker for iron deficiency anemia. * **Glutelin:** A major storage protein found in the endosperm of cereal grains (e.g., Glutenin in wheat). It provides nitrogen and amino acids during seed germination. **High-Yield Clinical Pearls for NEET-PG:** * **Other Storage Proteins:** Casein (Milk), Myoglobin (stores Oxygen in muscles), and Zein (Maize). * **Ricin Mechanism:** It acts as an N-glycosidase. Because it irreversibly halts protein synthesis, it is studied in targeted cancer immunotherapy (immunotoxins). * **Ferritin Levels:** Decreased in Iron Deficiency Anemia; increased in Hemochromatosis and as an **Acute Phase Reactant** during inflammation/infection.

Question 248: How many polypeptide chains does insulin compose of?

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

Explanation: **Explanation:** Insulin is a peptide hormone synthesized by the beta cells of the pancreatic islets. The mature, functional insulin molecule consists of **two polypeptide chains**: the **A-chain** (21 amino acids) and the **B-chain** (30 amino acids), totaling 51 amino acids. These two chains are held together by two interchain disulfide bridges, with an additional intrachain disulfide bridge within the A-chain. **Why the correct answer is right:** While insulin is synthesized as a single-chain precursor called **Preproinsulin**, it undergoes post-translational modification. In the endoplasmic reticulum, the "pre" signal sequence is cleaved to form **Proinsulin**. In the Golgi apparatus and secretory granules, the **C-peptide** (connecting peptide) is proteolytically removed, leaving only the A and B chains linked by disulfide bonds. Therefore, the mature hormone circulating in the blood consists of exactly **2 chains**. **Why incorrect options are wrong:** * **A (4 chains):** Hemoglobin is a classic example of a protein with 4 polypeptide chains (tetramer). * **B (3 chains):** Proinsulin can be thought of as having 3 segments (A, B, and C), but it is a single polypeptide chain. Collagen is a protein that consists of a triple helix (3 chains). * **D (5 chains):** There are no major human hormones or common biochemical markers studied in NEET-PG that consist of 5 polypeptide chains. **High-Yield Clinical Pearls for NEET-PG:** * **C-peptide:** It is secreted in 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). * **Disulfide Bonds:** There are **3 disulfide bonds** in total (2 interchain, 1 intrachain). * **Zinc:** Insulin is stored in the pancreas as a **hexamer** (6 insulin molecules) coordinated around a zinc ion.

Question 249: Which of the following best describes the typical structure of proteins like collagen and keratin?

A. Fibrous (Correct Answer)
B. Globular
C. Branched
D. Compound

Explanation: **Explanation:** Proteins are broadly classified based on their overall shape and solubility into **fibrous** and **globular** types. **Why Fibrous is correct:** Fibrous proteins, such as **collagen, keratin, and elastin**, consist of polypeptide chains arranged in long strands or sheets. They are characterized by their high tensile strength, structural stability, and insolubility in water. * **Collagen** provides structural integrity to connective tissues (skin, bone, tendons). * **Keratin** provides mechanical protection (hair, nails, outer layer of skin). Their structure is dominated by repetitive secondary motifs (like the triple helix in collagen) rather than complex tertiary folding. **Why other options are incorrect:** * **B. Globular:** These proteins (e.g., Hemoglobin, Myoglobin, Enzymes) are folded into compact, spherical shapes. They are generally water-soluble and perform dynamic functions like catalysis and transport. * **C. Branched:** Proteins are linear polymers of amino acids linked by peptide bonds. While glycans (carbohydrates) can be branched, protein backbones themselves are **never branched**. * **D. Compound:** Also known as conjugated proteins, these contain a protein molecule bound to a non-protein prosthetic group (e.g., Glycoproteins, Lipoproteins). While collagen is a glycoprotein, "Fibrous" is the more fundamental structural classification requested. **NEET-PG High-Yield Pearls:** * **Collagen:** The most abundant protein in the human body. It features a repeating sequence of **Gly-X-Y** (where X is often Proline and Y is Hydroxyproline). * **Vitamin C:** Essential for the hydroxylation of proline and lysine residues in collagen; deficiency leads to **Scurvy** (defective cross-linking). * **Keratin:** Rich in **Cysteine** residues; the degree of disulfide bonding determines the hardness of the tissue.

Question 250: Which protein chain precipitates at 50°C-60°C but disappears on heating?

A. Heavy chain
B. Light chain (Correct Answer)
C. Both
D. None of the above

Explanation: **Explanation:** The correct answer is **Light chain**, specifically referring to **Bence-Jones proteins**. These are free monoclonal immunoglobulin light chains (either kappa or lambda) produced in excess by neoplastic plasma cells. **1. Why Light Chain is Correct:** Bence-Jones proteins exhibit a unique **thermosolubility property** that serves as a classic diagnostic marker. When a urine sample containing these light chains is heated: * They **precipitate** (forming a cloudy appearance) at temperatures between **40°C and 60°C**. * Upon further heating to **100°C (boiling)**, the precipitate **redissolves** (disappears). * If the urine is cooled back down, the precipitate reappears at 60°C and dissolves again below 40°C. This occurs because the light chains undergo reversible thermal denaturation and aggregation. **2. Why Other Options are Incorrect:** * **Heavy Chain:** In conditions like Heavy Chain Disease, the fragments do not exhibit this specific reversible heat-precipitation property. They are generally larger and structurally different from the free light chains that characterize Bence-Jones proteinuria. * **Both:** Only the free light chains demonstrate this specific temperature-dependent solubility behavior. **Clinical Pearls for NEET-PG:** * **Associated Condition:** Bence-Jones proteinuria is a hallmark of **Multiple Myeloma**. * **Detection:** These proteins are **not** detected by routine urine dipsticks (which primarily sense albumin). They are detected via the **Sulfosalicylic Acid (SSA) test** or confirmed by **Urine Protein Electrophoresis (UPEP)** showing an 'M-spike'. * **Renal Impact:** These light chains can precipitate in the renal tubules, leading to "Myeloma Kidney" (cast nephropathy).

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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