Protein Structure and Function — MCQs

Protein Structure and Function Indian Medical PG Practice Questions and MCQs

Question 121: Which of the following is a folding protein chaperone?

A. GLUT-1
B. Calnexin (Correct Answer)
C. Cytochrome 450
D. Insulin receptor

Explanation: **Explanation:** **Correct Answer: B. Calnexin** Molecular chaperones are specialized proteins that assist in the correct folding of nascent polypeptide chains and prevent protein aggregation. **Calnexin** (along with its soluble homolog, Calreticulin) is a classic example of a chaperone located in the **Endoplasmic Reticulum (ER)**. It specifically binds to misfolded glycoproteins containing monoglucosylated N-linked oligosaccharides, ensuring they achieve their correct tertiary structure before being transported to the Golgi apparatus. This process is a vital part of the "ER Quality Control" system. **Analysis of Incorrect Options:** * **A. GLUT-1:** This is a glucose transporter protein found on the plasma membrane (highly expressed in RBCs and the blood-brain barrier). It facilitates the passive diffusion of glucose and is not involved in protein folding. * **C. Cytochrome P450:** This is a large family of hemeproteins primarily located in the smooth ER of hepatocytes. They function as enzymes for the oxidation of organic substances (Phase I metabolism of drugs and endogenous steroids), not as chaperones. * **D. Insulin Receptor:** This is a transmembrane receptor belonging to the **Tyrosine Kinase** family. It mediates the biological effects of insulin via signal transduction. **Clinical Pearls for NEET-PG:** * **Heat Shock Proteins (HSPs):** Most chaperones belong to the HSP family (e.g., HSP70, HSP60/Chaperonins). * **Prion Diseases:** These occur due to the failure of chaperones, leading to the accumulation of misfolded proteins (β-sheets). * **Cystic Fibrosis:** The most common mutation (ΔF508) results in a protein that is recognized as misfolded by ER chaperones and degraded, even if it is partially functional.

Question 122: Which of the following statements regarding a crystal is true?

A. Molecules are arranged in the same orientation with different conformation.
B. Molecules are arranged in different orientations with different conformations.
C. Molecules are arranged in the same orientation and same conformation. (Correct Answer)
D. Molecules are arranged in different orientations but with the same conformation.

Explanation: In biochemistry and structural biology (specifically X-ray crystallography), a **crystal** is defined by its highly ordered, repeating internal structure. ### **Explanation of the Correct Answer** For a substance to form a crystal, its constituent molecules must be arranged in a **highly ordered, periodic 3D lattice**. This requires every molecule within the unit cell to be in the **same conformation** (identical 3D shape/folding) and the **same orientation** (aligned in the same direction relative to the axes). If molecules had different shapes or were pointing in random directions, they could not pack into a repeating lattice, resulting in an amorphous solid rather than a crystal. This uniformity is what allows crystals to diffract X-rays predictably to determine protein structures. ### **Analysis of Incorrect Options** * **Option A & B:** If molecules had **different conformations**, they would be chemically identical but structurally distinct (like different folding states of a protein). Such heterogeneity prevents the precise, repeating "brick-like" stacking necessary for crystallization. * **Option D:** If molecules had the **same conformation but different orientations**, the symmetry of the lattice would be broken. While some complex crystals have multiple molecules per asymmetric unit, the overall crystal lattice still demands a repeating, identical pattern of those orientations. ### **High-Yield Clinical Pearls for NEET-PG** * **X-ray Crystallography:** The gold standard for determining the 3D structure of proteins (tertiary/quaternary structure). * **Precipitation vs. Crystallization:** Rapid loss of solubility leads to amorphous precipitation; slow, controlled loss leads to crystallization. * **Hemoglobin S (Sickle Cell):** Deoxygenated HbS forms **polymers (tactoids)**, which are "pseudo-crystalline" arrays. This is a classic example of pathological molecular ordering. * **Bence-Jones Proteins:** Can form crystals in renal tubular cells in Multiple Myeloma.

Question 123: A protein to be secreted from the cell is most likely to have what characteristic?

A. A hydrophilic signal sequence at its carboxyl terminus
B. Mannose-6-phosphate
C. A hydrophobic signal sequence at its amino terminus (Correct Answer)
D. A binding site for the mitochondrial membrane

Explanation: ### Explanation **1. Why Option C is Correct:** Proteins destined for secretion, membrane integration, or lysosomal transport follow the **Secretory Pathway**. This process begins with a **Signal Sequence** (Signal Peptide) located at the **N-terminus (Amino terminus)** of the nascent polypeptide chain. This sequence typically consists of 15–30 amino acids, the core of which is highly **hydrophobic**. As the protein is being synthesized, the **Signal Recognition Particle (SRP)** binds to this hydrophobic sequence, halting translation and docking the ribosome to the **Rough Endoplasmic Reticulum (RER)**. The protein is then co-translationally translocated into the RER lumen for further processing and eventual secretion via the Golgi apparatus. **2. Why Other Options are Incorrect:** * **Option A:** Signal sequences are located at the **N-terminus**, not the C-terminus, and must be **hydrophobic** to interact with the SRP and the lipid bilayer of the RER. * **Option B:** **Mannose-6-Phosphate (M6P)** is a specific tag added in the Golgi to proteins destined for the **Lysosomes**, not for general secretion outside the cell. * **Option D:** Proteins with mitochondrial targeting sequences (usually amphipathic alpha-helices) are directed to the mitochondria, not the secretory pathway. **3. Clinical Pearls & High-Yield Facts:** * **I-Cell Disease:** Caused by a deficiency in the enzyme *phosphotransferase*, leading to a failure to add M6P tags. Lysosomal enzymes are constitutively secreted instead of being sent to lysosomes, resulting in inclusion bodies. * **Zellweger Syndrome:** A defect in importing proteins into **peroxisomes** (uses C-terminal SKL signal). * **Signal Peptidase:** The enzyme that cleaves the signal sequence once the protein enters the RER lumen.

Question 124: All of the following are required for hydroxylation of proline in collagen synthesis except?

A. 2-oxoglutarate
B. Vitamin C
C. Dioxygenases
D. Pyridoxal phosphate (Correct Answer)

Explanation: **Explanation:** The hydroxylation of proline and lysine residues is a critical post-translational modification in collagen synthesis. This process occurs in the lumen of the endoplasmic reticulum and is catalyzed by the enzymes **prolyl hydroxylase** and **lysyl hydroxylase**. **Why Pyridoxal Phosphate (PLP) is the correct answer:** Pyridoxal phosphate (Vitamin B6) is a coenzyme primarily involved in amino acid metabolism (transamination, decarboxylation, and deamination) and glycogen phosphorylase activity. It plays **no role** in the hydroxylation of collagen. **Why the other options are incorrect:** The hydroxylation reaction is a complex process requiring specific co-factors: * **2-oxoglutarate (α-ketoglutarate):** Acts as a co-substrate; it is decarboxylated into succinate during the reaction. * **Vitamin C (Ascorbic Acid):** Acts as a reducing agent to maintain the iron atom in the enzyme's active site in the **ferrous state (Fe²⁺)**. Without it, the iron is oxidized to the ferric state (Fe³⁺), inactivating the enzyme. * **Dioxygenases:** Prolyl and lysyl hydroxylases belong to the class of dioxygenases (specifically, α-ketoglutarate-dependent dioxygenases) because they use molecular oxygen (O₂) as a source for the hydroxyl group. **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 is **intracellular**. * **Sequence:** Proline hydroxylation requires the sequence -X-Pro-Gly-. * **Iron:** Ferrous iron (Fe²⁺) is an essential metallic cofactor for these enzymes.

Question 125: Thyroxine is transported by?

A. Albumin (Correct Answer)
B. Transthyretin
C. Transferrin
D. Ceruloplasmin

Explanation: **Explanation:** Thyroxine ($T_4$) and Triiodothyronine ($T_3$) are hydrophobic hormones that require carrier proteins for transport in the plasma. While **Thyroxine-Binding Globulin (TBG)** is the primary carrier with the highest affinity, **Albumin** is the correct answer here as it serves as a major non-specific transporter. 1. **Why Albumin is correct:** Albumin has the **highest capacity** for binding thyroxine, even though it has the lowest affinity among the three main carriers (TBG, Transthyretin, and Albumin). Due to its high plasma concentration, it carries a significant fraction of circulating thyroid hormones and ensures a readily available reservoir. 2. **Why other options are incorrect:** * **Transthyretin (Pre-albumin):** While it does transport thyroxine (hence the name *trans-thy-retin*), it is not the primary answer in this specific MCQ context unless TBG is absent. It has a higher affinity than albumin but lower than TBG. * **Transferrin:** This is the specific transport protein for **Iron** ($Fe^{3+}$). * **Ceruloplasmin:** This is an alpha-2 globulin that functions as the primary carrier for **Copper** and acts as a ferroxidase. **High-Yield Clinical Pearls for NEET-PG:** * **Binding Hierarchy:** Affinity for $T_4$ follows the order: **TBG > Transthyretin > Albumin**. * **Capacity Hierarchy:** Capacity for $T_4$ follows the order: **Albumin > Transthyretin > TBG**. * Only the **free fraction** (0.03% of $T_4$ and 0.3% of $T_3$) is metabolically active. * In **Liver disease** or **Nephrotic syndrome**, decreased albumin and TBG levels lead to a decrease in total $T_4$, but the free $T_4$ levels usually remain euthyroid.

Question 126: Which type of collagen is primarily found in bones?

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

Explanation: **Explanation:** Collagen is the most abundant protein in the human body, organized into various types based on its structure and tissue distribution. **Type I Collagen** is the correct answer because it is the primary structural component of **Bone** (organic matrix/osteoid), **Skin, Tendons, Fascia, and Cornea**. It is designed to provide high tensile strength to resist pulling forces. A helpful mnemonic for NEET-PG is: **"Type One is in Bone."** **Analysis of Incorrect Options:** * **Type II:** Found primarily in **Cartilage** (including hyaline and elastic), vitreous humor, and the nucleus pulposus. Mnemonic: *"Type Two is in Car-two-lage."* * **Type III (Reticulin):** Found in **Blood vessels**, fetal skin, and distensible organs like the spleen and liver. It forms the reticular fibers of the granulation tissue during early wound healing. * **Type IV:** Found in the **Basement membrane** (basal lamina) and lens of the eye. It forms a mesh-like structure rather than fibrils. Mnemonic: *"Type Four is under the Floor (basement membrane)."* **Clinical Pearls for NEET-PG:** * **Osteogenesis Imperfecta:** Caused by a defect in **Type I** collagen synthesis, leading to brittle bones and blue sclera. * **Ehlers-Danlos Syndrome (Vascular type):** Associated with a defect in **Type III** collagen. * **Alport Syndrome:** A genetic disorder affecting **Type IV** collagen, manifesting as nephritis and sensorineural deafness. * **Scurvy:** Vitamin C deficiency leads to defective hydroxylation of proline and lysine residues, impairing stable collagen triple-helix formation.

Question 127: Which of the following amino acids can form an O-glycosidic linkage in an oligosaccharide molecule?

A. Asparagine
B. Glutamine
C. Serine (Correct Answer)
D. Cysteine

Explanation: ### Explanation **1. Why Serine is Correct:** Glycosylation is a post-translational modification where carbohydrate chains (oligosaccharides) are attached to specific amino acid side chains. **O-glycosidic linkages** occur when the sugar molecule attaches to the **hydroxyl (-OH) group** of an amino acid. Serine and Threonine are the primary amino acids that facilitate this bond because their side chains contain a free hydroxyl group. This process typically occurs in the Golgi apparatus and is vital for the synthesis of glycoproteins like mucins and blood group antigens. **2. Why the Other Options are Incorrect:** * **Asparagine (A):** This amino acid is involved in **N-glycosidic linkages**, not O-linked. The carbohydrate attaches to the amide nitrogen of the side chain. This occurs in the Endoplasmic Reticulum (ER). * **Glutamine (B):** Although it has an amide group similar to Asparagine, it is generally not involved in standard protein glycosylation. * **Cysteine (D):** This amino acid contains a sulfhydryl (-SH) group. While it can form disulfide bridges to stabilize protein folding, it does not form O-glycosidic bonds. **3. High-Yield Clinical Pearls for NEET-PG:** * **O-linked Glycosylation:** Occurs in the **Golgi apparatus**. Examples: Mucins, ABO blood group substances, and Proteoglycans. * **N-linked Glycosylation:** Occurs in the **Rough ER**. Requires a **Dolichol phosphate** lipid carrier. * **Collagen Exception:** In collagen, O-glycosylation occurs on **Hydroxylysine** residues. * **I-Cell Disease:** A high-yield pathology caused by a defect in N-linked glycosylation (failure to phosphorylate mannose residues), leading to lysosomal enzymes being secreted extracellularly rather than being targeted to lysosomes.

Question 128: Which of the following proteins is synthesized by the liver?

A. Prothrombin
B. Fibrinogen
C. Ceruloplasmin
D. All of the above (Correct Answer)

Explanation: **Explanation:** The liver is the primary metabolic hub of the body and serves as the central site for the synthesis of the majority of plasma proteins, with the notable exception of gamma-globulins (produced by plasma cells). * **Prothrombin (Factor II):** This is a vitamin K-dependent clotting factor synthesized in the liver. It is the precursor to thrombin, which is essential for the conversion of fibrinogen to fibrin during the coagulation cascade. * **Fibrinogen (Factor I):** This is a soluble plasma glycoprotein synthesized by hepatocytes. It is an acute-phase reactant and the final substrate of the coagulation pathway, forming the structural framework of a blood clot. * **Ceruloplasmin:** This is an alpha-2 globulin synthesized in the liver. It functions as the primary copper-carrying protein in the blood and possesses ferroxidase activity, which is crucial for iron metabolism (converting $Fe^{2+}$ to $Fe^{3+}$ for binding to transferrin). Since all three proteins are products of hepatic synthesis, **Option D** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **Albumin:** The most abundant protein synthesized by the liver; it is used as a marker of the liver's synthetic function (long half-life of ~20 days). * **Wilson’s Disease:** Characterized by low serum ceruloplasmin levels due to defective copper incorporation and excretion. * **Vitamin K Dependency:** Factors II, VII, IX, and X, as well as Proteins C and S, require Vitamin K for post-translational gamma-carboxylation in the liver. * **Negative Acute Phase Reactants:** Albumin and Transferrin (levels decrease during inflammation), whereas Fibrinogen and Ceruloplasmin are **Positive Acute Phase Reactants** (levels increase).

Question 129: The higher levels of protein structure are supported by which of the following bonds?

A. Hydrogen bonds
B. Disulphide bonds
C. Electrostatic interactions
D. All the above (Correct Answer)

Explanation: **Explanation:** Protein structure is organized into four levels: primary, secondary, tertiary, and quaternary. While the **primary structure** is maintained solely by covalent **peptide bonds**, the "higher levels" (secondary, tertiary, and quaternary) rely on a variety of non-covalent and covalent interactions to maintain their three-dimensional conformation. * **Hydrogen Bonds:** These are the primary stabilizers of the **secondary structure** ($\alpha$-helices and $\beta$-pleated sheets), occurring between the carbonyl oxygen and amide hydrogen of the peptide backbone. * **Disulphide Bonds:** These are strong covalent linkages between the sulfhydryl (-SH) groups of two **Cysteine** residues. They are crucial for stabilizing the **tertiary and quaternary structures**, especially in secreted proteins (e.g., Insulin). * **Electrostatic Interactions (Salt Bridges):** These occur between oppositely charged side chains (e.g., positively charged Lysine and negatively charged Aspartic acid) and contribute significantly to the stability of the protein's folded state. **Why "All the above" is correct:** Higher-order folding (Tertiary/Quaternary) is a collective result of all these forces, along with **hydrophobic interactions** (the driving force for folding) and Van der Waals forces. Since A, B, and C all contribute to stabilizing these levels, D is the correct choice. **High-Yield Clinical Pearls for NEET-PG:** * **Denaturation:** This process disrupts higher-order structures (secondary, tertiary, quaternary) but **never breaks the primary structure** (peptide bonds). * **Chaperones:** These are specialized proteins (Heat Shock Proteins) that assist in the correct folding of proteins, preventing lethal misfolding. * **Cystine vs. Cysteine:** Two Cysteine molecules joined by a disulphide bond form **Cystine**. This is a common point of confusion in biochemical pathways.

Question 130: Which of the following represents the strongest type of interaction?

A. Covalent bond (Correct Answer)
B. Hydrogen bond
C. Electrostatic interaction
D. Van der Waals forces

Explanation: **Explanation:** In the hierarchy of chemical bonds, the strength of an interaction is determined by the energy required to break it. **1. Why Covalent Bond is Correct:** Covalent bonds are the strongest chemical bonds because they involve the **sharing of electron pairs** between atoms. In protein structure, these are represented by **peptide bonds** (linking amino acids) and **disulfide bridges** (linking cysteine residues). These bonds typically have a bond energy of **200–400 kJ/mol**, making them significantly more stable than non-covalent interactions. They define the primary structure of proteins and are not disrupted by denaturation (except by specific enzymes or extreme chemical conditions). **2. Why Other Options are Incorrect:** * **Electrostatic Interactions (Ionic bonds/Salt bridges):** These occur between oppositely charged groups (e.g., $COO^-$ and $NH_3^+$). While strong, their energy (~40–80 kJ/mol) is much lower than covalent bonds, especially in an aqueous environment where water shields the charges. * **Hydrogen Bonds:** These are weak attractions between a hydrogen atom and an electronegative atom (O or N). They are vital for secondary structures ($\alpha$-helices and $\beta$-sheets) but have low energy (~12–30 kJ/mol). * **Van der Waals Forces:** These are the weakest intermolecular forces (~0.4–4 kJ/mol) arising from transient dipoles. They only become significant when many atoms are packed closely together. **High-Yield Clinical Pearls for NEET-PG:** * **Disulfide bonds** (Covalent) are the only covalent bonds that stabilize the **tertiary structure** of proteins. * **Denaturation** of proteins disrupts secondary, tertiary, and quaternary structures (non-covalent bonds) but **leaves the primary structure (covalent peptide bonds) intact.** * **Order of Strength:** Covalent > Ionic > Hydrogen > Van der Waals.

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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