Protein Structure and Function — MCQs

Protein Structure and Function Indian Medical PG Practice Questions and MCQs

Question 351: Which of the following best describes the characteristics of a glycoprotein?

A. Protein linked with a glycosidic bond
B. Core protein
C. Sugar residues are long in the carbohydrate portion of glycoproteins
D. Participate in cell surface recognition (Correct Answer)

Explanation: **Explanation:** **Glycoproteins** are proteins covalently bonded to carbohydrates. Their primary biological role involves **cell-to-cell interactions**, acting as receptors, ligands, and cell surface markers (e.g., MHC molecules and ABO blood group antigens). The carbohydrate chains project from the cell membrane, forming the glycocalyx, which is essential for **cell surface recognition**, signaling, and immune responses. **Analysis of Options:** * **Option D (Correct):** The oligosaccharide chains of glycoproteins are highly diverse, providing the structural specificity required for cells to recognize and adhere to one another. * **Option A (Incorrect):** While glycoproteins involve sugars, the linkage is specifically **N-glycosidic** (to Asparagine) or **O-glycosidic** (to Serine/Threonine). Simply stating "glycosidic bond" is a general chemical term; the functional hallmark is the branched carbohydrate side chain. * **Option B (Incorrect):** A "core protein" is a characteristic feature of **Proteoglycans**, not glycoproteins. In proteoglycans, numerous GAG chains radiate from a central core protein. * **Option C (Incorrect):** In glycoproteins, the carbohydrate portion consists of **short, branched oligosaccharides** (usually <15 sugar residues). Long, unbranched polysaccharide chains (Glycosaminoglycans) are characteristic of proteoglycans. **High-Yield Clinical Pearls for NEET-PG:** * **Glycoprotein vs. Proteoglycan:** Glycoproteins are mostly protein by weight; Proteoglycans are mostly carbohydrate (up to 95%). * **I-Cell Disease:** A deficiency in the enzyme required to tag glycoproteins with Mannose-6-Phosphate, leading to lysosomal storage issues. * **Erythropoietin:** A clinically significant glycoprotein used to treat anemia. * **Selectins:** Specific glycoproteins that mediate the "rolling" of WBCs during inflammation.

Question 352: Structural proteins are involved in maintaining the shape of a cell or in the formation of matrices in the body. What is the typical shape of these proteins?

A. Globular
B. Fibrous (Correct Answer)
C. Stretch of beads
D. Planar

Explanation: **Explanation:** Proteins are broadly classified into two categories based on their tertiary structure and solubility: **Fibrous** and **Globular**. **Why Fibrous is Correct:** Fibrous proteins are the primary structural components of the body. They are characterized by long, parallel polypeptide chains cross-linked at intervals to form stable fibers or sheets. These proteins are **insoluble in water** and possess high tensile strength, making them ideal for providing mechanical support and maintaining cellular/tissue architecture. Classic examples include **Collagen** (connective tissue), **Keratin** (hair/nails), and **Elastin**. **Analysis of Incorrect Options:** * **A. Globular:** These proteins are spherical or "globe-like" and are generally water-soluble. They serve functional roles rather than structural ones (e.g., enzymes, hemoglobin, and immunoglobulins). * **C. Stretch of beads:** This describes the "beads-on-a-string" appearance of **nucleosomes** (DNA wrapped around histones), which is a level of chromatin organization, not a classification of protein shape. * **D. Planar:** While peptide bonds have a partial double-bond character that makes them planar, proteins themselves do not exist as simple planar sheets in a 3D biological context. **High-Yield Clinical Pearls for NEET-PG:** * **Collagen** is the most abundant fibrous protein in the human body (Type I is most common). * **Scurvy** is a clinical condition caused by Vitamin C deficiency, leading to defective collagen synthesis because prolyl and lysyl hydroxylase enzymes require Vitamin C as a cofactor. * **Osteogenesis Imperfecta** (Brittle Bone Disease) most commonly results from mutations in the genes encoding Type I collagen. * **Elastin** lacks the regular hydroxylysine structure found in collagen but is rich in **Desmosine** cross-links.

Question 353: Which protein in muscle aids in relaxation?

A. Nebulin
B. Titin (Correct Answer)
C. Desmin
D. Calcineurin

Explanation: **Explanation:** The correct answer is **Titin** (also known as connectin). Titin is the largest known protein in the human body and acts as a molecular spring within the sarcomere. It extends from the Z-disk to the M-line, anchoring the thick (myosin) filaments. During muscle contraction, titin stores elastic potential energy; once the contraction stimulus ceases, this stored energy provides the **passive recoil force** necessary to return the sarcomere to its resting length, thereby aiding in muscle relaxation. **Analysis of Incorrect Options:** * **Nebulin (A):** An actin-binding protein that acts as a "molecular ruler." It regulates the length of thin filaments during assembly but does not provide elastic recoil. * **Desmin (B):** A type III intermediate filament that links Z-disks of adjacent myofibrils to each other and to the plasma membrane. It maintains structural integrity and alignment rather than facilitating relaxation. * **Calcineurin (D):** A calcium-dependent phosphatase involved in signaling pathways (e.g., T-cell activation and muscle fiber type switching). It is an enzyme, not a structural protein involved in the mechanical recoil of the sarcomere. **High-Yield NEET-PG Pearls:** * **Titin:** Mutations in the *TTN* gene are a leading cause of **Dilated Cardiomyopathy (DCM)**. * **Dystrophin:** Often confused with these proteins, it links the cytoskeleton of a muscle fiber to the surrounding extracellular matrix (defective in Duchenne Muscular Dystrophy). * **Tropomyosin:** In the resting state, it covers the myosin-binding sites on actin, preventing contraction. Relaxation requires the re-uptake of $Ca^{2+}$ into the sarcoplasmic reticulum by **SERCA** pumps.

Question 354: All of the following are globular proteins except:

A. Prolamines
B. Albumin
C. Globulin
D. Myosin (Correct Answer)

Explanation: **Explanation:** Proteins are broadly classified into two categories based on their molecular shape and solubility: **Globular** and **Fibrous** proteins. **1. Why Myosin is the Correct Answer:** **Myosin** is a **fibrous protein**. Unlike globular proteins, which are spherical and water-soluble, fibrous proteins are elongated, thread-like structures that are generally insoluble in water. Myosin, along with actin, forms the structural framework of muscle fibers. While the "head" of myosin has globular properties (enzymatic activity), the molecule as a whole is classified as a fibrous protein due to its long, alpha-helical tail and its primary role in providing structural integrity and contractile force. **2. Analysis of Incorrect Options:** * **Albumin:** The quintessential globular protein. It is highly soluble in water and functions primarily as a transport protein and a regulator of plasma oncotic pressure. * **Globulin:** As the name suggests, these are globular proteins. They are insoluble in pure water but soluble in dilute salt solutions (e.g., Immunoglobulins). * **Prolamines:** These are plant-derived globular proteins (e.g., Gliadin in wheat). They are unique because they are insoluble in water but soluble in 70-80% ethyl alcohol. **3. High-Yield Clinical Pearls for NEET-PG:** * **Fibrous Proteins (The "Big Four"):** Collagen (most abundant), Elastin, Keratin, and Myosin. * **Solubility Rule:** Globular proteins are usually soluble in aqueous media; Fibrous proteins are insoluble. * **Denaturation:** Globular proteins are easily denatured by heat or pH changes, whereas fibrous proteins are more stable and resistant to moderate environmental changes. * **Collagen Fact:** Remember that Collagen is a triple-helical fibrous protein and is the most abundant protein in the human body.

Question 355: All of the following are features of cardiac muscle EXCEPT:

A. Striated
B. Large T tubules
C. Troponin system present
D. Caldesmon is an important regulatory protein (Correct Answer)

Explanation: **Explanation:** The correct answer is **D**. **Caldesmon** is a regulatory protein found specifically in **smooth muscle**, not cardiac muscle. It functions by binding to actin and tropomyosin, thereby inhibiting the ATPase activity of myosin and preventing contraction. In smooth muscle, contraction is initiated when calcium-calmodulin binds to caldesmon, releasing it from actin. **Analysis of Options:** * **A. Striated:** Cardiac muscle, like skeletal muscle, contains highly organized sarcomeres (the functional units of contraction), which give it a characteristic striated appearance under a microscope. * **B. Large T tubules:** Cardiac muscle features T-tubules that are significantly wider (up to 5 times the diameter) than those in skeletal muscle. They are located at the **Z-discs** (unlike skeletal muscle, where they are at the A-I junction). * **C. Troponin system present:** Cardiac contraction is regulated by the troponin complex (TnC, TnI, and TnT). Calcium binds to **Troponin C**, causing a conformational change that moves tropomyosin away from the myosin-binding sites on actin. **High-Yield Clinical Pearls for NEET-PG:** * **Cardiac Biomarkers:** Cardiac-specific isoforms of **Troponin I and T** are the gold-standard markers for diagnosing Myocardial Infarction (MI). * **Triads vs. Diads:** Skeletal muscle has "Triads" (1 T-tubule + 2 terminal cisternae), while cardiac muscle typically has **"Diads"** (1 T-tubule + 1 terminal cisterna). * **Smooth Muscle Regulation:** Remember that smooth muscle lacks troponin. Instead, it uses **Caldesmon, Calponin,** and the **Calmodulin-MLCK** (Myosin Light Chain Kinase) pathway.

Question 356: What is the spatial relationship of every atom in a molecule known as?

A. Conformation (Correct Answer)
B. Configuration
C. Both of the above
D. None of the above

Explanation: ### Explanation **1. Why "Conformation" is Correct:** In biochemistry, **conformation** refers to the spatial arrangement of atoms in a molecule that can be altered by **rotation about single bonds** without breaking any covalent bonds. For proteins, this represents the three-dimensional shape (secondary, tertiary, and quaternary structures) that a polypeptide chain assumes. Because atoms are in constant motion but maintain specific spatial relationships to achieve a functional state (the "native conformation"), this term precisely describes the overall spatial architecture of the molecule. **2. Why the Other Options are Incorrect:** * **Configuration:** This refers to the fixed spatial arrangement of atoms that cannot be altered without **breaking and reforming covalent bonds**. Examples include geometric isomers (cis/trans) and optical isomers (L/D forms). While configuration is permanent, conformation is flexible. * **Both/None:** These are incorrect because the two terms are mutually exclusive in their chemical definitions regarding bond rotation versus bond breakage. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Native Conformation:** The specific 3D shape in which a protein is biologically active. Loss of this shape is called **denaturation**. * **Chaperones:** These are specialized proteins (e.g., Heat Shock Proteins) that assist in the correct folding of nascent polypeptides into their functional conformation. * **Prion Diseases:** These occur due to an alteration in protein conformation (e.g., alpha-helices changing to beta-pleated sheets), leading to neurodegeneration (e.g., Creutzfeldt-Jakob Disease). * **Ramachandran Plot:** A high-yield tool used to visualize allowed conformations of the polypeptide backbone based on the rotation of phi (φ) and psi (ψ) angles.

Question 357: What is the primary protein that binds to thyroxine?

A. Follistatin
B. Transthyretin
C. Thyroxine-binding globulin (Correct Answer)
D. Transferrin

Explanation: ***Thyroxine-binding globulin*** - **TBG** is a single chain glycoprotein that is the primary transporter, binding approximately 70-80% of circulating **T4** (thyroxine) and a smaller proportion of T3. - It serves as a high-affinity reservoir, maintaining hormonal homeostasis and contributing to the long **half-life** of T4. *Follistatin* - **Follistatin** binds and inhibits the activity of **activin** and is primarily involved in regulating the release of **FSH** (follicle-stimulating hormone) from the pituitary. - It has no functional role in the systemic transport or binding of thyroid hormones. *Transthyretin* - **Transthyretin** (or prealbumin) is the second most abundant carrier, binding about 10-15% of circulating T4, and is the principal carrier of **retinol** (Vitamin A). - Although it transports T4, its binding affinity and capacity are significantly lower than those of **TBG**, thus it is not the primary binder. *Transferrin* - **Transferrin** is the main plasma protein responsible for transporting **ferric iron (Fe3+)** throughout the body. - Its binding specificity is strictly for iron and does not involve the transport of **thyroxine**.

Question 358: Which among the following helps in the maturation of collagen?

A. Proline
B. Copper and zinc
C. Ascorbic acid (Correct Answer)
D. Phenylalanine

Explanation: ***Ascorbic acid***- Ascorbic acid (Vitamin C) is an essential cofactor for **prolyl hydroxylase** and **lysyl hydroxylase** enzymes, which hydroxylate proline and lysine residues in procollagen.- This **hydroxylation** is critical for stabilizing the collagen triple helix structure, a necessary step for proper collagen maturation and secretion.*Proline*- **Proline** (along with glycine and lysine) is a major amino acid component of the **collagen triple helix structure**.- While essential for collagen synthesis, proline itself is a substrate, not the **cofactor** required for the crucial hydroxylation steps necessary for maturation stabilization.*Copper and zinc*- **Copper** is a required cofactor for **lysyl oxidase**, the enzyme responsible for creating *covalent cross-links* between collagen molecules, which is a late-stage event for tensile strength.- Although cross-linking is part of the overall maturation process (extracellular), **ascorbic acid's** role in the intracellular hydroxylation of procollagen is a more fundamental step in maturation.*Phenylalanine*- **Phenylalanine** is an essential aromatic amino acid primarily used in protein synthesis and as a precursor for **tyrosine**.- It plays no direct role as a **cofactor** or required structural component in the specialized post-translational modification and subsequent maturation of collagen.

Question 359: Which of the following immunoglobulins shows the highest concentration in serum?

A. IgG (Correct Answer)
B. IgM
C. IgA
D. IgE

Explanation: ***IgG*** - **IgG** accounts for approximately **75-80%** of the total immunoglobulins in human serum, making it the highest in concentration.- Its high levels reflect its crucial roles in providing **long-term humoral immunity**, neutralizing toxins, and being the only class able to cross the **placenta** to confer passive immunity to the fetus.*IgM* - **IgM** is present at much lower concentrations (about 5-10%); it is primarily known for being the first antibody produced during a **primary immune response**. - Although it is the largest immunoglobulin (a pentamer) and effective at complement activation, its total serum concentration is substantially less than IgG.*IgA* - **IgA** constitutes about 10-15% of serum immunoglobulins, ranking second to IgG in concentration. - While abundant in serum, its primary functional significance lies in its dimeric form (**Secretory IgA**) found in mucosal secretions, protecting epithelial surfaces.*IgE* - **IgE** is the immunoglobulin present at the lowest serum concentration (trace amounts). - It is mainly bound to **mast cells** and **basophils** and is responsible for mediating immediate **Type I hypersensitivity** reactions (allergies).

Question 360: IGF-1 and IGF-2 are structurally most similar to which of the following molecules?

A. Preproinsulin
B. Insulin (Correct Answer)
C. Proinsulin
D. C-peptide

Explanation: ***Insulin*** - **Insulin-like growth factors (IGFs)**, including IGF-1 and IGF-2, belong to the same peptide growth factor superfamily as insulin, sharing notable **sequence homology** (~50% amino acid similarity) and a similar three-dimensional structure. - Both IGFs and insulin are small peptides stabilized by **disulfide bonds** forming A and B domains, and act via similar receptor tyrosine kinases (the insulin receptor and IGF-1 receptor can cross-react). - Mature insulin represents the **structurally closest molecule** to the secreted, functional IGFs. *Preproinsulin* - **Preproinsulin** is the earliest precursor and includes an N-terminal **signal peptide** (the 'pre' sequence) which is cleaved off upon entry into the endoplasmic reticulum. - Since IGFs mature into secreted proteins without this transient signal sequence in their final structure, preproinsulin is structurally less similar than mature insulin. *Proinsulin* - **Proinsulin** consists of the A chain and B chain linked by the connecting **C-peptide** domain. - Although IGFs are derived from a single precursor chain like proinsulin, **mature insulin** (two chains, A and B) is structurally closer to the secreted IGFs than proinsulin (which has three domains: A, B, and C). *C-peptide* - The **C-peptide** is the connecting segment linking the A and B chains of proinsulin, which is cleaved and removed before insulin becomes mature. - It is a short, linear peptide with little structural resemblance to the complex, disulfide-bonded domains of functional IGFs or insulin.

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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