Protein Structure and Function — MCQs
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Fibrinopeptide A and fibrinopeptide B are acidic due to the presence of which amino acids in their structure?
Which of the following substances is primarily found in tendons?
Which of these is not a part of extracellular matrix:
What is the molecular mass of Immunoglobulin G (IgG) in kilodaltons (kDa)?
What type of bond is involved in the side chain linkage of proteoglycans?
Conversion of prekallikrein to kallikrein requires which clotting factor?
Amide group is present in which part of protein?
Which of the following is not a part of extracellular matrix (ECM)?
GlcNAc-P-P-oligosaccharide is -
What is the classification of Carcinoembryonic Antigen (CEA)?
Protein Structure and Function Indian Medical PG Practice Questions and MCQs
Question 421: Fibrinopeptide A and fibrinopeptide B are acidic due to the presence of which amino acids in their structure?
- A. Serine and threonine
- B. Glutamate and aspartate (Correct Answer)
- C. Histidine and lysine
- D. Glutamine and valine
Explanation: **Glutamate and aspartate** - Both **glutamate** and **aspartate** are **acidic amino acids** due to the presence of an extra carboxyl group in their side chains. - The release of these fibrinopeptides from **fibrinogen** by thrombin exposes sites for fibrin polymerization in the final stage of coagulation. *Serine and threonine* - **Serine** and **threonine** are **polar, uncharged amino acids**, meaning they do not significantly contribute to the overall acidic nature of a peptide. - Their side chains contain hydroxyl groups, making them candidates for phosphorylation, but not acidity. *Histidine and lysine* - **Histidine** and **lysine** are **basic amino acids** due to the presence of nitrogenous groups in their side chains that can accept protons, thus imparting positive charges. - Therefore, their presence would make a peptide more basic, not acidic. *Glutamine and valine* - **Glutamine** is a **polar, uncharged amino acid**, derived from glutamate but with an amide group replacing one of the carboxyl oxygens. - **Valine** is a **nonpolar, aliphatic amino acid**, which does not contribute to the acidic nature of a peptide.
Question 422: Which of the following substances is primarily found in tendons?
- A. Collagen (Correct Answer)
- B. Fibrin
- C. Fibrillin
- D. Proteoglycans
Explanation: ***Collagen*** - **Type I collagen** is the predominant structural protein found in tendons, providing their characteristic **tensile strength** and resistance to stretch. - Its organized parallel bundles allow tendons to transmit forces effectively from muscle to bone. *Fibrin* - **Fibrin** is a protein involved in **blood clotting**, forming a meshwork that stops bleeding. - It is not a primary structural component of healthy tendons. *Fibrillin* - **Fibrillin** is a glycoprotein that forms microfibrils, which are crucial components of **elastic fibers**, providing elasticity to tissues like the skin, lungs, and blood vessel walls. - While present in some connective tissues, it does not provide the primary structural support of tendons. *Proteoglycans* - **Proteoglycans** are complex macromolecules that provide **hydration** and act as shock absorbers in many connective tissues, including cartilage. - While present in small amounts in tendons, they are not the primary structural component responsible for tensile strength.
Question 423: Which of these is not a part of extracellular matrix:
- A. Collagen
- B. Laminin
- C. Fibronectin
- D. Integrins (Correct Answer)
Explanation: ***Integrins*** - Integrins are **transmembrane receptors** on the cell surface that facilitate cell-extracellular matrix (ECM) adhesion and cell-cell adhesion. - They are part of the cell membrane, **not** an extracellular component. *Laminin* - **Laminin** is a major protein component of the **basal lamina**, a specialized extracellular matrix that underlies epithelial cells. - It plays a crucial role in cell adhesion, differentiation, and migration within the ECM. *Fibronectin* - **Fibronectin** is a large glycoprotein present in the **extracellular matrix** and in soluble form in blood plasma. - It mediates cell adhesion to the ECM by binding to integrins and various ECM components like collagen and proteoglycans. *Collagen* - **Collagen** is the most abundant protein in the human body and a primary structural component of the **extracellular matrix**. - It provides tensile strength and structural integrity to tissues like skin, bone, tendons, and cartilage.
Question 424: What is the molecular mass of Immunoglobulin G (IgG) in kilodaltons (kDa)?
- A. 150 (Correct Answer)
- B. 400
- C. 1000
- D. 1500
Explanation: **\*Correct Option: 150 kDa\*** - **Immunoglobulin G (IgG)** is the most abundant antibody in human serum and has a characteristic molecular mass of approximately **150 kDa**. - This mass is attributed to its structure, comprising two identical **heavy chains** (~50 kDa each) and two identical **light chains** (~25 kDa each). - IgG represents about **75-80% of total serum immunoglobulins** and is the main antibody involved in secondary immune responses. *Incorrect Option: 400 kDa* - A molecular mass of **400 kDa** is significantly higher than that of a monomeric IgG molecule. - This mass is closer to **IgM pentamers** (~900 kDa) or large protein complexes, but still does not match any standard immunoglobulin structure. *Incorrect Option: 1000 kDa* - A molecular mass of **1000 kDa (1 MDa)** is far too large for a single IgG molecule. - This weight typically corresponds to very large macromolecular structures or aggregates, such as **ribosomes** or large enzyme complexes. *Incorrect Option: 1500 kDa* - A molecular mass of **1500 kDa (1.5 MDa)** is extremely large for an individual antibody. - Such a mass would be characteristic of very large protein assemblies, viral capsids, or cellular components, not a soluble antibody.
Question 425: What type of bond is involved in the side chain linkage of proteoglycans?
- A. Covalent (Correct Answer)
- B. Hydrogen bond
- C. Electrostatic bond
- D. Van-der Waal's force
Explanation: ***Covalent*** - Proteoglycans are formed by **glycosaminoglycan (GAG)** chains that are covalently linked to a protein core. - Specifically, an **O-glycosidic bond** forms between a xylose residue on the GAG chain and a serine residue on the core protein. *Hydrogen bond* - **Hydrogen bonds** are weaker intermolecular forces that stabilize protein secondary structures and interactions between water molecules. - They are not strong enough to form the primary structural linkage between the GAG chains and the core protein in proteoglycans. *Electrostatic bond* - **Electrostatic bonds**, or ionic bonds, involve attraction between oppositely charged ions. While proteoglycans have many charged groups, these bonds are not the primary linkage connecting the GAG chains to the protein core. - They contribute to the overall structure and interactions of proteoglycans with other molecules but do not form the main side chain linkage. *Van-der Waal's force* - **Van der Waals forces** are weak, short-range intermolecular forces that arise from temporary fluctuations in electron distribution. - These forces play a role in tertiary and quaternary protein structure and molecular packing, but they are far too weak to establish the covalent attachments of GAG chains to the proteoglycan core protein.
Question 426: Conversion of prekallikrein to kallikrein requires which clotting factor?
- A. XIII
- B. XII (Correct Answer)
- C. X
- D. XI
Explanation: ***XII*** - **Factor XII (Hageman factor)** is crucial in the **intrinsic pathway** of coagulation. - It initiates the contact activation system, which includes the conversion of **prekallikrein to kallikrein**. *XIII* - **Factor XIII (fibrin-stabilizing factor)** is responsible for **cross-linking fibrin** monomers to form a stable clot. - It acts much later in the coagulation cascade, after thrombin has converted fibrinogen to fibrin. *XI* - **Factor XI (plasma thromboplastin antecedent)** is activated by factor XIIa and in turn activates factor IX in the intrinsic pathway. - While part of the intrinsic pathway, it does not directly convert prekallikrein to kallikrein. *X* - **Factor X (Stuart-Prower factor)** is a central component of the **common pathway**, activated by both intrinsic and extrinsic pathways. - Its primary role is to combine with factor Va, calcium, and phospholipids to form the **prothrombinase complex**, converting prothrombin to thrombin.
Question 427: Amide group is present in which part of protein?
- A. Amino-terminal
- B. Peptide bond (Correct Answer)
- C. Disulfide bond
- D. Carboxy-terminal
Explanation: ***Peptide bond*** - A **peptide bond** is formed between the **carboxyl group** of one amino acid and the **amino group** of another, releasing a water molecule. This bond has an **amide structure**. - The repeated formation of these amide (peptide) bonds links amino acids into long chains, forming a **polypeptide** or protein. *Amino-terminal* - The **amino-terminal (N-terminal)** end of a protein contains a free **amino group (-NH2)**, which is not part of an amide linkage within the polypeptide backbone. - It marks the beginning of the polypeptide chain and is typically involved in various cellular interactions and modifications. *Disulfide bond* - A **disulfide bond** is a covalent bond formed between two **sulfhydryl groups (-SH)** of **cysteine residues**, leading to the formation of a **cystine** residue. - This bond is crucial for stabilizing the **tertiary and quaternary structures** of proteins, but it does not contain an amide group. *Carboxy-terminal* - The **carboxy-terminal (C-terminal)** end of a protein contains a free **carboxyl group (-COOH)**, which is not part of an amide linkage within the polypeptide backbone. - It marks the end of the polypeptide chain and plays roles in protein processing, targeting, and regulation.
Question 428: Which of the following is not a part of extracellular matrix (ECM)?
- A. Lectins (Correct Answer)
- B. Fibronectin
- C. Laminin
- D. Proteoglycans
Explanation: ***Lectins*** - **Lectins** are carbohydrate-binding proteins involved in various cellular processes but are typically found **on cell surfaces** or within cells, not as a major structural component of the ECM. - While they can interact with ECM components, they are not considered a direct structural element of the extracellular matrix itself. *Fibronectin* - **Fibronectin** is a critical **glycoprotein** in the ECM, playing a vital role in cell adhesion, growth, migration, and differentiation. - It links cells to collagen fibers and other ECM components, forming an essential scaffold. *Laminin* - **Laminin** is a major **glycoprotein** component of the **basal lamina**, a specialized layer of the ECM found beneath epithelial cells. - It helps in cell attachment, differentiation, and migration. *Proteoglycans* - **Proteoglycans** are macromolecules consisting of a **core protein** covalently linked to one or more **glycosaminoglycan (GAG) chains**. - They are abundant in the ECM, where they contribute to its structural integrity, hydration, and can regulate the diffusion of molecules.
Question 429: GlcNAc-P-P-oligosaccharide is -
- A. Proteoglycan
- B. Glycoprotein (Correct Answer)
- C. Collagen
- D. Phospholipid
Explanation: ***Glycoprotein*** - **GlcNAc-P-P-oligosaccharide** refers to the **N-linked oligosaccharide precursor** that is synthesized on a **dolichol pyrophosphate** carrier (`-P-P`). This complex is characteristic of the initial stages of **N-linked glycosylation**, a process that forms glycoproteins. - **N-acetylglucosamine (GlcNAc)** is a crucial sugar residue found at the reducing end of this precursor, linking it to the dolichol carrier. *Proteoglycan* - Proteoglycans consist of a **core protein** covalently attached to long, unbranched **glycosaminoglycan (GAG)** chains, such as chondroitin sulfate or heparin. - While they contain sugar units, their structure and synthesis pathway are distinct from the GlcNAc-P-P-oligosaccharide described, which is specific to N-linked glycoprotein synthesis. *Collagen* - **Collagen** is a fibrous protein, primarily composed of a triple helix of polypeptide chains rich in **glycine, proline, and hydroxyproline**. - Although collagen undergoes some post-translational modifications like **glycosylation**, it does not involve the GlcNAc-P-P-oligosaccharide precursor in its typical synthesis. *Phospholipid* - **Phospholipids** are a major component of cell membranes, composed of a **hydrophilic head** (containing a phosphate group) and two **hydrophobic fatty acid tails**. - They are lipids and do not contain carbohydrate structures like GlcNAc-P-P-oligosaccharide.
Question 430: What is the classification of Carcinoembryonic Antigen (CEA)?
- A. Glycoprotein (Correct Answer)
- B. Lipoprotein
- C. Phosphoprotein
- D. Nucleoprotein
Explanation: ***Glycoprotein*** - Carcinoembryonic Antigen (CEA) is classified as a **glycoprotein** due to its structure, which consists of both **carbohydrate** and **protein** components. - This glycosylation is crucial for its function as a cell adhesion molecule and its recognition in diagnostic assays. *Lipoprotein* - **Lipoproteins** are complexes of lipids and proteins that function primarily in **lipid transport** in the blood. - CEA's primary role and structure are not related to lipid transport or being predominantly lipid-based. *Phosphoprotein* - A **phosphoprotein** is a protein that has been **covalently modified by the addition of a phosphate group**, a process crucial for cell signaling. - While proteins can be phosphorylated, the defining characteristic and major classification of CEA is its extensive glycosylation rather than phosphorylation state. *Nucleoprotein* - **Nucleoproteins** are proteins that are **structurally associated with nucleic acids** (DNA or RNA), such as histones or ribosomal proteins. - CEA does not have a structural or functional association with nucleic acids.
Practice by Chapter
Amino Acids: Structure and Properties
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Peptide Bond Formation
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Primary Structure of Proteins
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Secondary Structure of Proteins
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Tertiary and Quaternary Structures
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Protein Folding and Chaperones
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Protein Domains and Motifs
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Structure-Function Relationships
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Hemoglobin and Myoglobin
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Collagen and Elastin
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Albumin and Plasma Proteins
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Post-Translational Modifications
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