Protein Structure and Function — MCQs
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In which form is C-peptide primarily found during insulin synthesis?
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)?
Unfolded protein metabolism is associated with
What type of bond is involved in the side chain linkage of proteoglycans?
What is the classification of Carcinoembryonic Antigen (CEA)?
Conversion of prekallikrein to kallikrein requires which clotting factor?
Which of the following is not a part of extracellular matrix (ECM)?
Which of the following is not classified as a chaperone protein?
Protein Structure and Function Indian Medical PG Practice Questions and MCQs
Question 421: In which form is C-peptide primarily found during insulin synthesis?
- A. In Pre-proinsulin
- B. In Proinsulin (Correct Answer)
- C. As a combined entity with insulin after secretion
- D. A gastrointestinal bioactive molecule
Explanation: ***In Proinsulin*** - **C-peptide** is an integral part of **proinsulin**, the precursor molecule to insulin, which is synthesized in the **pancreatic beta cells**. - During the maturation process, **proinsulin** is cleaved, releasing both **insulin** and **C-peptide** in equimolar amounts. *In Pre-proinsulin* - **Pre-proinsulin** is the initial polypeptide chain synthesized on the ribosomes, containing a signaling **peptide sequence** that guides it into the endoplasmic reticulum. - The **signal peptide** is cleaved off during translocation into the ER, converting pre-proinsulin into **proinsulin**. *As a combined entity with insulin after secretion* - After secretion, **insulin** and **C-peptide** circulate as separate molecules in the bloodstream. - Their presence as distinct entities allows for the measurement of **endogenous insulin secretion**, as C-peptide has a longer half-life than insulin and is not removed by the liver to the same extent. *A gastrointestinal bioactive molecule* - **C-peptide** primarily functions as a marker for **endogenous insulin production** and does not have a significant role as a **gastrointestinal bioactive molecule**. - Its main utility is in distinguishing between type 1 diabetes (very low C-peptide) and type 2 diabetes or insulinoma (normal to high C-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: Unfolded protein metabolism is associated with
- A. Endoplasmic reticulum (Correct Answer)
- B. Golgi apparatus
- C. Mitochondria
Explanation: ***Endoplasmic reticulum*** - The **endoplasmic reticulum (ER)** is the primary site for protein folding, modification, and assembly, especially for secreted and transmembrane proteins. - When misfolded proteins accumulate, the ER triggers the **unfolded protein response (UPR)** to restore homeostasis or induce apoptosis. *Golgi apparatus* - The Golgi apparatus is primarily involved in **further processing**, sorting, and packaging of proteins and lipids synthesized in the ER. - It does not directly manage the initial folding of proteins or the response to widespread protein misfolding. *Mitochondria* - **Mitochondria** are known for their role in **energy production** (ATP synthesis) through cellular respiration. - While they possess their own protein synthesis machinery for some essential mitochondrial proteins, they are not involved in the overall cellular management of unfolded protein metabolism from the ER.
Question 426: 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 427: 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.
Question 428: 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 429: 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 430: Which of the following is not classified as a chaperone protein?
- A. Calnexin
- B. Protein disulfide isomerase
- C. Calreticulin
- D. Calbindin (Correct Answer)
Explanation: ***Calbindin*** - **Calbindin** is a **calcium-binding protein** that helps regulate intracellular calcium levels, particularly in the brain and intestines. - It does not assist in **protein folding** or assembly like chaperone proteins. *Calnexin* - **Calnexin** is a **chaperone protein** located in the endoplasmic reticulum (ER). - It assists in the proper folding and quality control of newly synthesized **glycoproteins**. *Protein disulfide isomerase* - **Protein disulfide isomerase (PDI)** is an ER enzyme that **catalyzes the formation and rearrangement of disulfide bonds** in newly synthesized proteins, which is crucial for proper folding. - Due to its role in enabling correct protein folding, it is considered a **chaperone-like protein**. *Calreticulin* - **Calreticulin** is another **calcium-binding chaperone protein** found in the endoplasmic reticulum. - It works synergistically with calnexin to ensure the **proper folding of glycoproteins**.
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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