Protein Structure and Function Practice Questions

Q: In which form is C-peptide primarily found during insulin synthesis?

In Proinsulin. ***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).

Q: Which of the following substances is primarily found in tendons?

Collagen. ***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.

Q: Which of these is not a part of extracellular matrix:

Integrins. ***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.

Q: What is the molecular mass of Immunoglobulin G (IgG) in kilodaltons (kDa)?

150. **\*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.

Q: Unfolded protein metabolism is associated with

Endoplasmic reticulum. ***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.

Q: What type of bond is involved in the side chain linkage of proteoglycans?

Covalent. ***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.

Q: What is the classification of Carcinoembryonic Antigen (CEA)?

Glycoprotein. ***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.

Q: Conversion of prekallikrein to kallikrein requires which clotting factor?

XII. ***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.

Q: Which of the following is not a part of extracellular matrix (ECM)?

Lectins. ***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.

Q: Which of the following is not classified as a chaperone protein?

Calbindin. ***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**.

Question 1

In which form is C-peptide primarily found during insulin synthesis?

Accepted Answer

In Proinsulin

Answer

In Pre-proinsulin

Answer

As a combined entity with insulin after secretion

Answer

A gastrointestinal bioactive molecule

Question 2

Which of the following substances is primarily found in tendons?

Accepted Answer

Collagen

Answer

Fibrin

Answer

Fibrillin

Answer

Proteoglycans

Question 3

Which of these is not a part of extracellular matrix:

Accepted Answer

Integrins

Answer

Collagen

Answer

Laminin

Answer

Fibronectin

Question 4

What is the molecular mass of Immunoglobulin G (IgG) in kilodaltons (kDa)?

Accepted Answer

150

Answer

400

Answer

1000

Answer

1500

Question 5

Unfolded protein metabolism is associated with

Accepted Answer

Endoplasmic reticulum

Answer

Golgi apparatus

Answer

Mitochondria

Question 6

What type of bond is involved in the side chain linkage of proteoglycans?

Accepted Answer

Covalent

Answer

Hydrogen bond

Answer

Electrostatic bond

Answer

Van-der Waal's force

Question 7

What is the classification of Carcinoembryonic Antigen (CEA)?

Accepted Answer

Glycoprotein

Answer

Lipoprotein

Answer

Phosphoprotein

Answer

Nucleoprotein

Question 8

Conversion of prekallikrein to kallikrein requires which clotting factor?

Accepted Answer

XII

Answer

XIII

Answer

X

Answer

XI

Question 9

Which of the following is not a part of extracellular matrix (ECM)?

Accepted Answer

Lectins

Answer

Fibronectin

Answer

Laminin

Answer

Proteoglycans

Question 10

Which of the following is not classified as a chaperone protein?

Accepted Answer

Calbindin

Answer

Calnexin

Answer

Protein disulfide isomerase

Answer

Calreticulin

Protein Structure and Function — MCQs

Protein Structure and Function — MCQs

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