Protein Structure and Function Practice Questions

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: 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: 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: What is the primary role of calnexin and calreticulin in the endoplasmic reticulum?

Act as chaperones. ***Act as chaperones*** - **Calnexin** and **calreticulin** are **chaperone proteins** located in the **endoplasmic reticulum (ER)**. - They bind to unfolded or misfolded glycoproteins to assist in their proper folding and assembly. - They are part of the **ER quality control system**, ensuring only properly folded proteins proceed to the Golgi apparatus. *Degrade misfolded proteins* - While misfolded proteins are eventually degraded through **ER-associated degradation (ERAD)**, this is not the primary function of calnexin and calreticulin. - These chaperones first attempt to **rescue and refold** proteins; degradation is a separate process involving other machinery. *Serve as tumor markers* - **Calnexin** and **calreticulin** are not typically used as **tumor markers** in clinical practice. - Their functions are related to protein quality control within the cell, not cancer detection. *Facilitate enzymatic reactions* - While some proteins in the ER are enzymes, **calnexin** and **calreticulin** themselves are not enzymes, nor do they primarily facilitate enzymatic reactions. - Their function is to ensure correct protein folding, distinct from direct catalytic activity.

Q: Abnormal proteins which are bound to ubiquitin are degraded in -

Proteasomes. ***Proteasomes*** - **Proteasomes** are multi-subunit protein complexes responsible for degrading **ubiquitin-tagged proteins**. - This degradation is a tightly regulated process essential for cell cycle control, gene expression, and immune response. *Golgi apparatus* - The **Golgi apparatus** primarily functions in modifying, sorting, and packaging proteins and lipids synthesized in the Endoplasmic Reticulum. - It does not directly participate in the degradation of **ubiquitin-bound proteins**. *Smooth ER* - The **smooth endoplasmic reticulum (SER)** is involved in lipid synthesis, detoxification of drugs and poisons, and storage of calcium ions. - It lacks ribosomes and is not directly implicated in the degradation of misfolded proteins tagged with ubiquitin. *Lysosomes* - **Lysosomes** are organelles containing various hydrolytic enzymes that break down waste materials and cellular debris, as well as foreign invaders like bacteria. - While they degrade proteins, they primarily target **extracellular proteins** taken up by endocytosis or cellular components via **autophagy**, not specifically ubiquitin-bound proteins.

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**.

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: GlcNAc-P-P-oligosaccharide is -

Glycoprotein. ***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.

Q: Amide group is present in which part of protein?

Peptide bond. ***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.

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.

Question 1

Which of these is not a part of extracellular matrix:

Accepted Answer

Integrins

Answer

Collagen

Answer

Laminin

Answer

Fibronectin

Question 2

Which of the following substances is primarily found in tendons?

Accepted Answer

Collagen

Answer

Fibrin

Answer

Fibrillin

Answer

Proteoglycans

Question 3

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

Accepted Answer

150

Answer

400

Answer

1000

Answer

1500

Question 4

What is the primary role of calnexin and calreticulin in the endoplasmic reticulum?

Accepted Answer

Act as chaperones

Answer

Degrade misfolded proteins

Answer

Serve as tumor markers

Answer

Facilitate enzymatic reactions

Question 5

Abnormal proteins which are bound to ubiquitin are degraded in -

Accepted Answer

Proteasomes

Answer

Golgi apparatus

Answer

Smooth ER

Answer

Lysosomes

Question 6

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

Accepted Answer

Calbindin

Answer

Calnexin

Answer

Protein disulfide isomerase

Answer

Calreticulin

Question 7

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

Accepted Answer

Lectins

Answer

Fibronectin

Answer

Laminin

Answer

Proteoglycans

Question 8

GlcNAc-P-P-oligosaccharide is -

Accepted Answer

Glycoprotein

Answer

Proteoglycan

Answer

Collagen

Answer

Phospholipid

Question 9

Amide group is present in which part of protein?

Accepted Answer

Peptide bond

Answer

Amino-terminal

Answer

Disulfide bond

Answer

Carboxy-terminal

Question 10

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

Protein Structure and Function — MCQs

Protein Structure and Function — MCQs

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