Protein Structure and Function Practice Questions

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: 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: 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: 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: 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: 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 proteins is primarily responsible for marking other proteins for degradation?

Ubiquitin. **Ubiquitin** - **Ubiquitin** is a small regulatory protein that marks proteins for degradation by targeting them to the **proteasome**. - The ubiquitination process involves a cascade of enzymes (E1, E2, E3) that sequentially attach ubiquitin to the target protein, forming a **polyubiquitin chain**. *RNAse* - **RNAse** (Ribonuclease) is an enzyme that catalyzes the degradation of **RNA into smaller components**. - Its primary function is in **RNA processing** and turnover, not protein degradation. *Zymase* - **Zymase** is a complex of enzymes that catalyzes the **fermentation of sugar into ethanol and carbon dioxide**. - It is commonly found in yeast and is essential for **alcoholic fermentation**, with no role in protein degradation. *Chaperone* - **Chaperone proteins** assist in the **folding of newly synthesized proteins** and the refolding of misfolded or denatured proteins. - Their role is to ensure proper protein structure and function, preventing aggregation, rather than marking proteins for destruction.

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

Which of the following substances is primarily found in tendons?

Accepted Answer

Collagen

Answer

Fibrin

Answer

Fibrillin

Answer

Proteoglycans

Question 2

Which of these is not a part of extracellular matrix:

Accepted Answer

Integrins

Answer

Collagen

Answer

Laminin

Answer

Fibronectin

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 classification of Carcinoembryonic Antigen (CEA)?

Accepted Answer

Glycoprotein

Answer

Lipoprotein

Answer

Phosphoprotein

Answer

Nucleoprotein

Question 5

Amide group is present in which part of protein?

Accepted Answer

Peptide bond

Answer

Amino-terminal

Answer

Disulfide bond

Answer

Carboxy-terminal

Question 6

Unfolded protein metabolism is associated with

Accepted Answer

Endoplasmic reticulum

Answer

Golgi apparatus

Answer

Mitochondria

Question 7

GlcNAc-P-P-oligosaccharide is -

Accepted Answer

Glycoprotein

Answer

Proteoglycan

Answer

Collagen

Answer

Phospholipid

Question 8

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

Accepted Answer

Lectins

Answer

Fibronectin

Answer

Laminin

Answer

Proteoglycans

Question 9

Which of the following proteins is primarily responsible for marking other proteins for degradation?

Accepted Answer

Ubiquitin

Answer

RNAse

Answer

Zymase

Answer

Chaperone

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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