Protein Structure and Function Practice Questions

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: 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: What is the number of variable regions present on each light and heavy chain of an antibody?

1. ***1*** - Each **light chain** and **heavy chain** within an antibody molecule contains **one variable region (V domain)**. - These variable regions are crucial for **antigen binding specificity**, as they combine to form the antigen-binding site. - The variable domain is located at the **N-terminal end** of each chain. *2* - While a complete antibody molecule has **two antigen-binding sites** (bivalent), each formed by pairing of VH and VL domains, individual chains possess only **one variable region each**. - The number '2' refers to the total number of identical binding sites on the intact antibody, not the number of variable regions per chain. *3* - The number **3** does not correspond to the number of variable regions on individual chains. - This might be confused with the **three complementarity-determining regions (CDRs)** present within each variable domain (CDR1, CDR2, CDR3), which are hypervariable loops that directly contact the antigen. *4* - The number **4** is incorrect for variable regions. - This number corresponds to the total number of **polypeptide chains** in a complete IgG antibody (2 heavy + 2 light chains), or the number of **constant domains** in some heavy chain isotypes (IgM, IgE have 4 CH domains).

Q: Which of the following amino acids is not classified as polar?

Methionine. ***Methionine*** - Methionine is classified as a **nonpolar**, **hydrophobic** amino acid due to its side chain containing a **thioether group** (-S-CH3), which has no significant dipole moment. - Its nonpolar nature means it is less likely to interact with water and tends to be found in the interior of proteins. *Glutamic acid* - Glutamic acid is a **polar**, **acidic** amino acid because its side chain contains a **carboxyl group** (-COOH) that is deprotonated (COO-) at physiological pH, carrying a negative charge. - This charged group allows it to participate in **ionic interactions** and hydrogen bonding. *Histidine* - Histidine is a **polar**, **basic** amino acid due to its **imidazole ring** in the side chain, which can be protonated or deprotonated depending on the pH. - Its pKa is close to physiological pH, making it an important **buffer** and active site residue in many enzymes. *Glutamine* - Glutamine is a **polar**, **uncharged** amino acid with an **amide group** (-CONH2) in its side chain. - The carbonyl oxygen and the amine hydrogens in the amide group allow for significant **hydrogen bonding** with water and other polar molecules.

Q: Which amino acid has two chiral centers?

Threonine. ***Threonine*** - Threonine is unique among the standard 20 amino acids because it possesses **two chiral centers**: one at the **alpha-carbon** and another at the **beta-carbon**. - The presence of two chiral centers means that threonine can exist as **four stereoisomers** (2^n, where n is the number of chiral centers). *Tryptophan* - Tryptophan has only **one chiral center**, which is the **alpha-carbon** bonded to the amino group, carboxyl group, hydrogen atom, and the side chain. - Its side chain, an **indole ring**, does not contain an additional chiral center. *Tyrosine* - Tyrosine, like most amino acids, possesses only **one chiral center** at its **alpha-carbon**. - The aromatic ring system (phenol group) in its side chain does not introduce another chiral center. *Phenylalanine* - Phenylalanine also has only **one chiral center** located at its **alpha-carbon**. - Its benzyl side chain, consisting of a methylene group and a benzene ring, is not chiral.

Q: Which of the following statements about Glutathione is false?

All of the above are true statements. ***All of the above are true statements*** - Since the question asks which statement is **FALSE**, and all the listed properties of glutathione are **TRUE**, the correct answer indicates that none of the statements are false. - All three statements accurately describe glutathione's structure and function. *Tripeptide* - Glutathione is indeed a **tripeptide** composed of three amino acids: **γ-glutamyl-cysteinyl-glycine**. - The unique γ-peptide bond (between glutamate's γ-carboxyl and cysteine's amino group) makes it resistant to peptidases. *Act as antioxidant in reduced state* - Glutathione functions as an **antioxidant** in its **reduced form (GSH)**, donating electrons to neutralize reactive oxygen species (ROS). - The **thiol group (-SH) of cysteine** is the active site for antioxidant activity. - Enzyme **glutathione reductase** maintains GSH levels by reducing oxidized glutathione (GSSG). *Formed from glutamic acid, glycine, cysteine* - Glutathione is synthesized from **glutamate, cysteine, and glycine** in two ATP-dependent steps. - First, **γ-glutamylcysteine synthetase** links glutamate and cysteine. - Then, **glutathione synthetase** adds glycine to form the complete tripeptide.

Q: Which type of bond is primarily responsible for the primary structure of a protein?

Peptide bond. ***Peptide bond*** - The **primary structure** of a protein is defined by the unique linear sequence of **amino acids** linked together by **peptide bonds**. - These are **amide bonds** formed between the carboxyl group of one amino acid and the amino group of another, with the elimination of water. *Hydrogen bond* - **Hydrogen bonds** are crucial for the **secondary structure** (e.g., alpha-helices and beta-sheets) and **tertiary/quaternary structures** of proteins, stabilizing their 3D folds. - They involve interactions between polar atoms, not the direct linkage of amino acids in the primary sequence. *Disulfide bond* - **Disulfide bonds** are **covalent bonds** formed between the sulfur atoms of two **cysteine residues**, contributing to the **tertiary** and sometimes **quaternary structure** stability. - They are not involved in forming the linear sequence of amino acids, which is the primary structure. *Electrostatic bond* - **Electrostatic bonds**, or **ionic bonds**, occur between oppositely charged amino acid side chains and are important for **tertiary** and **quaternary structure** stability. - They do not form the backbone of the protein's primary sequence.

Question 1

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

Accepted Answer

Lectins

Answer

Fibronectin

Answer

Laminin

Answer

Proteoglycans

Question 2

GlcNAc-P-P-oligosaccharide is -

Accepted Answer

Glycoprotein

Answer

Proteoglycan

Answer

Collagen

Answer

Phospholipid

Question 3

Abnormal proteins which are bound to ubiquitin are degraded in -

Accepted Answer

Proteasomes

Answer

Golgi apparatus

Answer

Smooth ER

Answer

Lysosomes

Question 4

What is the number of variable regions present on each light and heavy chain of an antibody?

Accepted Answer

1

Answer

2

Answer

3

Answer

4

Question 5

Which of the following amino acids is not classified as polar?

Accepted Answer

Methionine

Answer

Glutamic acid

Answer

Histidine

Answer

Glutamine

Question 6

Which amino acid has two chiral centers?

Accepted Answer

Threonine

Answer

Tyrosine

Answer

Tryptophan

Answer

Phenylalanine

Question 7

Which of the following statements about Glutathione is false?

Accepted Answer

All of the above are true statements

Answer

Tripeptide

Answer

Act as antioxidant in reduced state

Answer

Formed from glutamic acid, glycine, cysteine

Question 8

What is the primary action of metalloproteinases in the extracellular matrix?

Accepted Answer

Degradation of extracellular matrix components, including collagen

Answer

Modification of collagen structure

Answer

Formation of collagen

Answer

Activation of collagen synthesis

Question 9

Which type of bond is primarily responsible for the primary structure of a protein?

Accepted Answer

Peptide bond

Answer

Hydrogen bond

Answer

Disulfide bond

Answer

Electrostatic bond

Question 10

What is the process of Hofmann elimination in organic chemistry?

Accepted Answer

E2 elimination reaction producing the least substituted alkene

Answer

E1 elimination reaction favoring tertiary substrates

Answer

SN1 substitution reaction with carbocation intermediate

Answer

SN2 substitution reaction with inversion of configuration

Protein Structure and Function — MCQs

Protein Structure and Function — MCQs

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