Protein Structure and Function Practice Questions

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 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: 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 statements regarding collagen synthesis is incorrect?

Hydroxylation of proline occurs in Golgi apparatus. ***Hydroxylation of proline occurs in Golgi apparatus*** - This statement is incorrect because the **hydroxylation of proline** residues occurs in the **endoplasmic reticulum** (ER), not the Golgi apparatus. - This step is critical for forming stable **triple helix** structures of collagen and requires **vitamin C**. *Synthesized in ribosomes as preprocollagen* - This statement is correct. Collagen synthesis begins in the cytoplasm, where mRNA is translated by **ribosomes** into **preprocollagen**, which contains a signal peptide. - The signal peptide directs the nascent polypeptide chain into the lumen of the **endoplasmic reticulum**. *Hydroxylation of lysine occurs in ER* - This statement is correct. Following entry into the ER, specific **lysine** residues are hydroxylated by **lysyl hydroxylase** to form hydroxylysine. - This hydroxylation, along with that of proline, is crucial for **cross-linking** and stability of the collagen molecule. *Triple helix assembly occurs in ER* - This statement is correct. After hydroxylation and glycosylation of some residues, three procollagen alpha chains self-assemble to form a **triple helix** within the **endoplasmic reticulum**. - This assembly is stabilized by **disulfide bonds** at the C-terminal ends and molecular chaperones.

Q: Amino acid with aliphatic side chain is?

Leucine. ***Leucine*** - Leucine has an **isobutyl group** (-CH2CH(CH3)2) as its side chain, making it a **nonpolar aliphatic amino acid**. - **Aliphatic amino acids** (glycine, alanine, valine, leucine, isoleucine, proline) have side chains consisting of only carbon and hydrogen atoms in straight or branched chains, with **no polar functional groups**. - These amino acids are **hydrophobic** and typically found in the interior of proteins. *Serine* - Serine has a **hydroxyl group** (-OH) in its side chain (-CH2OH), classifying it as a **polar uncharged amino acid**, not an aliphatic amino acid. - The hydroxyl group makes the side chain **hydrophilic** and capable of hydrogen bonding. - The presence of the polar functional group distinguishes it from aliphatic amino acids. *Threonine* - Threonine also contains a **hydroxyl group** (-OH) in its side chain (-CH(OH)CH3), making it a **polar uncharged amino acid**, not an aliphatic amino acid. - Like serine, the hydroxyl group provides **polarity and hydrogen bonding capacity**. - This functional group places it in a different classification from aliphatic amino acids. *Aspartate* - Aspartate has a **carboxyl group** (-COOH) in its side chain (-CH2COOH), making it an **acidic (negatively charged) amino acid**. - At physiological pH, this group is deprotonated (COO⁻), making aspartate **negatively charged**. - This clearly distinguishes it from nonpolar aliphatic amino acids.

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

Are lipid in nature. ***Are lipid in nature*** - Chaperones are **proteins** (typically **heat shock proteins** or **chaperonins**), not lipids. - Their function involves assisting in the proper **folding and assembly of other proteins**, and they are composed of amino acids. *Cause folding of proteins* - Chaperones **do not cause** proteins to fold; rather, they **assist in proper folding** and refolding by preventing aggregation or misfolding. - They bind to nascent or partially unfolded proteins to guide them towards their correct three-dimensional structure. *May have ATPase activity* - Many chaperones, especially **Hsp70** and **chaperonins** like GroEL/GroES, utilize **ATP hydrolysis** for their function. - This **ATPase activity** drives conformational changes essential for binding, release, and refolding of their client proteins. *Include heat shock proteins* - The **heat shock protein (Hsp)** families (e.g., Hsp70, Hsp90, Hsp60) are a major class of chaperones. - Hsps are upregulated in response to stress (like heat) to help refold damaged proteins and prevent aggregation.

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.

Question 1

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 2

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 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 statements regarding collagen synthesis is incorrect?

Accepted Answer

Hydroxylation of proline occurs in Golgi apparatus

Answer

Hydroxylation of lysine occurs in ER

Answer

Synthesized in ribosomes as preprocollagen

Answer

Triple helix assembly occurs in ER

Question 6

Amino acid with aliphatic side chain is?

Accepted Answer

Leucine

Answer

Serine

Answer

Threonine

Answer

Aspartate

Question 7

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 8

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

Question 9

Which of the following statements about chaperones is false?

Accepted Answer

Are lipid in nature

Answer

Cause folding of proteins

Answer

Include heat shock proteins

Answer

May have ATPase activity

Question 10

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

Protein Structure and Function — MCQs

Protein Structure and Function — MCQs

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