Protein Structure and Function — MCQs

Protein Structure and Function Indian Medical PG Practice Questions and MCQs

Question 431: Which of the following proteins is primarily responsible for marking other proteins for degradation?

A. Ubiquitin (Correct Answer)
B. RNAse
C. Zymase
D. Chaperone

Explanation: **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.

Question 432: Unfolded protein metabolism is associated with

A. Endoplasmic reticulum (Correct Answer)
B. Golgi apparatus
C. Mitochondria

Explanation: ***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.

Question 433: Abnormal proteins which are bound to ubiquitin are degraded in -

A. Proteasomes (Correct Answer)
B. Golgi apparatus
C. Smooth ER
D. Lysosomes

Explanation: ***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.

Question 434: What is the primary role of calnexin and calreticulin in the endoplasmic reticulum?

A. Degrade misfolded proteins
B. Act as chaperones (Correct Answer)
C. Serve as tumor markers
D. Facilitate enzymatic reactions

Explanation: ***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.

Question 435: Which of the following is not classified as a chaperone protein?

A. Calnexin
B. Protein disulfide isomerase
C. Calreticulin
D. Calbindin (Correct Answer)

Explanation: ***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 436: What is the number of variable regions present on each light and heavy chain of an antibody?

A. 1 (Correct Answer)
B. 2
C. 3
D. 4

Explanation: ***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).

Question 437: Which immunoglobulin is known to be heat-labile?

A. IgA
B. IgG
C. IgM (Correct Answer)
D. IgE

Explanation: ***IgM*** - **IgM** is known for its **heat lability** and is readily denatured at 56°C within a few minutes. - This characteristic is due to its **pentameric structure** held together by disulfide bonds and J chains, which are sensitive to thermal denaturation. - Heat lability of IgM is clinically important in complement fixation tests and other laboratory assays where heat inactivation is performed. - IgM is the first antibody produced in primary immune response and its heat sensitivity distinguishes it from other immunoglobulins. *IgA* - **IgA** exists in monomeric (serum) and dimeric (secretory) forms and shows moderate stability to heat. - Secretory IgA is relatively stable as it needs to function in harsh mucosal environments, though not as heat-resistant as IgG. - Does not exhibit the pronounced heat lability characteristic of IgM. *IgG* - **IgG** is the most stable immunoglobulin and is highly resistant to heat denaturation. - Can withstand temperatures up to 60-70°C without significant loss of activity. - Its monomeric structure with strong intramolecular bonds provides exceptional thermal stability. - Most abundant antibody in serum and has the longest half-life. *IgE* - **IgE** is actually quite stable to heat and can withstand 56°C for extended periods. - While it has a short half-life in serum (2-3 days), this is due to receptor binding rather than heat instability. - Important in type I hypersensitivity reactions and parasitic infections. - Does not show the characteristic heat lability that defines IgM.

Question 438: What is the major site of protein glycosylation?

A. Ribosome and Golgi body
B. ER and Ribosome
C. Ribosome and Cytoplasm
D. ER and Golgi body (Correct Answer)

Explanation: ***ER and Golgi body*** - The **endoplasmic reticulum (ER)** is the primary site for **N-linked glycosylation**, where carbohydrates are added to the asparagine residues of nascent proteins. - The **Golgi apparatus** is crucial for further modification and processing of these N-linked glycans, as well as the site for **O-linked glycosylation**, where sugars are added to serine or threonine residues. *Ribosome and Golgi body* - **Ribosomes** are responsible for **protein synthesis (translation)** but do not directly perform glycosylation, which is a post-translational modification. - While the **Golgi body** is a site of glycosylation, the ribosome's inclusion makes this option incorrect as the ribosome's role precedes glycosylation. *ER and Ribosome* - The **ER** is a major site of protein glycosylation, especially N-linked glycosylation. - However, **ribosomes** are involved in protein synthesis and lack the enzymatic machinery for adding sugar moieties to proteins. *Ribosome and Cytoplasm* - **Ribosomes** synthesize proteins, but glycosylation does not occur there. - The **cytoplasm** is the site for many metabolic pathways, but major protein glycosylation events mostly occur within the ER and Golgi.

Question 439: Amino acid with aliphatic side chain is?

A. Serine
B. Leucine (Correct Answer)
C. Threonine
D. Aspartate

Explanation: ***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.

Question 440: Which type of bond is primarily responsible for the primary structure of a protein?

A. Hydrogen bond
B. Disulfide bond
C. Peptide bond (Correct Answer)
D. Electrostatic bond

Explanation: ***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.

Practice by Chapter

Amino Acids: Structure and Properties

Practice Questions

Peptide Bond Formation

Practice Questions

Primary Structure of Proteins

Practice Questions

Secondary Structure of Proteins

Practice Questions

Tertiary and Quaternary Structures

Practice Questions

Protein Folding and Chaperones

Practice Questions

Protein Domains and Motifs

Practice Questions

Structure-Function Relationships

Practice Questions

Hemoglobin and Myoglobin

Practice Questions

Collagen and Elastin

Practice Questions

Albumin and Plasma Proteins

Practice Questions

Post-Translational Modifications

Practice Questions

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