Protein Folding and Chaperones — MCQs
All are true about prions EXCEPT:
Which of the following reagents would be most useful in determining the N-terminal amino acid of a polypeptide?
Which factor stabilizes the alpha-helical structure of proteins?
All are true regarding G protein Except?
What type of bond is involved in the side chain linkage of proteoglycans?
What is the primary role of calnexin and calreticulin in the endoplasmic reticulum?
Which of the following statements regarding prion diseases is NOT true?
In response to changes in Ca2+ concentration, which of the following Ca2+ binding proteins can modify the activity of many enzymes & proteins?
What do chaperones assist in?
Which of the following is a polar amino acid?
Protein Folding and Chaperones Indian Medical PG Practice Questions and MCQs
Question 1: All are true about prions EXCEPT:
- A. Contain nucleic acid (Correct Answer)
- B. Protease resistant
- C. Not affected by radiation
- D. Cause spongiform changes
Explanation: ***Contain nucleic acid*** - This statement is **FALSE** - prions do NOT contain nucleic acid, making this the correct answer for an EXCEPT question. - Prions are unique infectious agents composed solely of **abnormally folded proteins (PrPSc)**, completely lacking any genetic material such as **DNA or RNA**. - This fundamental characteristic differentiates them from all conventional pathogens including **viruses, bacteria, fungi, and parasites**. *Protease resistant* - This statement is TRUE about prions. - The **abnormal folding** of prion proteins (β-pleated sheet conformation) renders them highly **resistant to degradation** by proteases. - This resistance contributes to their **accumulation in neurological tissue** and the progressive pathogenesis of transmissible spongiform encephalopathies. *Not affected by radiation* - This statement is TRUE about prions. - Because prions lack nucleic acid, they are **resistant to inactivation** by treatments like **ultraviolet (UV) radiation** and **ionizing radiation**. - These forms of radiation primarily damage genetic material (DNA/RNA), which prions completely lack. - Prions require **autoclaving at 134°C for extended periods** or treatment with strong alkalis for effective inactivation. *Cause spongiform changes* - This statement is TRUE about prions. - Prion diseases are characterized by **vacuolation** of brain tissue, giving it a distinctive **spongy appearance** on microscopic examination. - These spongiform changes are pathognomonic features of prion diseases such as **Creutzfeldt-Jakob disease (CJD)**, **kuru**, **Gerstmann-Sträussler-Scheinker syndrome**, and **bovine spongiform encephalopathy (BSE)**.
Question 2: Which of the following reagents would be most useful in determining the N-terminal amino acid of a polypeptide?
- A. Trypsin
- B. Carboxypeptidase
- C. Phenylisothiocyanate (Correct Answer)
- D. Cyanogen bromide
Explanation: ***Phenylisothiocyanate*** - **Phenylisothiocyanate** (PITC), also known as Edman's reagent, is used in the **Edman degradation** method to identify the N-terminal amino acid. - It sequentially cleaves the **N-terminal amino acid** without hydrolyzing the rest of the peptide chain, allowing for identification by chromatography. *Trypsin* - **Trypsin** is a protease that cleaves peptide bonds at the carboxyl side of **lysine** and **arginine** residues. - It is used for peptide fragmentation, not for determining the N-terminal amino acid. *Carboxypeptidase* - **Carboxypeptidases** are exopeptidases that cleave amino acids from the **C-terminal end** of a polypeptide chain. - They are used to identify the C-terminal amino acid, not the N-terminal. *Cyanogen bromide* - **Cyanogen bromide (CNBr)** is a chemical reagent that specifically cleaves peptide bonds on the C-terminal side of **methionine** residues. - It is used for specific peptide fragmentation and not for N-terminal sequencing.
Question 3: Which factor stabilizes the alpha-helical structure of proteins?
- A. Disulfide bonds
- B. Hydrophobic forces
- C. Ionic interactions
- D. Hydrogen bonds (Correct Answer)
Explanation: ***Hydrogen bonds*** - Hydrogen bonds form between the **carbonyl oxygen (C=O)** of one peptide bond and the **amide hydrogen (N-H)** of a peptide bond **four residues away** along the polypeptide backbone. - These regularly spaced **intramolecular hydrogen bonds** are the primary force maintaining the characteristic **3.6 residues per turn helical structure** and stability of the alpha-helix. - This represents the fundamental stabilizing force of **secondary protein structure**. *Disulfide bonds* - Disulfide bonds are **covalent linkages** between cysteine residues that primarily stabilize **tertiary and quaternary structures**. - They are not involved in the regular, repetitive backbone structure of an alpha-helix. *Hydrophobic forces* - Hydrophobic interactions arise from **nonpolar amino acid side chains** clustering together to avoid water. - These forces are critical for **tertiary structure** stabilization and protein core formation, not secondary structure. *Ionic interactions* - Ionic interactions (salt bridges) occur between **oppositely charged side chains** (e.g., lysine and aspartate). - They contribute to **tertiary and quaternary structure** stability but are not the primary force in alpha-helix formation.
Question 4: All are true regarding G protein Except?
- A. It has a trimeric structure
- B. Alpha subunit has GTPase activity
- C. It is activated by GTP binding
- D. Active conformation has all three subunits (Correct Answer)
Explanation: ***Active conformation has all three subunits*** - The **active conformation** of a G protein typically involves the dissociation of the **alpha subunit** from the **beta-gamma dimer** upon GTP binding. - The alpha subunit, now bound to GTP, and the beta-gamma dimer then activate downstream effector proteins independently; thus, the fully trimeric structure is not the active conformation. *It has a trimeric structure* - In its **inactive state**, a G protein is indeed a **heterotrimer** composed of alpha (α), beta (β), and gamma (γ) subunits, with GDP bound to the alpha subunit. - This **trimeric structure** is a defining characteristic of G proteins before activation by a G protein-coupled receptor. *Alpha subunit has GTPase activity* - The alpha subunit of a G protein possesses intrinsic **GTPase activity**, which is crucial for terminating signaling. - This activity allows it to **hydrolyze bound GTP back to GDP**, returning the G protein to its inactive state. *It is activated by GTP binding* - G proteins are activated when a **guanosine diphosphate (GDP)** molecule bound to the alpha subunit is exchanged for a **guanosine triphosphate (GTP)** molecule. - This **GTP binding** is triggered by the activation of a G protein-coupled receptor (GPCR) by its ligand, leading to conformational changes that facilitate nucleotide exchange.
Question 5: What type of bond is involved in the side chain linkage of proteoglycans?
- A. Covalent (Correct Answer)
- B. Hydrogen bond
- C. Electrostatic bond
- D. Van-der Waal's force
Explanation: ***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 6: 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 7: Which of the following statements regarding prion diseases is NOT true?
- A. Myoclonus found only in 10% cases (Correct Answer)
- B. They are caused by infectious proteins
- C. Brain biopsy is diagnostic
- D. Neurodegenerative disease
Explanation: ***Myoclonus found only in 10% cases*** - This statement is incorrect because **myoclonus (sudden, involuntary muscle jerks)** is a very common and prominent feature of **Creutzfeldt-Jakob disease (CJD)** and other prion diseases, occurring in more than 90% of cases as the disease progresses. - The prevalence of myoclonus is significantly higher than 10%, making this an inaccurate representation of the clinical presentation. *They are caused by infectious proteins* - This statement is true; prion diseases are unique neurodegenerative disorders caused by the misfolding and aggregation of a host-encoded protein called the **prion protein (PrP)**. - These rogue proteins, known as **prions (PrPSc)**, can induce normal PrP molecules to misfold, leading to a chain reaction of protein aggregation and neuronal damage [1]. *Brain biopsy is diagnostic* - This statement is true; **brain biopsy** and post-mortem examination are considered definitive diagnostic methods for prion diseases like CJD. - These procedures allow for the detection of **spongiform changes**, **neuronal loss**, and the presence of **PrPSc aggregates** in brain tissue [1]. *Neurodegenerative disease* - This statement is true; prion diseases are characterized by progressive **neurodegeneration**, meaning they cause irreversible damage and loss of neurons in the brain. - This leads to a decline in cognitive function, motor control, and ultimately, death.
Question 8: In response to changes in Ca2+ concentration, which of the following Ca2+ binding proteins can modify the activity of many enzymes & proteins?
- A. Collagen
- B. Calmodulin (Correct Answer)
- C. Kinesin
- D. Elastin
Explanation: ***Calmodulin*** - **Calmodulin** is a highly conserved, 148-amino acid protein with four **calcium-binding EF-hand motifs**. - Upon binding to **calcium ions (Ca2+)**, it undergoes a conformational change that enables it to interact with and regulate the activity of a wide variety of enzymes and proteins, including **kinases, phosphatases, and ion channels**, mediating many Ca2+-dependent cellular processes. *Collagen* - **Collagen** is a major structural protein in the extracellular matrix, providing **tensile strength** to tissues. - Its primary function is structural support, rather than acting as a calcium-sensing regulatory protein for enzyme activity. *Kinesin* - **Kinesin** is a **motor protein** involved in intracellular transport, moving cargo along microtubules. - While its activity can be modulated, it is not primarily known as a calcium-binding protein that directly regulates a broad range of enzymes in response to calcium concentration changes. *Elastin* - **Elastin** is a highly elastic protein found in connective tissue, allowing tissues to **recoil after stretching**. - Like collagen, its main role is structural, contributing to the elasticity of tissues, rather than signaling or enzyme regulation via calcium binding.
Question 9: What do chaperones assist in?
- A. Protein Cleavage
- B. Protein Degradation
- C. Protein Modification
- D. Protein Folding (Correct Answer)
Explanation: ***Protein Folding*** - **Chaperone proteins** bind to newly synthesized polypeptide chains and unfolded proteins, helping them achieve their **correct three-dimensional structure**. - They also prevent **misfolding** and **aggregation** of proteins, which can be detrimental to cellular function. *Protein Cleavage* - **Protein cleavage** involves the enzymatic hydrolysis of peptide bonds, often performed by **proteases**. - This process is not directly facilitated by chaperones; chaperones primarily function in structural maturation, not degradation or processing. *Protein Degradation* - **Protein degradation** is carried out by systems like the **ubiquitin-proteasome pathway** or lysosomes, which break down damaged or unwanted proteins. - While chaperones can triage misfolded proteins for degradation, they do not directly perform the degradation themselves. *Protein Modification* - **Protein modification** involves the covalent attachment of chemical groups (e.g., phosphorylation, glycosylation) or other molecules to a protein. - This process is performed by specific enzymes like kinases or glycosyltransferases; chaperones’ role is more structural than enzymatic modification.
Question 10: Which of the following is a polar amino acid?
- A. Tryptophan
- B. Serine (Correct Answer)
- C. Valine
- D. Tyrosine
Explanation: ***Serine*** - Serine is a **polar, uncharged amino acid** due to the presence of a **hydroxyl group (-OH)** in its side chain. - The hydroxyl group allows serine to participate in hydrogen bonding, making it hydrophilic and thus polar. - Among the polar amino acids, serine is **unambiguously classified** as polar with no aromatic characteristics. *Tryptophan* - Tryptophan is an **aromatic amino acid** with a large, nonpolar indole ring in its side chain. - While it contains a nitrogen atom, the overall bulk and structure of its side chain render it largely **nonpolar** or slightly amphipathic. *Valine* - Valine is a **branched-chain amino acid** with an isopropyl group in its side chain. - This side chain consists entirely of **hydrocarbon atoms**, making it distinctly **nonpolar** and hydrophobic. *Tyrosine* - Tyrosine is technically a **polar amino acid** due to its phenolic hydroxyl group that can participate in hydrogen bonding. - However, it is primarily classified as an **aromatic amino acid** due to its benzene ring structure. - In many classification schemes, tyrosine's **dual nature** (aromatic + polar) makes it less straightforward than serine, which is unambiguously polar. - In this context, **serine** is the clearest example of a polar amino acid without aromatic characteristics.
Want unlimited practice?
Get full access to all questions, explanations, and performance tracking.
Start For Free