Protein Structure and Function — MCQs
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Which of the following statements regarding sulfhydryl groups is false?
Fibrin is degraded by which enzyme?
Which of the following is a component of intermediate filaments?
Which one of the following can be a homologous substitution for isoleucine in a protein sequence?
What is the primary reason for the difference in the oxygen dissociation curves of myoglobin and hemoglobin?
Which of the following is present in cornea ?
Which of the following statements about protein structures is most accurate?
Addition of which Amino Acid will increase UV absorption
Fibrinopeptide A and fibrinopeptide B are acidic due to the presence of which amino acids in their structure?
What do chaperones assist in?
Protein Structure and Function Indian Medical PG Practice Questions and MCQs
Question 411: Which of the following statements regarding sulfhydryl groups is false?
- A. They are present in coenzyme A and lipoic acid
- B. They are not involved in reduction of peroxides (Correct Answer)
- C. They are present in cysteine
- D. They are present in Captopril and penicillamine
Explanation: ***They are not involved in reduction of peroxides*** - This statement is **false** because **sulfhydryl groups**, particularly in **glutathione**, play a crucial role in the **reduction of hydrogen peroxide** and organic peroxides, protecting cells from oxidative damage. - **Glutathione peroxidase**, an enzyme that contains selenium, uses glutathione (with its sulfhydryl group) to catalyze this reaction, converting peroxides into water or less harmful alcohols. *They are present in coenzyme A and lipoic acid* - This statement is **true**; **Coenzyme A** contains a terminal **sulfhydryl group (-SH)** that is essential for its role in transferring acyl groups in metabolic reactions, such as the **Krebs cycle** and fatty acid metabolism. - **Lipoic acid** also contains a **disulfide bond** that can be reduced to two sulfhydryl groups, enabling its function as a coenzyme in **pyruvate dehydrogenase** and alpha-ketoglutarate dehydrogenase complexes. *They are present in Captopril and penicillamine* - This statement is **true**; **Captopril** is an **ACE inhibitor** that contains a **sulfhydryl group**, which is critical for its inhibitory activity against **angiotensin-converting enzyme**. - **Penicillamine** is a chelating agent and immunosuppressant, also containing a **sulfhydryl group**, used in conditions like **Wilson's disease** and **rheumatoid arthritis**. *They are present in cysteine* - This statement is **true**; **Cysteine** is an **amino acid** uniquely characterized by its **sulfhydryl group (-SH)**, making it a key component in **protein structure** (forming disulfide bonds with other cysteine residues) and in the synthesis of **glutathione**. - The sulfhydryl group in cysteine is highly reactive and contributes to its **redox properties** and metal-binding capabilities.
Question 412: Fibrin is degraded by which enzyme?
- A. Fibrinogen
- B. Thrombin
- C. Plasmin (Correct Answer)
- D. None of the options
Explanation: ***Plasmin*** - Fibrin is cleaved and degraded by **plasmin**, an enzyme that plays a crucial role in the fibrinolytic pathway [1]. - Plasminogen is activated to plasmin by tissue plasminogen activator (tPA), leading to the dissolution of fibrin clots in the process [1]. *Fibrin* - Fibrin is the **end product of coagulation**, not an enzyme; it serves as a scaffold for blood clot formation. - It does not have the ability to degrade itself; rather, it is broken down by **plasmin**. *Thrombin* - Thrombin is primarily involved in the **conversion of fibrinogen to fibrin** but does not degrade fibrin. - It promotes clot formation and stabilization rather than their dissolution. *None* - This option suggests that fibrin is not degraded by any substance, which is incorrect. - Fibrin degradation is specifically mediated by **plasmin**, contradicting the assertion that no agents are involved. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Hemodynamic Disorders, Thromboembolic Disease, and Shock, pp. 130-132.
Question 413: Which of the following is a component of intermediate filaments?
- A. Tubulin
- B. Actin
- C. Myosin
- D. Keratin (Correct Answer)
Explanation: ***Keratin*** - **Keratin** is the primary protein component of **intermediate filaments** found in epithelial cells, providing structural integrity and mechanical support. - It forms a diverse family of fibrous proteins essential for cell and tissue toughness, particularly prominent in skin, hair, and nails. *Actin* - **Actin** is the main component of **microfilaments** (also known as actin filaments), which are involved in cell motility, muscle contraction, and maintaining cell shape. - It does not form intermediate filaments but rather forms dynamic, thin filaments that work with myosin for various cellular functions. *Tubulin* - **Tubulin** is the protein subunit that polymerizes to form **microtubules**, which are part of the cytoskeleton involved in intracellular transport, cell division, and ciliary/flagellar movement. - Microtubules are distinct from intermediate filaments in their structure, function, and protein composition. *Myosin* - **Myosin** is a motor protein that interacts with actin to facilitate muscle contraction and other forms of cell motility. - It is not a component of intermediate filaments but rather a key player in the assembly and function of **contractile fibers** within cells.
Question 414: Which one of the following can be a homologous substitution for isoleucine in a protein sequence?
- A. Methionine
- B. Aspartic acid
- C. Valine (Correct Answer)
- D. Arginine
Explanation: ***Valine*** - Valine is a **branched-chain amino acid** with a similar nonpolar aliphatic side chain to isoleucine, making it a common **homologous substitute** due to their similar size and chemical properties. - Due to their structural resemblance, valine can often substitute for isoleucine without significantly altering the protein's overall **structure or function**, especially if the substitution occurs in a less critical region. *Methionine* - Methionine contains a **sulfur atom** in its side chain, making it chemically distinct from isoleucine, which has an all-carbon aliphatic side chain. - While both are nonpolar, the presence of sulfur in methionine changes its **electronic properties** and **reactivity**, making it a less homologous substitution than valine. *Aspartic acid* - Aspartic acid is an **acidic amino acid** with a negatively charged carboxyl group in its side chain at physiological pH. - This charged and polar nature is fundamentally different from the **nonpolar, branched-chain aliphatic side chain** of isoleucine, making it a non-homologous substitution that would likely disrupt protein structure. *Arginine* - Arginine is a **basic amino acid** with a positively charged guanidinium group in its side chain at physiological pH. - Its large, charged, and polar side chain is completely different from the **small, nonpolar, branched-chain aliphatic side chain** of isoleucine, and such a substitution would almost certainly cause significant structural and functional changes in a protein.
Question 415: What is the primary reason for the difference in the oxygen dissociation curves of myoglobin and hemoglobin?
- A. Hemoglobin has lower oxygen affinity than myoglobin
- B. Hemoglobin can bind to 4 oxygen molecules
- C. Cooperative binding in hemoglobin (Correct Answer)
- D. Myoglobin has a high affinity for oxygen
Explanation: ***Cooperative binding in hemoglobin*** - **Cooperative binding** means that the binding of one oxygen molecule to hemoglobin increases its affinity for subsequent oxygen molecules, resulting in its characteristic **sigmoidal dissociation curve**. - This property allows hemoglobin to efficiently **load oxygen in the lungs** (high partial pressure) and **unload it in the tissues** (low partial pressure), a function not performed by myoglobin. *Hemoglobin can bind to 4 oxygen molecules* - While true that hemoglobin can bind to four oxygen molecules, this is a statement of its capacity, not the primary reason for the difference in the *shape* of the dissociation curves. - Myoglobin only binds to one oxygen molecule, which contributes to its hyperbolic curve, but the dynamic sigmoidal shape of hemoglobin's curve is due to cooperative binding. *Myoglobin has a high affinity for oxygen* - Myoglobin does indeed have a **higher oxygen affinity** than hemoglobin, especially at lower oxygen partial pressures, which is essential for its role in oxygen storage in muscle tissue. - However, its *constant* high affinity, combined with binding only one oxygen, leads to a **hyperbolic (not sigmoidal)** dissociation curve, which is distinct from hemoglobin's sigmoidal curve. *Hemoglobin has lower oxygen affinity than myoglobin* - This statement is generally true when comparing the overall affinity of hemoglobin to myoglobin, particularly at lower oxygen partial pressures in the tissues. - However, stating that hemoglobin has simply "lower affinity" doesn't explain the sophisticated, physiologically crucial sigmoidal shape of its dissociation curve, which is dictated by cooperative binding.
Question 416: Which of the following is present in cornea ?
- A. Hyaluronic acid
- B. Chondroitin sulfate (Correct Answer)
- C. Heparan sulfate
- D. Dermatan sulfate
Explanation: ***Chondroitin sulfate*** - **Chondroitin sulfate** is one of the two major **glycosaminoglycans (GAGs)** present in the **corneal stroma**, comprising approximately 35-45% of corneal GAGs. - Along with keratan sulfate, it maintains **collagen fibril spacing and organization**, which is critical for **corneal transparency**. - The regular arrangement of these GAGs between collagen fibrils prevents light scattering and maintains corneal clarity. *Hyaluronic acid* - **Hyaluronic acid** is abundant in the **vitreous humor** and **synovial fluid** but is present only in minimal amounts in the cornea. - Its primary functions are lubrication and hydration in other tissues, not structural support in the cornea. *Heparan sulfate* - **Heparan sulfate** is found in **basement membranes** throughout the body, including Descemet's membrane of the cornea. - However, it is not a major component of the **corneal stroma** where structural transparency is maintained. *Dermatan sulfate* - **Dermatan sulfate** is predominantly found in the **sclera**, **skin**, and **blood vessels**, not in significant amounts in the cornea. - The cornea specifically requires **keratan sulfate** and **chondroitin sulfate** for its unique optical properties.
Question 417: Which of the following statements about protein structures is most accurate?
- A. Secondary structure is stabilized by hydrogen bonds.
- B. Denaturation primarily affects secondary and tertiary structures, leaving the primary structure intact.
- C. The sequence of amino acids determines the secondary and tertiary structures of proteins. (Correct Answer)
- D. The three-dimensional structure of a protein is referred to as its tertiary structure.
Explanation: ***The sequence of amino acids determines the secondary and tertiary structures of proteins.*** - This represents **Anfinsen's principle**, the most fundamental concept in protein folding: the **primary structure (amino acid sequence) contains all the information necessary** to determine the final three-dimensional structure of a protein. - This was demonstrated by **Nobel Prize-winning experiments** showing that denatured proteins can spontaneously refold into their native structure based solely on their amino acid sequence. - This is the **foundational principle** from which all other structural concepts derive - the sequence dictates everything else about protein structure. *Secondary structure is stabilized by hydrogen bonds.* - While this statement is **factually correct**, it describes a *mechanism* of structural stabilization rather than the fundamental principle of protein structure determination. - Hydrogen bonds are **one type of interaction** that stabilizes already-formed secondary structures, but the formation pattern itself is determined by the amino acid sequence. *Denaturation primarily affects secondary and tertiary structures, leaving the primary structure intact.* - This statement is also **factually correct** and describes what happens during denaturation (loss of 3D structure while peptide bonds remain intact). - However, it describes a *consequence* or phenomenon rather than the fundamental organizing principle of protein structure. *The three-dimensional structure of a protein is referred to as its tertiary structure.* - This is a **correct definition** but merely terminology rather than a principle. - It defines what tertiary structure means but doesn't explain the underlying mechanism of how protein structures are determined.
Question 418: Addition of which Amino Acid will increase UV absorption
- A. Tryptophan (Correct Answer)
- B. Leucine
- C. Proline
- D. Arginine
Explanation: ***Tryptophan*** - **Tryptophan** contains an **indole ring** with a conjugated pi system responsible for strong **UV light absorption** at approximately **280 nm**. - Its unique aromatic structure allows it to absorb UV light, making it a key amino acid for protein quantification using **spectrophotometry**. *Leucine* - **Leucine** is an **aliphatic amino acid** with a non-polar side chain and lacks chromophores. - It does not significantly absorb UV light in the typical range used for protein analysis. *Proline* - **Proline** is an **imino acid** with a unique cyclic structure, but it lacks aromatic rings or conjugated double bonds. - It does not absorb UV light significantly at wavelengths above 230 nm. *Arginine* - **Arginine** is a **basic amino acid** with a guanidinium group, but this functional group does not contribute to UV absorption in the 280 nm range. - Its presence does not enhance the UV absorbance of proteins.
Question 419: Fibrinopeptide A and fibrinopeptide B are acidic due to the presence of which amino acids in their structure?
- A. Serine and threonine
- B. Glutamate and aspartate (Correct Answer)
- C. Histidine and lysine
- D. Glutamine and valine
Explanation: **Glutamate and aspartate** - Both **glutamate** and **aspartate** are **acidic amino acids** due to the presence of an extra carboxyl group in their side chains. - The release of these fibrinopeptides from **fibrinogen** by thrombin exposes sites for fibrin polymerization in the final stage of coagulation. *Serine and threonine* - **Serine** and **threonine** are **polar, uncharged amino acids**, meaning they do not significantly contribute to the overall acidic nature of a peptide. - Their side chains contain hydroxyl groups, making them candidates for phosphorylation, but not acidity. *Histidine and lysine* - **Histidine** and **lysine** are **basic amino acids** due to the presence of nitrogenous groups in their side chains that can accept protons, thus imparting positive charges. - Therefore, their presence would make a peptide more basic, not acidic. *Glutamine and valine* - **Glutamine** is a **polar, uncharged amino acid**, derived from glutamate but with an amide group replacing one of the carboxyl oxygens. - **Valine** is a **nonpolar, aliphatic amino acid**, which does not contribute to the acidic nature of a peptide.
Question 420: 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.
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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