Protein Structure and Function — MCQs

Protein Structure and Function Indian Medical PG Practice Questions and MCQs

Question 391: Not a monomeric intermediate filament:

A. Tubulin (Correct Answer)
B. Desmin
C. Keratin
D. Vimentin

Explanation: ***Tubulin*** - **Tubulin** is the monomeric building block of **microtubules**, which are distinct from intermediate filaments. - Microtubules are involved in cell motility, intracellular transport, and maintaining cell shape, but they do not belong to the intermediate filament family. *Desmin* - **Desmin** is a type III **intermediate filament** that is primarily expressed in muscle cells. - It plays a crucial role in organizing the **sarcomeres** and providing structural integrity to muscle fibers. *Keratin* - **Keratin** is the most diverse family of **intermediate filaments** and is primarily found in epithelial cells. - It provides **mechanical strength** to cells and tissues, forming structures like hair, nails, and the outer layer of skin. *Vimentin* - **Vimentin** is a type III **intermediate filament** that is widely expressed in cells of **mesenchymal origin**, such as fibroblasts, endothelial cells, and leukocytes. - It contributes to cell shape, motility, and the integrity of the **cytoskeleton**.

Question 392: Which amino acid does not include post translational modification?

A. Selenocysteine (Correct Answer)
B. Hydroxy-lysine
C. γ-Carboxyglutamate
D. Hydroxy-proline

Explanation: ***Selenocysteine*** - **Selenocysteine** is considered the 21st proteinogenic amino acid, directly incorporated into proteins **during translation** via a specialized tRNA and UGA codon (usually a stop codon). - It uses a **co-translational mechanism** involving SECIS elements (Selenocysteine Insertion Sequence) in the mRNA. - It is **NOT** formed by post-translational modification of another amino acid. *Hydroxy-lysine* - **Hydroxy-lysine** is formed by **post-translational hydroxylation** of lysine residues in collagen. - This modification requires **lysyl hydroxylase** enzyme and **Vitamin C** as a cofactor. - Essential for collagen cross-linking and stability. *γ-Carboxyglutamate* - **γ-Carboxyglutamate** is formed by **post-translational carboxylation** of glutamate residues. - Found in clotting factors (II, VII, IX, X) and requires **Vitamin K** as a cofactor. - Critical for calcium binding and proper coagulation function. *Hydroxy-proline* - **Hydroxy-proline** is formed by **post-translational hydroxylation** of proline residues in collagen. - This modification requires **prolyl hydroxylase** enzyme and **Vitamin C** as a cofactor. - Essential for the thermal stability and structural integrity of collagen.

Question 393: Which component is NOT involved in collagen synthesis?

A. Ascorbic acid
B. Hydroxyproline
C. Glycine
D. Pyridoxal phosphate (Correct Answer)

Explanation: ***Pyridoxal phosphate*** - **Pyridoxal phosphate** (PLP) is the active form of **vitamin B6** and serves as a coenzyme in various metabolic reactions, particularly those involving **amino acid metabolism**, but it is **not directly involved in collagen synthesis**. - While it plays a role in general protein metabolism, it does not participate in the hydroxylation of proline and lysine or the formation of the collagen triple helix, which are the critical steps in collagen synthesis. *Ascorbic acid* - **Ascorbic acid** (vitamin C) is a crucial cofactor for **prolyl hydroxylase** and **lysyl hydroxylase**, enzymes essential for the hydroxylation of proline and lysine residues in collagen. - This hydroxylation is critical for forming stable **cross-links** and the proper folding of the collagen triple helix. - Deficiency leads to scurvy with defective collagen. *Hydroxyproline* - **Hydroxyproline** is a modified amino acid formed **during collagen synthesis** through post-translational hydroxylation of proline residues by prolyl hydroxylase. - It is an essential structural component of mature collagen, strengthening the **collagen triple helix** through additional hydrogen bonding. - Its presence is used as a marker of collagen synthesis and degradation. *Glycine* - **Glycine** is the most abundant amino acid in collagen, accounting for about one-third of its total amino acid content, occurring at every third position in the α-chain sequence (Gly-X-Y). - Its small size allows the close packing of the three α-chains, which is essential for the formation of the characteristic **collagen triple helix**.

Question 394: 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 395: Which enzyme is involved in the conversion of fibrinogen to fibrin, a key step in the blood clotting process?

A. Thrombin (Correct Answer)
B. Plasmin
C. Elastase
D. Collagenase

Explanation: ***Thrombin*** - **Thrombin** is a crucial serine protease in the coagulation cascade that catalyzes the conversion of soluble **fibrinogen** into insoluble **fibrin monomers**. - These fibrin monomers then polymerize to form a stable **fibrin mesh**, which is the structural basis of a blood clot, trapping red blood cells and platelets. *Plasmin* - **Plasmin** is an enzyme involved in **fibrinolysis**, the process of breaking down blood clots, rather than forming them. - It cleaves **fibrin** into soluble degradation products, helping to dissolve the clot once its purpose is served. *Elastase* - **Elastase** is a protease that breaks down **elastin**, a protein that provides elasticity to tissues. - It is primarily involved in tissue remodeling and inflammation and does not play a direct role in the conversion of fibrinogen to fibrin. *Collagenase* - **Collagenase** refers to enzymes that break down **collagen**, a major structural protein in connective tissues. - These enzymes are involved in tissue repair and remodeling but are not directly involved in the final steps of blood coagulation.

Question 396: A mutation in a protein alters its tertiary structure, making it more prone to aggregation. What type of mutation is most likely responsible for this alteration?

A. Nonsense mutation
B. Missense mutation causing substitution with a hydrophobic amino acid (Correct Answer)
C. Frameshift mutation leading to premature stop
D. Splicing mutation

Explanation: **Missense mutation causing substitution with a hydrophobic amino acid** - A **missense mutation** changes a single amino acid, and if this change introduces a **hydrophobic amino acid** into a region that was previously hydrophilic or vice versa, it can significantly alter the protein's folding and **tertiary structure**. - **Hydrophobic amino acids** tend to aggregate in aqueous environments to minimize contact with water, leading to protein misfolding and **aggregation**. *Nonsense mutation* - A **nonsense mutation** results in a **premature stop codon**, leading to a truncated, non-functional protein. - While this can lead to loss of function, it typically *prevents* the formation of a full-length protein capable of extensive misfolding and **aggregation** in the same manner as a protein with altered tertiary structure. *Frameshift mutation leading to premature stop* - A **frameshift mutation** alters the reading frame, usually leading to a completely different sequence of **amino acids** downstream and often an early **stop codon**. - Similar to **nonsense mutations**, this results in a severely truncated and likely non-functional protein, rather than a full-length protein with a subtly altered and aggregation-prone tertiary structure. *Splicing mutation* - A **splicing mutation** affects the removal of introns and ligation of exons during **mRNA processing**. - This can lead to the inclusion of introns or exclusion of exons, resulting in an altered protein sequence, but the direct cause of **aggregation** due to tertiary structure alteration is more specifically linked to a specific amino acid change from a missense mutation.

Question 397: What is the primary role of ceruloplasmin in copper metabolism?

A. It stores excess copper in the liver
B. It catalyzes the oxidation of ferrous to ferric iron (Correct Answer)
C. It facilitates the intestinal absorption of copper
D. It aids in the export of copper from cells

Explanation: ***It catalyzes the oxidation of ferrous to ferric iron*** - **Ceruloplasmin** is a multicopper ferroxidase enzyme that serves as the PRIMARY copper-carrying protein in plasma (contains ~95% of serum copper). - Its **principal enzymatic function** is oxidizing Fe²⁺ to Fe³⁺, which is essential for iron loading onto **transferrin** and iron mobilization from tissues. - This ferroxidase activity is critical for **iron homeostasis** and prevents iron-mediated oxidative damage. - In **aceruloplasminemia** (genetic deficiency of ceruloplasmin), patients develop iron accumulation in tissues, demonstrating the importance of this function. *It aids in the export of copper from cells* - Copper export from cells is primarily mediated by **ATP7A** (intestinal and peripheral cells) and **ATP7B** (hepatocytes), not ceruloplasmin. - **ATP7B** transports copper into bile for excretion; defects cause **Wilson's disease** with copper accumulation. - Ceruloplasmin is synthesized in the liver with incorporated copper but does not actively transport copper OUT of cells. *It stores excess copper in the liver* - Intracellular copper storage is the function of **metallothionein**, not ceruloplasmin. - Metallothionein binds excess copper within cells, providing protection against **copper toxicity**. - Ceruloplasmin is an extracellular plasma protein, not an intracellular storage protein. *It facilitates the intestinal absorption of copper* - Intestinal copper absorption is mediated by **copper transporter 1 (CTR1)** at the apical membrane. - **ATP7A** then transports copper across the basolateral membrane into the bloodstream. - Ceruloplasmin functions in plasma after copper has already been absorbed and released from intestinal cells.

Question 398: Which condition is associated with a defect in collagen synthesis?

A. Osteogenesis imperfecta (Correct Answer)
B. Marfan syndrome
C. Cystic fibrosis
D. Tay-Sachs disease

Explanation: ***Osteogenesis imperfecta*** - This condition is primarily caused by an inherited defect in the synthesis of **Type I collagen**, leading to fragile bones and frequent fractures. - The abnormal collagen structure compromises the integrity of bone matrix, sclera, and other connective tissues. *Marfan syndrome* - This is a genetic disorder affecting **fibrillin-1**, a glycoprotein essential for elastic fiber formation in connective tissue, not collagen synthesis directly. - It leads to problems in the heart, blood vessels, eyes, and skeleton due to weak connective tissues. *Cystic fibrosis* - This is an autosomal recessive disorder caused by a mutation in the **CFTR gene**, primarily affecting chloride ion transport and resulting in thick, viscous secretions. - It mainly impacts the lungs, pancreas, and other exocrine glands, with no direct link to collagen synthesis defects. *Tay-Sachs disease* - This is a rare, inherited neurodegenerative disorder caused by a deficiency of the enzyme **hexosaminidase A**, leading to the accumulation of GM2 gangliosides in nerve cells. - It affects the central nervous system, resulting in progressive neurological deterioration, and is unrelated to collagen.

Question 399: Which protein is not synthesized in the liver?

A. Immunoglobulins (Correct Answer)
B. Albumin
C. Acute phase proteins
D. Coagulation factors

Explanation: ***Immunoglobulins*** - **Immunoglobulins** (antibodies) are primarily synthesized and secreted by **plasma cells**, which are differentiated B lymphocytes, not hepatocytes in the liver. - While the liver plays a role in processing some immune components, it is not the site of immunoglobulin synthesis. *Acute phase proteins* - The liver is a major producer of **acute phase proteins** (e.g., C-reactive protein, fibrinogen) in response to inflammation. - These proteins are synthesized by **hepatocytes** to modulate the inflammatory response. *Albumin* - **Albumin** is the most abundant protein in human plasma and is exclusively synthesized by the **liver**. - It plays crucial roles in maintaining **oncotic pressure** and transporting various substances. *Coagulation factors* - Most **coagulation factors** (e.g., factors II, VII, IX, X, fibrinogen) are synthesized by the **liver**. - These proteins are essential for the complex process of **blood clotting**.

Question 400: What is the primary mechanism of action of alpha-1 antitrypsin?

A. inhibition of elastase
B. inhibition of neutrophil elastase (Correct Answer)
C. inhibition of proteases
D. inhibition of serine proteases

Explanation: ***inhibition of neutrophil elastase*** - Alpha-1 antitrypsin (A1AT) is a **serine protease inhibitor** whose **primary and most clinically significant target is neutrophil elastase**. - Neutrophil elastase is released by neutrophils during inflammation and can degrade **elastin** in alveolar walls, leading to emphysema. - In **A1AT deficiency**, the lack of inhibition of neutrophil elastase results in progressive destruction of lung tissue, causing **early-onset emphysema**. - This is the most specific and medically accurate answer regarding A1AT's primary mechanism. *inhibition of elastase* - While technically correct, this is too **non-specific** as there are multiple elastases in the body (pancreatic elastase, macrophage elastase, etc.). - The **clinically relevant** mechanism is specifically the inhibition of **neutrophil elastase**, which is responsible for lung pathology in A1AT deficiency. - Medical literature and teaching specifically emphasize neutrophil elastase as the primary target. *inhibition of serine proteases* - This is too **broad and general**, as A1AT does not inhibit all serine proteases. - While A1AT is indeed a serine protease inhibitor, it has **specific targets**, with neutrophil elastase being the primary one. - Other serine proteases like thrombin, trypsin, and many others are not significantly inhibited by A1AT. *inhibition of proteases* - This is the **most general and least specific** option. - A1AT is a specific inhibitor with defined targets, not a general protease inhibitor. - This option lacks the precision needed to describe A1AT's primary mechanism of action.

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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