Protein Structure and Function — MCQs
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C-reactive protein is classified as which of the following?
Which of the following statements about collagen is incorrect?
Predominant type of immunoglobulin in human milk is:
What is the molecule that attaches to a protein to signal its degradation?
Which of the following is not classified as a molecular motor?
Which protein is characterized by a triple helix structure?
Which of the following amino acids is found in keratin?
Most abundant collagen in the body is
What percentage of total serum proteins do immunoglobulins typically represent?
Albumin is synthesized by:
Protein Structure and Function Indian Medical PG Practice Questions and MCQs
Question 471: C-reactive protein is classified as which of the following?
- A. Gamma globulin
- B. Beta-1 globulin (Correct Answer)
- C. Non-specific inflammatory protein
- D. Alpha-1 globulin
Explanation: ***Beta-1 globulin*** - **C-reactive protein (CRP)** is classified as an **acute-phase reactant** and migrates in the **beta globulin region** during serum protein electrophoresis. - CRP is a **pentameric protein** synthesized by the liver in response to inflammatory stimuli, particularly **IL-6**. - Its location in the beta fraction reflects its molecular characteristics and electrophoretic mobility. *Alpha-1 globulin* - The **alpha-1 globulin** fraction contains proteins such as **alpha-1 antitrypsin**, **alpha-1 acid glycoprotein (orosomucoid)**, and **alpha-1 fetoprotein**. - While these are also acute-phase proteins, CRP has different electrophoretic properties and migrates separately in the beta region. *Gamma globulin* - The **gamma globulin** fraction is predominantly composed of **immunoglobulins (antibodies)**, which are crucial for adaptive immunity. - CRP is part of the **innate immune response** and belongs to the pentraxin family, not the immunoglobulin family. - During severe acute inflammation, CRP levels can be so elevated that it may appear in the gamma region, but its standard classification is as a beta globulin. *Non-specific inflammatory protein* - While CRP is indeed a **non-specific marker of inflammation**, this option describes its **function** rather than its **electrophoretic classification**. - The correct classification based on serum protein electrophoresis mobility is **beta globulin**.
Question 472: Which of the following statements about collagen is incorrect?
- A. Collagen has a triple helix structure.
- B. Vitamin C is essential for collagen synthesis.
- C. Collagen contains glycine at every third position.
- D. Collagen primarily exhibits a beta-pleated structure. (Correct Answer)
Explanation: ***Collagen primarily exhibits a beta-pleated structure.*** - This statement is **INCORRECT** and is therefore the correct answer to this question. - Collagen primarily exhibits a **triple helix structure**, not a beta-pleated structure. - **Beta-pleated sheets** are characteristic of proteins like **keratin** and **fibroin**, not collagen. - The unique triple helical conformation gives collagen **high tensile strength** and is crucial for its function in connective tissues. *Collagen has a triple helix structure.* - This statement is **correct**, making it an incorrect answer choice. - The **triple helix structure** is a defining characteristic of collagen, formed by three polypeptide alpha-chains coiling around each other. - This structure is essential for collagen's **mechanical strength** and ability to form robust fibers in connective tissues. *Vitamin C is essential for collagen synthesis.* - This statement is **correct**, making it an incorrect answer choice. - **Vitamin C** (ascorbic acid) is a crucial **cofactor for prolyl hydroxylase and lysyl hydroxylase**, enzymes vital for hydroxylating proline and lysine residues in collagen. - Without adequate vitamin C, collagen cannot be properly cross-linked, leading to unstable collagen and conditions like **scurvy**. *Collagen contains glycine at every third position.* - This statement is **correct**, making it an incorrect answer choice. - The repetitive sequence **Gly-X-Y** (where X and Y are often proline and hydroxyproline) is characteristic of collagen. - **Glycine's small size** allows for the tight packing of the three alpha-chains in the triple helix, which is critical for its stability.
Question 473: Predominant type of immunoglobulin in human milk is:
- A. Ig A (Correct Answer)
- B. Ig G
- C. Ig M
- D. Ig D
Explanation: ***Ig A*** - **Secretory IgA (sIgA)** is the predominant immunoglobulin found in human milk, present in high concentrations. - **sIgA** provides crucial **passive immunity** to the infant's mucosal surfaces, protecting against infections in the gastrointestinal and respiratory tracts. *Ig G* - While present in human milk, **IgG** is not the predominant immunoglobulin type; its primary transfer to the fetus occurs **transplacentally**. - **IgG** in milk offers some systemic immunity but is quantitatively less significant than **sIgA** in mucosal protection via breastfeeding. *Ig M* - **IgM** is found in very low concentrations in human milk compared to **sIgA**. - Its role in passive immunity through breastfeeding is minimal, as **IgM** is primarily involved in the **primary immune response** within the infant's own immune system post-birth. *Ig D* - **IgD** is found in extremely low or negligible amounts in human milk. - Its main function is largely unexplained but is associated with **B cell activation** and is predominantly found on the surface of naive B cells, not in secretions.
Question 474: What is the molecule that attaches to a protein to signal its degradation?
- A. Ubiquitin (Correct Answer)
- B. RNAseF
- C. Zymase
- D. Chaperone
Explanation: ***Ubiquitin*** - **Ubiquitin** is a small regulatory protein found in most eukaryotic cells that attaches to target proteins, marking them for degradation via the **proteasome system**. - This process, known as **ubiquitination**, is crucial for regulating protein levels and cellular processes by removing damaged or unneeded proteins. *RNAseF* - **RNAseF** is not a generally recognized or standardized term in molecular biology for a molecule involved in protein degradation signaling. - **RNases** are typically enzymes that degrade RNA, not proteins. *Zymase* - **Zymase** is a complex of enzymes, primarily found in yeast, that catalyzes the fermentation of sugar into ethanol and carbon dioxide. - It is involved in metabolic pathways, not in signaling protein degradation. *Chaperone* - **Chaperone proteins** assist in the proper folding of other proteins, prevent aggregation, and help transport proteins within the cell. - They are involved in maintaining protein conformation and function, not in directly tagging proteins for degradation.
Question 475: Which of the following is not classified as a molecular motor?
- A. Kinesin
- B. Dynein
- C. Actin (Correct Answer)
- D. Myosin
Explanation: ***Actin*** - **Actin** is a globular multi-functional protein that forms **microfilaments**, which are a crucial component of the cytoskeleton in eukaryotic cells and play a vital role in cellular processes such as muscle contraction, cell motility, and cytokinesis. - While actin interacts with molecular motors like myosin to generate force and movement, actin itself is a **filamentous track** upon which motors move, not a motor protein. *Kinesin* - **Kinesin** is a motor protein that moves along **microtubules**, typically transporting cargo towards the **plus end** of the microtubule. - It uses **ATP hydrolysis** to power its movement, acting as a molecular motor in various cellular processes. *Dynein* - **Dynein** is another molecular motor protein that also moves along **microtubules**, but typically transports cargo towards the **minus end** of the microtubule. - It is crucial for processes like **ciliary and flagellar movement** and intracellular transport. *Myosin* - **Myosin** is a family of **motor proteins** best known for its role in muscle contraction, where it interacts with actin filaments. - It uses **ATP hydrolysis** to move along actin filaments, generating contractile force and movement.
Question 476: Which protein is characterized by a triple helix structure?
- A. Cystine
- B. Pectin
- C. DNA
- D. Collagen (Correct Answer)
Explanation: ***Collagen*** - **Collagen** is the most abundant protein in mammals and is uniquely characterized by its **triple helix structure**, formed by three polypeptide chains wound around each other. - This distinctive structure provides **high tensile strength** and flexibility, crucial for its role in connective tissues, skin, bones, and cartilage. *Cystine (an amino acid)* - **Cystine** is a dimer of two cysteine amino acids linked by a **disulfide bond**, not a triple helix structure. - As a single amino acid, it primarily serves as a building block for proteins and is involved in forming cross-links within protein structures. *Pectin (a plant polysaccharide)* - **Pectin** is a complex carbohydrate found in plant cell walls and is a **polysaccharide**, not a protein. - Its structure consists of a backbone of **galacturonic acid units**, often with side chains, and does not form a triple helix. *DNA (a nucleic acid)* - **DNA** (deoxyribonucleic acid) is a nucleic acid composed of two polynucleotide strands forming a **double helix structure**, not a triple helix. - The double helix is stabilized by **hydrogen bonds** between complementary base pairs.
Question 477: Which of the following amino acids is found in keratin?
- A. Lysine
- B. Histidine
- C. Arginine
- D. All of the options (Correct Answer)
Explanation: ***All of the options*** - **Keratin** is a fibrous structural protein that forms the main component of hair, skin, and nails - All three amino acids listed - **Histidine, Lysine, and Arginine** - are indeed found in keratin's composition - **Lysine** and **Arginine** are basic amino acids that contribute to keratin's structural stability and are involved in ionic interactions - **Histidine** is also present and plays a role in the protein's functional and structural aspects - While keratin is particularly rich in **cysteine** (which forms disulfide bonds responsible for its strength), it also contains significant amounts of these other amino acids - The complete amino acid composition of keratin includes all of these and many other amino acids working together to provide its characteristic properties *Why individual options alone are incomplete* - Selecting only **Histidine**, **Lysine**, or **Arginine** individually would be incorrect because it would imply the other amino acids are NOT found in keratin - Since the question asks which amino acid "is found" in keratin and all three ARE present, the correct answer must acknowledge all of them
Question 478: Most abundant collagen in the body is
- A. Type I (Correct Answer)
- B. Type II
- C. Type V
- D. Type VI
Explanation: ***Type I*** - **Type I collagen** is the most abundant type in the human body, constituting about 90% of the body's total collagen. - It is primarily found in **skin, tendons, ligaments, bone, dentin, and intervertebral discs**, providing mechanical strength and structural integrity. *Type II* - **Type II collagen** is the main collagen found in **cartilage**, especially hyaline and elastic cartilage. - It provides resistance to pressure and is crucial for the structure of the **intervertebral disc nucleus pulposus** and the **vitreous humor of the eye**. *Type V* - **Type V collagen** is a minor fibrillar collagen that associates with **type I collagen** to regulate fibril diameter and organization. - It is found in **cornea, bone, and interstitial matrices**, playing a role in tissue development and integrity. *Type VI* - **Type VI collagen** is a microfibrillar collagen that forms bead-like microfibrils and is found in most **interstitial tissues**. - It plays a significant role in anchoring other extracellular matrix components and is particularly abundant in the **basement membranes** of blood vessels and muscles.
Question 479: What percentage of total serum proteins do immunoglobulins typically represent?
- A. 15-20% (Correct Answer)
- B. 5-10%
- C. 10-15%
- D. 20-25%
Explanation: ***15-20%*** - Immunoglobulins (gamma globulins), also known as **antibodies**, typically constitute **15-20%** of total serum proteins in healthy adults. - This represents the combined contribution of all immunoglobulin classes (**IgG, IgA, IgM, IgD, and IgE**), with IgG being the most abundant. - This range is well-established in clinical biochemistry and represents normal adaptive immune function. *10-15%* - This range is at the lower end of normal and may be seen in some individuals. - However, it does not represent the typical or average range cited in standard biochemistry references. - Values in this range might suggest **hypogammaglobulinemia** if on the lower end. *5-10%* - This percentage is significantly too low for normal immunoglobulin representation in serum. - Such low levels would indicate **hypogammaglobulinemia** or **immunodeficiency disorders**. - While **albumin** comprises 55-60% of total protein, immunoglobulins are normally much higher than 5-10%. *20-25%* - This range is at the upper limit of normal and may indicate a **hypergammaglobulinemia**. - Such elevated levels might suggest conditions like chronic infections, **monoclonal gammopathy**, autoimmune diseases, or **multiple myeloma**. - While possible in disease states, this exceeds the typical physiological range.
Question 480: Albumin is synthesized by:
- A. Liver (Correct Answer)
- B. Kidney
- C. Muscle
- D. Spleen
Explanation: ***Liver*** - The **liver** is the primary organ responsible for the synthesis of **albumin**, a crucial plasma protein. - This synthesis occurs in the **hepatocytes**, the main functional cells of the liver. *Kidney* - The **kidneys** primarily function in filtering waste products from the blood and regulating fluid and electrolyte balance, not **protein synthesis**. - While they can excrete small amounts of albumin in disease states (e.g., **albuminuria**), they do not produce it. *Muscle* - **Muscle tissue** is mainly involved in movement through contraction and acts as a major site for **protein storage** and amino acid metabolism, but it does not synthesize albumin. - Muscle cells synthesize various structural and functional proteins for their own use, but not plasma proteins like albumin. *Spleen* - The **spleen** is a lymphatic organ involved in immune responses, filtering blood, and recycling old red blood cells. - It does not have a role in the synthesis of **albumin**; its primary protein synthesis is related to its immune functions (e.g., producing antibodies).
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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