Secondary Structure of Proteins — MCQs
Most abundant collagen in the body is
Which of the following reagents would be most useful in determining the N-terminal amino acid of a polypeptide?
The α-helix and β-pleated sheet in proteins are examples of which level of protein structure?
What type of protein is keratin classified as?
In amyloidosis, beta pleated sheets can be directly visualized at the molecular level using:
Which of the following is a type of covalent bond?
What sequence on the template strand of DNA corresponds to the first amino acid inserted into a protein?
Which of the following is not a hemoprotein?
Which of the following statements about protein structures is most accurate?
Which type of bond is primarily responsible for the primary structure of a protein?
Secondary Structure of Proteins Indian Medical PG Practice Questions and MCQs
Question 1: 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 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: The α-helix and β-pleated sheet in proteins are examples of which level of protein structure?
- A. Primary structure
- B. Secondary structure (Correct Answer)
- C. Tertiary structure
- D. Quaternary structure
Explanation: ***Secondary structure*** - The **α-helix** and **β-pleated sheet** are formed by **hydrogen bonding** between the backbone atoms of amino acids within a polypeptide chain. - This level of structure describes the regular, recurring arrangements of **local regions** of the polypeptide backbone. *Primary structure* - This refers to the **linear sequence of amino acids** in a polypeptide chain, determined by the genetic code. - It does not involve the folding patterns of the polypeptide backbone but rather the order of its constituent monomers. *Tertiary structure* - This describes the **overall three-dimensional shape** of a single polypeptide chain, including the folding of helices and sheets and the arrangement of side chains. - It is stabilized by various interactions, including **hydrophobic interactions**, ionic bonds, hydrogen bonds, and disulfide bridges. *Quaternary structure* - This applies to proteins composed of **multiple polypeptide subunits**, describing how these subunits associate and are arranged in space. - It is established through interactions between different polypeptide chains, such as in **hemoglobin**.
Question 4: What type of protein is keratin classified as?
- A. Conjugated protein
- B. Globular protein
- C. Cylindrical protein
- D. Fibrous protein (Correct Answer)
Explanation: ***Fibrous protein*** - **Keratin** is a structural protein characterized by its **elongated, filament-like structure**, which is typical of fibrous proteins. - Fibrous proteins like keratin provide **mechanical strength** and play a significant role in the structure of tissues such as skin, hair, and nails. - Other examples of fibrous proteins include collagen, elastin, and myosin. *Globular protein* - **Globular proteins** have a **compact, spherical shape** and are often water-soluble, serving functions like enzymes, transporters, or receptors (e.g., hemoglobin or albumin). - Keratin's primary role is structural, not catalytic or transport, and its shape is not compact or spherical. *Cylindrical protein* - While some proteins might have a somewhat elongated or tube-like structure, **"cylindrical protein" is not a standard biochemical classification** of protein type. - This term does not accurately describe the characteristic fibrous nature and function of keratin. *Conjugated protein* - **Conjugated proteins** contain a non-protein component (prosthetic group) such as a carbohydrate, lipid, or metal ion attached to the protein (e.g., glycoproteins, lipoproteins, hemoglobin). - Keratin is a **simple fibrous protein** composed only of amino acids without prosthetic groups, so it is not classified as a conjugated protein.
Question 5: In amyloidosis, beta pleated sheets can be directly visualized at the molecular level using:
- A. Electron microscope
- B. X-ray crystallography (Correct Answer)
- C. Congo red stain
- D. Cryo-electron microscopy
Explanation: ***Congo red stain*** - Congo red stain is **specific** for detecting amyloid deposits, showing a characteristic **apple-green birefringence** under polarized light [1]. - The presence of **beta-pleated sheets** is a key feature of the amyloid fibrils that this stain highlights, confirming amyloidosis [1]. *Spiral electron microscope* - The **spiral electron microscope** is not a standard technique used for identifying amyloid structures or deposits. - It does not provide the **specificity** required to visualize amyloid-related beta-pleated sheets. *Electron microscope* - While electron microscopy can visualize amyloid fibrils [2], it does not specifically confirm the **beta-pleated sheet** structure like Congo red does. - This technique requires more complex preparations and does not have the same **ease of interpretation** for diagnosing amyloidosis. *X-ray crystallography* - X-ray crystallography is primarily used to determine the **three-dimensional** structure of crystalline substances, not specific to amyloid detection. - It does not provide direct evidence of **amyloid deposits** like Congo red staining does. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 268-269. [2] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. (Basic Pathology) introduces the student to key general principles of pathology, both as a medical science and as a clinical activity with a vital role in patient care. Part 2 (Disease Mechanisms) provides fundamental knowledge about the cellular and molecular processes involved in diseases, providing the rationale for their treatment. Part 3 (Systematic Pathology) deals in detail with specific diseases, with emphasis on the clinically important aspects., pp. 135-136.
Question 6: Which of the following is a type of covalent bond?
- A. Hydrogen bond
- B. Disulfide bond (Correct Answer)
- C. Ionic bond
- D. Electrostatic bond
Explanation: ***Correct: Disulfide bond*** - A **disulfide bond** is formed by the oxidation of two **thiol** (sulfhydryl) groups, creating a strong **covalent bond** between two sulfur atoms. - These bonds are crucial for stabilizing the **tertiary and quaternary structures of proteins**, contributing significantly to their overall shape and function. *Incorrect: Hydrogen bond* - A **hydrogen bond** is a **weak electrostatic attraction** between a hydrogen atom covalently bonded to a highly electronegative atom (like oxygen or nitrogen) and another electronegative atom. - It is an **intermolecular force** or a weak intramolecular force, not a covalent bond that involves the sharing of electrons. *Incorrect: Ionic bond* - An **ionic bond** is formed by the **complete transfer of electrons** from one atom to another, resulting in the formation of oppositely charged ions that attract each other. - This bond involves **electrostatic attraction** between ions, rather than the sharing of electrons characteristic of covalent bonds. *Incorrect: Electrostatic bond* - An **electrostatic bond** is a general term for the attractive force between oppositely charged particles, encompassing **ionic bonds** and other weaker interactions. - This term describes the **nature of the attraction** rather than the specific type of chemical bond (like covalent, which involves electron sharing).
Question 7: What sequence on the template strand of DNA corresponds to the first amino acid inserted into a protein?
- A. 3' TAC 5' (Correct Answer)
- B. 3' TAG 5'
- C. 3' TAA 5'
- D. 3' ATG 5'
Explanation: ***3' TAC 5'*** - The **start codon** for protein synthesis on **mRNA** is **5'-AUG-3'**, which codes for **methionine** (or N-formylmethionine in prokaryotes) and signals the initiation of translation. - To produce an mRNA codon of **5'-AUG-3'**, the complementary sequence on the **template DNA strand** must be **3'-TAC-5'** (adenine pairs with uracil/thymine, guanine pairs with cytosine, and the strands are antiparallel). - During transcription, RNA polymerase reads the template strand in the 3' to 5' direction and synthesizes mRNA in the 5' to 3' direction. *3' TAG 5'* - This template DNA sequence would be transcribed to produce the mRNA codon **5'-AUC-3'**, which codes for **isoleucine**, not methionine. - Therefore, this sequence does not correspond to the first amino acid inserted into a protein. *3' TAA 5'* - This template DNA sequence would be transcribed to produce the mRNA codon **5'-AUU-3'**, which also codes for **isoleucine**, not methionine. - This is not the initiation codon sequence. *3' ATG 5'* - While **ATG** appears in this sequence, when presented as the **template strand** in the 3' to 5' orientation, it would be transcribed to produce mRNA **5'-UAC-3'**, which codes for **tyrosine**, not methionine. - The sequence **ATG** on the **coding strand** (non-template strand) corresponds to the start codon, but this option incorrectly presents it as the template strand sequence.
Question 8: Which of the following is not a hemoprotein?
- A. Myoglobin
- B. Elastin (Correct Answer)
- C. Cytochrome P450
- D. Catalase
Explanation: ***Correct: Elastin*** - **Elastin** is a structural protein primarily found in **connective tissues** that provides elasticity to organs and tissues (skin, blood vessels, lungs) - It does NOT contain a **heme group** and is therefore not classified as a hemoprotein - Functions purely as a structural component without any prosthetic groups *Incorrect: Myoglobin* - **Myoglobin** is an iron- and oxygen-binding protein found in muscle tissue - Contains a single **heme group** with an iron atom, making it a quintessential hemoprotein - Functions in oxygen storage and delivery in muscle cells *Incorrect: Cytochrome P450* - **Cytochrome P450** enzymes are a superfamily of hemoproteins containing a **heme prosthetic group** - The heme iron is crucial for their role in **drug metabolism** and detoxification - Involved in metabolism of endogenous and exogenous compounds in the liver *Incorrect: Catalase* - **Catalase** is an enzyme that catalyzes decomposition of hydrogen peroxide (H₂O₂) into water and oxygen - Contains a **heme prosthetic group** with an **iron atom** essential for its enzymatic activity - One of the most efficient enzymes, protecting cells from oxidative damage
Question 9: 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 10: 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.
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