Protein Structure and Function — MCQs
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Which amino acid among the following has significant UV absorption at 280 nm used in protein quantification?
Which protein is commonly referred to as the "master regulator of the cell cycle"?
All of the following are true about connexons, except which of the following?
Haemoglobin, unlike myoglobin, demonstrates a unique characteristic in its oxygen binding.
Which of the following is NOT a chromoprotein?
Which of the following protein molecules is responsible for cell-to-cell adhesion?
Proteins targeted for destruction in eukaryotes are covalently linked to which of the following?
The α-helix and β-pleated sheet in proteins are examples of which level of protein structure?
Which amino acid has the maximum buffering capacity?
Which amino acid contains a guanidinium group?
Protein Structure and Function Indian Medical PG Practice Questions and MCQs
Question 461: Which amino acid among the following has significant UV absorption at 280 nm used in protein quantification?
- A. Tyrosine (Correct Answer)
- B. Alanine
- C. Histidine
- D. Arginine
Explanation: ***Correct Option: Tyrosine*** - Tyrosine contains a **phenol functional group** (aromatic ring with hydroxyl group), giving it **significant UV absorption at 280 nm** (specifically ~274 nm) - Along with **tryptophan** and **phenylalanine**, tyrosine is one of the three aromatic amino acids used for **protein quantification via UV spectroscopy** - The aromatic side chain with conjugated double bonds enables strong UV light absorption *Incorrect Option: Alanine* - Alanine has a **methyl group** as its side chain (non-polar, aliphatic) - **Lacks aromatic rings** or conjugated systems - Does **not absorb UV light** at 280 nm *Incorrect Option: Histidine* - Histidine has an **imidazole ring** (heterocyclic aromatic) in its side chain - While technically aromatic, it has **minimal UV absorption at 280 nm** (weak absorption around 210-230 nm) - **Not used for protein quantification** at 280 nm due to insignificant absorption at this wavelength *Incorrect Option: Arginine* - Arginine contains a **guanidinium group** (highly basic, polar) - **Non-aromatic structure** without conjugated double bonds - Does **not exhibit UV absorption** at wavelengths used for protein analysis
Question 462: Which protein is commonly referred to as the "master regulator of the cell cycle"?
- A. APC
- B. Patched
- C. RB
- D. TP53 (Correct Answer)
Explanation: ***RB*** - The **retinoblastoma protein (RB)** is widely recognized as the "governor of proliferation" as it regulates the cell cycle [1]. - It acts by **inhibiting cell division** and promoting differentiation, particularly at the G1/S checkpoint [1][2]. *Patched* - This protein primarily functions in the **Hedgehog signaling pathway** and is involved in **tumor suppression** but is not known as the "governor of proliferation." - Its role is more about **cell growth regulation** in specific contexts rather than general proliferation control. *APC* - The **adenomatous polyposis coli (APC)** protein is crucial in the **Wnt signaling pathway** and is involved in controlling cell adhesion and proliferation but is not termed as the "governor of proliferation." - It primarily functions as a **tumor suppressor**, particularly in colorectal cancers, rather than a general regulator of proliferation. *TP53* - The **TP53 protein** is known as the "guardian of the genome," mainly involved in responding to DNA damage and regulating the cell cycle, but not specifically as the "governor of proliferation." - Its primary role is in **apoptosis** and **DNA repair**, rather than directly controlling cell proliferation like RB. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Neoplasia, pp. 301-302. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Neoplasia, pp. 300-301.
Question 463: All of the following are true about connexons, except which of the following?
- A. Gap junction unit
- B. It is a protein
- C. Has five subunits (Correct Answer)
- D. Charcot marie tooth disease
Explanation: ***Has five subunits*** - A **connexon** is formed from **six protein subunits** called **connexins**, not five. - These six connexins assemble to form a single functional connexon channel. *It is a protein* - A **connexon** is indeed a **protein complex** that forms the channels within **gap junctions**. - It consists of multiple protein subunits called connexins. *Gap junction unit* - A **connexon** is the fundamental **transmembrane channel unit** of a **gap junction**. - Two opposing connexons (one from each adjacent cell) align to form a complete gap junction channel. *Charcot marie tooth disease* - Mutations in certain **connexin genes** (e.g., **connexin 32**) are associated with neurodegenerative diseases like **Charcot-Marie-Tooth disease type 1X**. - These mutations impair the proper function of connexons in myelinated nerve fibers.
Question 464: Haemoglobin, unlike myoglobin, demonstrates a unique characteristic in its oxygen binding.
- A. Parabolic curve of oxygen association
- B. Co-operative index of 81
- C. Hill's coefficient of 1
- D. Co-operative effect of combined O2 (Correct Answer)
Explanation: ***Co-operative effect of combined O2*** - Haemoglobin exhibits **cooperative binding**, meaning the binding of one oxygen molecule to a heme group increases the affinity of the remaining heme groups for oxygen. This results in a **sigmoidal oxygen dissociation curve**. - This **cooperative binding** ensures efficient oxygen uptake in the lungs (high oxygen tension) and efficient oxygen release in the tissues (low oxygen tension). *Parabolic curve of oxygen association* - A **parabolic curve** typically describes processes with a squared relationship and is not characteristic of oxygen binding in haemoglobin. The actual curve for haemoglobin is **sigmoidal**. - This option does not accurately represent the unique binding kinetics of haemoglobin. *Co-operative index of 81* - While haemoglobin does show **cooperativity**, an index of "81" is not a standard or accurate measure for the cooperative effect in haemoglobin. - The **Hill coefficient** is used to quantify cooperativity, and for haemoglobin, it is typically around 2.8 to 3. *Hill's coefficient of 1* - A **Hill's coefficient of 1** indicates no cooperativity, meaning that the binding of one ligand does not affect the binding of subsequent ligands. - This is characteristic of **myoglobin**, which has only one binding site and thus a hyperbolic oxygen-binding curve, not haemoglobin.
Question 465: Which of the following is NOT a chromoprotein?
- A. Cytochrome
- B. Myoglobin
- C. Hemoglobin
- D. Serum albumin (Correct Answer)
Explanation: ***Serum albumin*** - Serum albumin is primarily a **transport protein** and a major contributor to **oncotic pressure** in the blood. - It does not contain a prosthetic colored group and is therefore **not classified as a chromoprotein**. - It is a **simple protein** composed only of amino acids without any non-protein prosthetic group. *Hemoglobin* - Hemoglobin is a **chromoprotein** because it contains a **heme prosthetic group** which is responsible for its red color and oxygen-binding capabilities. - It is found in red blood cells and is essential for **oxygen transport**. *Cytochrome* - Cytochromes are **chromoproteins** involved in the **electron transport chain**. - They contain **heme prosthetic groups** similar to hemoglobin, giving them characteristic absorption spectra. - They play a crucial role in **cellular respiration and ATP production**. *Myoglobin* - Myoglobin is a **chromoprotein** found in muscle tissue; like hemoglobin, it contains a **heme prosthetic group**. - This heme group gives myoglobin its characteristic reddish-brown color and allows it to **store oxygen in muscle cells**.
Question 466: Which of the following protein molecules is responsible for cell-to-cell adhesion?
- A. Laminin
- B. Fibronectin
- C. Collagen
- D. Cadherin (Correct Answer)
Explanation: ***Cadherin*** - **Cadherins** are transmembrane proteins that mediate **direct cell-to-cell adhesion** in a calcium-dependent manner - They form **adherens junctions** and **desmosomes**, which are essential for maintaining tissue integrity - Cadherins on adjacent cells bind to each other (**homophilic binding**), creating strong cell-cell connections - Critical for **embryonic development**, tissue architecture, and **epithelial barrier function** *Fibronectin* - **Fibronectin** is an extracellular matrix glycoprotein that mediates **cell-to-ECM adhesion**, not direct cell-to-cell adhesion - It binds to **integrins** on the cell surface, facilitating cell attachment to the extracellular matrix - Important for cell migration, wound healing, and embryonic development - Does not directly connect cells to each other *Collagen* - **Collagen** is the most abundant structural protein providing **tensile strength** to connective tissues - Primarily functions as **extracellular scaffolding**, not as an adhesion molecule - Provides mechanical support but does not mediate cell-cell adhesion *Laminin* - **Laminins** are major components of the **basal lamina** (basement membrane) - Mediate **cell-to-basal lamina adhesion** through integrin receptors - Important for cell differentiation, migration, and tissue organization - Function in cell-to-ECM adhesion, not cell-to-cell adhesion
Question 467: Proteins targeted for destruction in eukaryotes are covalently linked to which of the following?
- A. Clathrin
- B. Pepsin
- C. Laminin
- D. Ubiquitin (Correct Answer)
Explanation: ***Ubiquitin*** - **Ubiquitin** is a small regulatory protein that tags proteins for degradation via the **ubiquitin-proteasome system** in eukaryotes. - This process, known as **ubiquitination**, marks abnormal, misfolded, or short-lived proteins for destruction by the **proteasome**. *Clathrin* - **Clathrin** is a protein involved in the formation of vesicles for **endocytosis** and intracellular trafficking, not protein degradation. - It forms a triskelion structure that drives the budding of vesicles from the cell membrane or trans-Golgi network. *Pepsin* - **Pepsin** is a **digestive enzyme** found in the stomach that breaks down proteins into smaller peptides, primarily in the digestive tract. - It functions in a highly **acidic environment** and is not involved in intracellular protein tagging for degradation. *Laminin* - **Laminin** is a large glycoprotein that is a major component of the **basal lamina**, a part of the extracellular matrix. - It plays a crucial role in cell adhesion, migration, and differentiation, and is not involved in intracellular protein degradation.
Question 468: 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 469: Which amino acid has the maximum buffering capacity?
- A. Histidine (Correct Answer)
- B. Cysteine
- C. Tyrosine
- D. Arginine
Explanation: ***Histidine*** - Histidine possesses an **imidazole ring** in its side chain, which has a pKa of approximately 6.0. - This pKa value is close to the physiological pH (7.4), allowing histidine to effectively **donate and accept protons**, thus buffering against pH changes within biological systems. *Cysteine* - Cysteine contains a **thiol group (-SH)** in its side chain, with a pKa around 8.3. - While it can participate in buffering, its pKa is further from physiological pH, making its buffering capacity **less effective** than histidine at pH 7.4. *Tyrosine* - Tyrosine contains a **phenolic hydroxyl group (-OH)** in its side chain, which has a pKa of about 10.1. - This pKa is significantly **higher than physiological pH**, rendering tyrosine a poor buffer under normal physiological conditions. *Arginine* - Arginine has a **guanidinium group** in its side chain, which is highly basic with a pKa of approximately 12.5. - Due to its very high pKa, arginine is **fully protonated and positively charged** at physiological pH, meaning it primarily acts as a base and has limited buffering capacity in the physiological range.
Question 470: Which amino acid contains a guanidinium group?
- A. Arginine (Correct Answer)
- B. Tyrosine
- C. Histidine
- D. Lysine
Explanation: ***Arginine*** - Arginine is one of the 20 common amino acids and is unique for containing a **guanidinium group** in its side chain. - The guanidinium group is a highly basic functional group, which contributes to arginine's role as a **positively charged** amino acid at physiological pH. *Tyrosine* - Tyrosine contains a **phenolic hydroxyl group** in its side chain, making it an aromatic amino acid. - It does not contain a guanidinium group. *Histidine* - Histidine contains an **imidazole ring** in its side chain, which can be protonated or deprotonated near physiological pH. - This allows histidine to act as a **proton donor or acceptor** in enzymatic reactions, but it lacks a guanidinium group. *Lysine* - Lysine contains a **primary amine group** at the end of its long aliphatic side chain. - Like arginine, it is a positively charged amino acid, but its basicity comes from the amine group, not a guanidinium group.
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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