Protein Structure and Function Practice Questions

Q: Which amino acid among the following has significant UV absorption at 280 nm used in protein quantification?

Tyrosine. ***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

Q: Which protein is commonly referred to as the "master regulator of the cell cycle"?

TP53. ***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.

Q: All of the following are true about connexons, except which of the following?

Has five subunits. ***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.

Q: Haemoglobin, unlike myoglobin, demonstrates a unique characteristic in its oxygen binding.

Co-operative effect of combined O2. ***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.

Q: Which of the following is NOT a chromoprotein?

Serum albumin. ***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**.

Q: Which of the following protein molecules is responsible for cell-to-cell adhesion?

Cadherin. ***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

Q: Proteins targeted for destruction in eukaryotes are covalently linked to which of the following?

Ubiquitin. ***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.

Q: The α-helix and β-pleated sheet in proteins are examples of which level of protein structure?

Secondary structure. ***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**.

Q: Which amino acid has the maximum buffering capacity?

Histidine. ***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.

Q: Which amino acid contains a guanidinium group?

Arginine. ***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.

Question 1

Which amino acid among the following has significant UV absorption at 280 nm used in protein quantification?

Accepted Answer

Tyrosine

Answer

Alanine

Answer

Histidine

Answer

Arginine

Question 2

Which protein is commonly referred to as the "master regulator of the cell cycle"?

Accepted Answer

TP53

Answer

APC

Answer

Patched

Answer

RB

Question 3

All of the following are true about connexons, except which of the following?

Accepted Answer

Has five subunits

Answer

Gap junction unit

Answer

It is a protein

Answer

Charcot marie tooth disease

Question 4

Haemoglobin, unlike myoglobin, demonstrates a unique characteristic in its oxygen binding.

Accepted Answer

Co-operative effect of combined O2

Answer

Parabolic curve of oxygen association

Answer

Co-operative index of 81

Answer

Hill's coefficient of 1

Question 5

Which of the following is NOT a chromoprotein?

Accepted Answer

Serum albumin

Answer

Cytochrome

Answer

Myoglobin

Answer

Hemoglobin

Question 6

Which of the following protein molecules is responsible for cell-to-cell adhesion?

Accepted Answer

Cadherin

Answer

Laminin

Answer

Fibronectin

Answer

Collagen

Question 7

Proteins targeted for destruction in eukaryotes are covalently linked to which of the following?

Accepted Answer

Ubiquitin

Answer

Clathrin

Answer

Pepsin

Answer

Laminin

Question 8

The α-helix and β-pleated sheet in proteins are examples of which level of protein structure?

Accepted Answer

Secondary structure

Answer

Primary structure

Answer

Tertiary structure

Answer

Quaternary structure

Question 9

Which amino acid has the maximum buffering capacity?

Accepted Answer

Histidine

Answer

Cysteine

Answer

Tyrosine

Answer

Arginine

Question 10

Which amino acid contains a guanidinium group?

Accepted Answer

Arginine

Answer

Tyrosine

Answer

Histidine

Answer

Lysine

Protein Structure and Function — MCQs

Protein Structure and Function — MCQs

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