Protein Structure and Function Practice Questions

Q: Which salivary protein is known to exhibit properties that may inhibit the transmission of human immunodeficiency virus (HIV)?

Salivary leukocyte proteinase inhibitor (SLPI). ***Salivary leukocyte proteinase inhibitor (SLPI)*** - **SLPI** is an **anti-inflammatory** and **anti-microbial protein** found in saliva and other mucosal secretions. - Research suggests **SLPI** can **inhibit HIV-1 replication** at various stages of the viral life cycle, including **blocking viral entry** into target cells. *Sialoperoxidase* - **Sialoperoxidase** is an enzyme involved in the **innate immune system** that catalyzes the oxidation of thiocyanate into hypothiocyanite, an antimicrobial agent. - While it has antimicrobial properties, its primary role is not directly linked to **HIV inhibition**. *Secretory IgA* - **Secretory IgA (sIgA)** is a primary **antibody** found in mucosal secretions that provides **immune protection** against pathogens by preventing their adherence and invasion. - While **sIgA** can bind to **HIV**, its effectiveness in **preventing HIV transmission** through mucosal surfaces is limited and debated in research, unlike the more direct antiviral action of SLPI. *Histidine-rich proteins* - **Histidine-rich proteins** (e.g., histatins) in saliva have **antifungal** and **antibacterial properties**, primarily by disrupting microbial cell membranes. - Their antimicrobial spectrum primarily targets fungi and bacteria, with no significant direct role or robust evidence of **HIV inhibition**.

Q: In a mutation, if valine is replaced by which of the following, would it not result in any change in the function of the protein?

Leucine. ***Leucine*** - Both valine and leucine are **hydrophobic, branched-chain amino acids** with similar chemical properties and side-chain structures. - Due to their chemical similarity, replacing valine with leucine is less likely to cause a significant change in **protein folding, stability, or function**, often behaving as a conservative substitution. *Proline* - Proline is unique due to its **cyclic side chain**, which introduces a rigid kink into the polypeptide backbone, significantly affecting protein secondary structure (e.g., disrupting alpha-helices). - Substituting valine with proline would cause a **major structural disruption** and likely alter protein function. *Glycine* - Glycine is the **smallest amino acid** with only a hydrogen atom as its side chain, making it very flexible. - Replacing valine (a bulkier, branched amino acid) with glycine would dramatically increase local flexibility and potentially create **empty space**, which could alter protein folding and function. *Aspartic acid* - Aspartic acid is a **negatively charged, acidic amino acid**, featuring a carboxyl group in its side chain. - Substituting valine (a nonpolar, hydrophobic amino acid) with aspartic acid would introduce a significant **charge and polarity difference**, drastically altering the protein's electrostatic interactions, solubility, and overall structure.

Q: Which of the following amino acids is suitably accommodated within the first turn of an alpha helix?

Alanine. ***Alanine*** - Alanine's **small, nonpolar side chain** (-CH3) makes it ideal for alpha-helix formation as it minimizes steric hindrance and fits well within the helix's compact structure. - Its intrinsic helical propensity is among the highest, promoting the formation of **hydrogen bonds** that stabilize the alpha helix. *Aspartic acid* - Aspartic acid has a **negatively charged side chain** (-CH2COO⁻) that can cause electrostatic repulsion within the helix, making it less favorable, especially at physiological pH. - The charge can also interfere with the formation of the crucial **hydrogen bonds** in the backbone, destabilizing the helix. *Tyrosine* - Tyrosine possesses a **large, bulky aromatic side chain** that can create significant steric hindrance within the tightly packed structure of an alpha helix. - The **hydroxyl group** on its side chain can potentially form hydrogen bonds, but its overall size and rigidity disfavor its inclusion in the initial turns of a helix. *Glycine* - Glycine has the **smallest side chain (a hydrogen atom)**, which gives it too much conformational flexibility, making it a **helix breaker**. - Its high flexibility allows for many conformations, making it difficult to maintain the rigid helical structure and form stable **hydrogen bonds**.

Question 1

Which salivary protein is known to exhibit properties that may inhibit the transmission of human immunodeficiency virus (HIV)?

Accepted Answer

Salivary leukocyte proteinase inhibitor (SLPI)

Answer

Sialoperoxidase

Answer

Secretory IgA

Answer

Histidine-rich proteins

Question 2

Crystallin protein aggregation and modification are key mechanisms in cataract formation across various species, including humans. Which of the following protein pairs are known to be associated with cataract development through aggregation or cross-linking?

Accepted Answer

β-crystallin aggregates and γ-crystallin complexes

Answer

γ-crystallin complexes and damaged lens fiber proteins

Answer

α-crystallin aggregates and β-crystallin aggregates

Answer

β-crystallin aggregates and damaged lens fiber proteins

Question 3

In a mutation, if valine is replaced by which of the following, would it not result in any change in the function of the protein?

Accepted Answer

Leucine

Answer

Proline

Answer

Aspartic acid

Answer

Glycine

Question 4

Which of the following statements about protein denaturation is true?

Accepted Answer

Disulfide bonds contribute to the tertiary structure stability of proteins

Answer

Enzyme active sites are rigid structures that cannot change conformation

Answer

All of the above

Answer

Protein denaturation is always irreversible under physiological conditions

Question 5

Which of the following amino acids is suitably accommodated within the first turn of an alpha helix?

Accepted Answer

Alanine

Answer

Tyrosine

Answer

Glycine

Answer

Aspartic acid

Protein Structure and Function — MCQs

Protein Structure and Function — MCQs

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