Question

Given below are two statements, one is labelled as Assertion A and the other is labelled as Reason R
Assertion A : Although most of the animals cannot digest $\alpha$-keratin but clothes moth larvae can very easily do so.
Reason R : The clothes moth larvae has a very high concentration of mercaptans in their digestive tract that helps them to cleave disulfide bonds abundantly present in $\alpha$-keratin.
In light of the above statements, choose the correct answer from the options given below:

• A. Both A and R are true and R is the correct explanation of A
• B. Both A and R are true but R is NOT the correct explanation of A
• C. A is true but R is false
• D. A is false but R is true

Hint:

The chemical resistance of $\alpha$-keratin is due to covalent disulfide ($-\text{S-S}-$) bonds.
Reducing agents like mercaptans in the moth gut break these bonds, allowing proteolytic enzymes to digest the protein.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
$\alpha$-Keratin is an insoluble, fibrous structural protein found in hair, wool, feathers, and nails.
Its structure is stabilized by extensive hydrophobic interactions and covalent disulfide cross-links between cysteine residues, making it highly resistant to enzymatic digestion by most animals.

Step 2: Detailed Explanation:
Let us evaluate both statements:
1. Assertion A: Most animals, including humans, cannot digest $\alpha$-keratin because their digestive enzymes cannot break down the heavily cross-linked protein matrix.
However, the larvae of the clothes moth (Tineola bisselliella) feed on wool and hair and can digest $\alpha$-keratin. Thus, Assertion A is true.
2. Reason R: The digestive tract of the clothes moth larvae maintains a highly reducing environment containing high concentrations of thiol compounds (mercaptans or hydrogen sulfide).
This reducing environment cleaves the disulfide bonds ($-\text{S-S}-$) in $\alpha$-keratin, converting them into free sulfhydryl groups ($-\text{SH}$):
\[ \text{Keratin-S-S-Keratin} + 2\text{R-SH} \rightarrow 2\text{Keratin-SH} + \text{R-S-S-R} \]
Once these disulfide cross-links are broken, the fibrous structure of the keratin unwinds, exposing the peptide backbone to digestive proteases for complete hydrolysis. Thus, Reason R is true.
Since the presence of reducing mercaptans in the digestive tract explains the biochemical mechanism by which moth larvae digest otherwise indigestible keratin, the reason is the correct explanation for the assertion.

Step 3: Final Answer:
Both Assertion A and Reason R are true, and Reason R is the correct explanation of Assertion A, which corresponds to Option (A).