Question

Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A): When an individual amino acid is part of a peptide or protein it is called amino acid residue. Reason (R): When an amino acid becomes part of a protein or peptide through peptide bond it loses a molecule of water and is called amino acid residue. In the light of the above statements, choose the most appropriate answer from the options given below:

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A) \
• B. Both (A) and (R) are correct but (R) is not the correct explanation of (A) \
• C. (A) is correct but (R) is not correct \
• D. (A) is not correct but (R) is correct

Hint:

Residue = Remainder. An amino acid loses water ($\text{H}_{2}\text{O}$) when joining a chain, so only a "residue" or remainder of the original molecule is left behind.

Answer 1

The Correct Option is A

Step 1: Concept A peptide bond forms through a condensation reaction joining the $\alpha$-carboxyl group of one amino acid to the $\alpha$-amino group of another amino acid.

Step 2: Meaning The term "residue" reflects what remains of the individual amino acid molecules after structural atoms are lost during the polymerization assembly process.

Step 3: Analysis * Assertion (A) is correct: Once bonded within a polypeptide chain, the individual building blocks are formally termed amino acid residues. * Reason (R) is correct: During the enzymatic creation of each peptide bond, a dehydration synthesis occurs where a hydroxyl group ($-\text{OH}$) is lost from the carboxyl terminal and a hydrogen proton ($-\text{H}$) is lost from the amino terminal, releasing a net molecule of water ($\text{H}_{2}\text{O}$). This chemical loss is the direct reason they are called "residues".

Step 4: Conclusion Both statements are accurate, and the structural mechanism described in (R) directly explains the naming convention outlined in (A). Final Answer: (A)