Question

pI value of an ionizable 'R'-group of an amino acid residue in a protein may vary from its original value
A. Due to interaction with other ionizable groups present in the surrounding
B. Due to the loss of charge of \(\alpha\)-amino and \(\alpha\)-carboxyl groups of amino acid residue
C. Due to the non-interactive nature of ionizable R - group
D. Due to the presence of ionizable R - group in peptide backbone
Choose the correct answer from the option given below:

• A. A & B only
• B. B & C only
• C. C & D only
• D. D & A only

Hint:

pKa Perturbation in Proteins: Caused by local electrostatic microenvironments (adjacent charges, dielectric changes) and neutralization of terminal $\alpha$-charges in peptide bonds.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The ionization constant ($pK_a$) and effective isoelectric behavior of amino acid side chains shift due to local electrostatic microenvironments and peptide bond incorporation.

Step 2: Detailed Explanation:
Factors shifting the $pK_a$ ionization of an amino acid residue within a folded protein:
1. A (True): Neighboring charged residues, hydrogen bonding networks, local dielectric constants, and hydrophobic pockets strongly alter the $pK_a$ of ionizable R-groups (e.g., catalytic dyad/triad shifts).
2. B (True): In a polypeptide chain, the $\alpha$-amino and $\alpha$-carboxyl groups lose their ionic charges by participating in neutral covalent peptide bonds, fundamentally altering the total electrostatic field surrounding the R-groups.
Therefore, statements A and B are the primary driving mechanisms.

Step 3: Final Answer:
Thus, the correct answer is A & B only, matching option (A).