22. Which one of the following amino acid substitutions is likely to cause the largest change in
protein conformation?
(A) Phe → Ile
(B) Ser → Thr
(C) Gln → Tyr
(D) Glu → Val

Amino acid substitutions impact protein conformation based on differences in size, charge, hydrophobicity, and secondary structure propensity. The substitution most likely to disrupt folding and stability involves the greatest physicochemical disparity. Among the options, Glu → Val causes the largest change due to switching from a large, charged, hydrophilic residue to a smaller, neutral, hydrophobic one.

Correct Answer

(D) Glu → Val
This substitution replaces glutamic acid (Glu), a negatively charged side chain that forms salt bridges and hydrogen bonds essential for protein structure, with valine (Val), a branched hydrophobic residue. Such a change drastically alters local electrostatics, packing density, and solvent interactions, often destabilizing the native fold as shown in structure-based energy calculations.

Why Glu → Val Has the Largest Impact

Proteins rely on side-chain properties for conformational stability. Glu’s carboxylate group (volume ~109 ų, hydrophilic, pKa ~4.3) participates in ionic interactions, while Val’s isopropyl group (volume ~117 ų but compact/branched, hydrophobic) prefers buried cores. This polarity reversal can shift energy landscapes significantly, with studies showing charged-to-hydrophobic swaps increase unfolding free energy barriers by up to 0.9 ln(k) units. In evolutionary terms, such distant replacements rarely occur without functional loss.

Option Analysis

• (A) Phe → Ile: Both are large hydrophobic residues (Phe aromatic ring, Ile branched chain). Substitutions here minimally affect core packing or conformation, as seen in tolerated evolutionary changes.

• (B) Ser → Thr: Similar small polar residues (Ser -CH2OH, Thr -CH(OH)CH3) with comparable hydrogen-bonding and beta-branching. They often substitute conservatively in BLOSUM matrices without structural shifts.

• (C) Gln → Tyr: Gln (polar uncharged amide) to Tyr (polar phenolic with aromatic ring). Shares hydrogen-bonding but adds bulk and pi-stacking; moderate impact, less disruptive than charge loss.

Key Properties Comparison