Polarity dictates amino acid placement in enzymes

Steps:

• Polar R-groups are hydrophilic and interact with water on the enzyme's surface.
• Non-polar R-groups are hydrophobic and stabilize the core by avoiding water.
• Histidine's polar imidazole group positions it externally for potential catalysis or binding.
• Proline's non-polar pyrrolidine ring buries it internally to maintain structure.

Why D is correct:

• Hydrophilic polar residues like histidine reside on the surface to solvate in aqueous environments, while hydrophobic non-polar residues like proline pack inside to minimize exposure per the hydrophobic effect.

Why the others are wrong:

• A: Reverses placements; polar histidine avoids the hydrophobic interior.
• B: Places non-polar proline on the surface, disrupting stability in water.
• C: Buries polar histidine inside, preventing favorable water interactions.

Final answer: D