Valine and glutamic acid differ in which property relevant to protein structure?

• A. Valine is hydrophilic; Glutamic acid is hydrophobic
• B. Valine is hydrophobic; Glutamic acid is hydrophilic
• C. Both are hydrophobic
• D. Both are hydrophilic

Answer: (B)

Proteins fold based on how amino acid side chains interact with water: nonpolar, hydrophobic groups tend to bury themselves inside the protein, while polar or charged groups interact with the solvent. Valine has a nonpolar, branched hydrocarbon side chain, making it hydrophobic. Glutamic acid has a carboxylate group that is negatively charged at physiological pH, making it hydrophilic. This difference helps valine to be part of the protein’s interior and glutamic acid to be on the surface where it can form interactions with water and other polar residues. So the property that differs is hydrophobicity/charge: valine is hydrophobic; glutamic acid is hydrophilic. The other descriptions conflict with the known polarity and charge of these side chains.