Amino Acids, Peptides, and Protein Structure (5D) Practice Test

A short coiled-coil protein mediates dimerization in a signaling complex. The heptad repeat places hydrophobic residues at positions that pack at the dimer interface, while charged residues are typically solvent-exposed. A single substitution introduces a charged residue at an interface position, and the dimer dissociation constant increases markedly in 150 mM NaCl. Which alteration to the amino acid sequence would most likely disrupt protein function by weakening coiled-coil association?

A soluble, globular protein contains a buried ion pair (Glu-Lys) that contributes to tertiary stability. In a physiological context of febrile temperature (40b0C), the wild-type protein remains folded. A mutant replaces the buried glutamate with glutamine (Eb2Q) without changing overall size or expression level. Differential scanning calorimetry shows a lower $T_m$ and increased aggregation propensity. Given the described structure, which interaction is most directly lost in the mutant, leading to reduced stability?