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

A 55-residue intracellular protein binds a small anionic metabolite. The binding site includes a Lys side chain that forms a key electrostatic interaction with the ligands carboxylate. At pH 10.5 the protein retains its folded structure by CD but shows markedly reduced binding. Which environmental interpretation best explains the reduced binding at high pH?

A 20-residue peptide is synthesized with the sequence Ac-Ala-Lys-Gly-Asp-Ser-Leu-Val-Phe-Gly-Lys-Leu-Ala-Asp-Gly-Ser-Val-Leu-Lys-NH$_2$. The peptide is designed to be net neutral near physiological pH due to N-terminal acetylation and C-terminal amidation. At pH 7.4, which alteration to the amino acid sequence would most likely increase the peptides net positive charge?