Figure 3.

Contribution of acidic residues to the pH-dependent binding stability of HdeA. In the binding stability curves, ΔΔG = 0 at the reference pH (pH_ref = 4). A decrease in ΔΔG from pH_ref to a given target pH represents destabilization of the dimer complex and thus favors the monomeric state. An increase in ΔΔG from pH_ref to a target pH stabilizes binding in the complex and thus favors the dimeric state. The black curve in the upper left corresponds to the binding stability profile with the contributions from all acidic residues in the dimer. Red and cyan curves denote acidic residues that destabilize and stabilize the dimer upon protonation at low pH, respectively. In the bottom right is a cartoon representation of the HdeA dimer crystal structure⁴⁷. In the structure, lines are drawn between residue pairs that are in close contact in the crystal structure and that involve an acidic amino acid. Red and cyan lines denote contacts involving acidic residues that destabilize and stabilize, respectively, the dimer at low pH. The location of the acidic residues that exhibit the largest destabilizing and stabilizing contributions are denoted by red (Asp20, Asp25, and Asp51) and cyan (Glu37) beads, respectively. For clarity, labels for these residues are only shown in one of the two monomers. Asp2, Asp8, and Asp88 are not resolved in the HdeA crystal structure, and thus were not considered in our calculations.