FIGURE 9.

Charged residues whose protonations may trigger A-state formation. The charged residues with pK_a values of 2.5 ± 1.0 are shown in red in the three-dimensional structure of native PrP^C (Protein Data Bank entry 1AG2) (Asp¹⁴⁴, Asp¹⁴⁷, Glu¹⁵², and Glu¹⁹⁶). The salt bridges between these residues and neighboring residues are represented as green dotted lines (Asp¹⁴⁴–Arg¹⁴⁸, Asp¹⁴⁷–Arg¹⁵¹, and Arg¹⁵⁸–Glu¹⁹⁶). The S1-H1-S2 region is shown as a blue ribbon diagram, whereas the H2-H3 region is shown as a white surface diagram.