Fig. 3.

NMR investigation of HdeA secondary structure upon pH variation. (a) Residue specific combined C^α and C^β chemical shift deviations from their respective random coil values (positive values indicates α-helices, negative values indicate β-sheets) at pHs ranging from 6.9 (red) to 3.1 (purple). Vertical grey bars indicate Asp or Glu that can be protonated in this pH range. Apart from these residues, combined C^α and C^β chemical shift deviations show only minor changes, suggesting that the previously observed increased H/D exchange upon lowering the pH is mainly due to a non-directly visible intermediate. (b) Residues disappearing (no longer observable) upon pH increase from 5.6 to 6.9 are shown in orange on the HdeA dimeric structure (left, right: two different views) (PDB ID: 1BG8) [20]. The affected residues cluster closely around Glu37 (side-chain is labeled), suggesting a key role for this residue in HdeA stabilization at mildly acidic pH.