Structural elements of DsbAs and the catalytic site of MtbDsbA. (a) Structure-based sequence alignment of DsbAs. Structurally equivalent positions (upper case), variable regions (lower case) and insertions (dashes) are shown. PknE DsbA sequence alignment is based on MtbDsbA and BsDsbA (see Supplementary Fig. S6 for the PknE hypothetical model). Secondary-structure assignments for MtbDsbA (top green), EcDsbA (bottom blue) and topological variations originating at strand β1 in MtbDsbA and EcDsbA (orange) are presented. The catalytic motif and cis-Pro motif are highlighted in cyan and the equivalent EcDsbA residues involved in partner/substrate interaction are highlighted in orange. A negatively charged residue (Glu165) in the vicinity of the catalytic cysteine, a residue blocking the hydrophobic groove (Trp226) and a noncatalytic structural disulfide bond are marked with red arrows (see text for details). (b) The structure of the catalytic face of MtbDsbA. Residues forming the putative binding surface and negatively charged residues neighbouring the catalytic cysteine are shown. The inset displays the 2mFo− DFc electron-density map around the catalytic motif, the cis-Pro loop and ordered active-site water molecules (1.0σ contour level).