Primary, secondary and tertiary structure of Ssp1. (a) The amino-acid sequence of Ssp1 with assigned elements of secondary structure. Helices are depicted as cylinders and β-strands as arrows. An alignment of Ssp1 and Ssp2 is also shown with strictly conserved residues shown on a black background. Residues involved in disulfide-bond formation are coloured yellow. (b) Cartoon representation of Ssp1 with secondary-structure elements labelled. The disulfide bond is shown in stick representation with C-atom positions in yellow and S-atom positions in gold. His133 and Cys50 (sticks) mark the active site.