Overall structure of the catalytic domain: SseI (137–322). (a) Overall fold shown as a cartoon diagram. Each of the eight helices (blue) and six β-strands (red) are numbered, with the catalytic triad Cys178, His216 and Asp231 shown as sticks in orange. The missing residues 264–266 are shown as a green line. (b) Topology of the catalytic domain. (c) Secondary-structure alignment with the sequence of the catalytic domain generated by PDBSum (Laskowski, 2009). Helices are numbered with an H and strands with a β. β-Turns are indicated by β, γ-turns by γ and β-hairpins by .