(a) A sequence alignment based on the crystal structures of PGABN and SlCE4 (PDB entry 2cc0
). Conserved residues are shown in white on a red background. Red triangles show residues that coordinate the active-site zinc ion and blue triangles show nearby residues involved in catalysis. The residue numbering refers to PGABN. The first β-strand of PGABN (residues 48–55) matches the last strand of SlCE4, the sequence of which has been shifted from the C-terminus to the N-terminus in this alignment. His55 of PGABN matches His155 of SlCE4. The figure was produced by ESPript (Gouet et al., 1999). (b) Overlay of the Cα traces of the PgaB N-domain and SlCE4 (PDB entry 2cc0
), showing the helices and strands of both structures, but not coil. Alignment of the models was carried out with SSM (Krissinel & Henrick, 2004). Each structure is coloured from blue to red (N-terminus to C-terminus) as in (a), but circular permutation of the sequences leads to a marked difference in colouring, despite the close structural similarity. His55 of PGABN lies close to the active site, overlapping His155 of SlCE4, which is a conserved histidine in the CE4 family.