(a) A stereo overlay of PGABN (green) and PdaA (light blue) showing the active site. Labels in black indicate the residues in PGABN. O atoms are coloured red and N atoms blue. The models were fitted by overlapping 128 Cα atoms of the conserved core residues. The zinc ion of PGABN is shown as a grey sphere. PdaA (PDB entry 1w17
) does not bind metal ions owing to the loss of the coordinating aspartate residue. Instead, Asn74 points away from the active site. Arg163 forms a salt bridge to Asp73, but the equivalent arginine in PGABN is Arg289, which sits on a different β-strand to Arg163 of PdaA. (b) Stereo overlay of the active sites of PGABN (green) and SlCE4 (brown). Both structures have a bound zinc ion with very similar coordination geometry. Other residues around the active site show low sequence conservation between the two structures, and Tyr117 of PGABN has no counterpart in SlCE4.