Fig. 3. Overall CuGE structure, CE15 subgroups, and relation to the classical α/β-hydrolase fold.

a Structure of the catalytic domain of CuGE (residues 79–458) with selected residues shown as sticks: the catalytic triad (S270, E293, and H404, black), disulfide bridges (C81–C116, C269–C405, C391–C377, green), and carbohydrates (cyan), indicating the location of N- and O-glycosylation sites (N104-NAG, S86-MAN, and T87-MAN). The break in strand β3 is caused by a proline residue (P169) in CuGE. b, c The two crystallographically independent molecules A and B from the dΔS270A:U^m4X complex colored by temperature factors to visualize flexibility. b Molecule A. c Ligand-bound molecule B. d Topology diagram for a classical α/β-hydrolase fold. The illustration is based on information in Ollis et al.²³, Carr et al.²⁴, and Dimitrou et al.²⁵. The location of the catalytic residues is shown (nucleophile serine in green, base histidine in blue, and acid glutamate in red). The α/β-hydrolase group A and B are distinguished by different configurations of the catalytic triad with the catalytic acid located either in the turn after β7 (A_A) or after β6 (A_B), respectively²⁵. e Topology diagram for CE15-A. I1-I3 indicate inserts observed in bacterial CE15-A structures^13–15. f Topology diagram for CE15-B. For clarity, only the major secondary structure elements have been included in the topology diagrams in e, f; structures from both CE15 subgroups comprise additional small secondary structure elements, including multiple helical elements inserted after the strands β5 and β7 (Supplementary Fig. 4).