Mercury-soaked crystals of mature legumain diffracted to beyond 2.5 Å resolution. Super-activated legumain was crystallized in complex with the covalent legumain-specific inhibitor AAN-CMK in a tetragonal lattice. (a) Legumain crystals are about 100 × 100 × 100 µm in size. (b) Diffraction image taken at ID14-4 (ESRF). Resolution at the edge, 2.47 Å; resolution at the corner, 1.86 Å. The diffraction patterns of native and Hg-derivative crystals are isomorphous (<1%) and similar in diffraction quality, e.g. resolution and mosaicity.