As observed for MG-132, the ligand occupies only one active site in non-saturating conditions (Fig. ). The presence of the empty cavity is evident by Fourier difference map Fo-Fc, with reduced mobility in the ligand-bound subunit, again emerging by the values of the b-factors (Fig. ). The ligand occupies both sites when in excess (Fig. ) as it does in the reported X-ray structure (PDBid 6W63). However, also in this case, the b-factors of chain B are higher (Fig. and Fig. ).