Figure 3.

Inactive and active CAPN2/S1[m-calpain] 3D structures. Surface-type schematic 3D structures (cross-eyed view) of active (Ca²⁺- and calpastatin-bound) CAPN2/S1[m-calpain], using the PDB data 3BOW.⁴⁸ Oligopeptides, represented by the yellow surface + ball-and-stick inside, indicate calpastatin tightly bound to CAPN2/S1[m-calpain]; due to its soft structure, some parts of calpastatin are not visible. The active protease domain (CysPc) is formed by the fusion of the PC1 and PC2 subdomains upon the binding of one Ca²⁺ to each subdomain. Though not visible here, the active site exists deep inside at the site circled in black. None of the 10 Ca²⁺ is visible here.