Definition of active site residues of caspases. A, Alignments of active site residues of caspases. The residues were identified by inspecting hydrogen bonding and van der Waals interactions in the X-ray structures of caspases (codes 1PAU, 1F1J, 1F9E, and 1JXQ) and using examples as described , and then assigning the particular subsites (Sx1-x2, where x2 is the position in the Sx1 subsite). B, Schematic representation showing the locations of the subsites on the active site with Ac-DEVD-CHO. The underlined subsite has a conserved residue except the S1–5(S/A) subsite.