1SSX:
reference: The 0.83A Resolution Crystal Structure of alpha-Lytic Protease Reveals the Detailed Structure of the Active Site and Identifies a Source of Conformational Strain., Fuhrmann CN, Kelch BA, Ota N, Agard DA, J Mol Biol 2004 May 14;338(5):999-1013. PMID: 15111063

This OCA Structure page
uses Jmol, developed by the Jmol Development Team (documentation).