3PBH:
reference: Crystal structure of the wild-type human procathepsin B at 2.5 A resolution reveals the native active site of a papain-like cysteine protease zymogen., Podobnik M, Kuhelj R, Turk V, Turk D, J Mol Biol. 1997 Sep 5;271(5):774-88. PMID: 9299326

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