2WUW:
reference: Crystallographic analysis of counterion effects on subtilisin enzymatic action in acetonitrile., Cianci M, Tomaszewski B, Helliwell JR, Halling PJ, J Am Chem Soc. 2010 Feb 24;132(7):2293-300. PMID: 20099851

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