1C5I:
reference: Hydrogen bonding and catalysis: a novel explanation for how a single amino acid substitution can change the pH optimum of a glycosidase., Joshi MD, Sidhu G, Pot I, Brayer GD, Withers SG, McIntosh LP, J Mol Biol 2000 May 26;299(1):255-79. PMID: 10860737

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