4GPG:
reference: Neutron and X-ray crystallographic analysis of Achromobacter protease I at pD 8.0: protonation states and hydration structure in the free-form., Ohnishi Y, Yamada T, Kurihara K, Tanaka I, Sakiyama F, Masaki T, Niimura N, Biochim Biophys Acta. 2013 Aug;1834(8):1642-7. doi: 10.1016/j.bbapap.2013.05.012., Epub 2013 May 25. PMID: 23714114

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