4RKO:
reference: Why ser and not thr brokers catalysis in the trypsin fold., Pelc LA, Chen Z, Gohara DW, Vogt AD, Pozzi N, Di Cera E, Biochemistry. 2015 Feb 24;54(7):1457-64. doi: 10.1021/acs.biochem.5b00014. Epub, 2015 Feb 11. PMID: 25664608

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