3VPK:
reference: Crystal structure of 6-guanidinohexanoyl trypsin near the optimum pH reveals the acyl-enzyme intermediate to be deacylated., Masuda Y, Nitanai Y, Mizutani R, Noguchi S, Proteins. 2013 Mar;81(3):526-30. doi: 10.1002/prot.24206. Epub 2012 Dec 24. PMID: 23161653

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