4Z1L:
reference: A Minimal beta-Lactone Fragment for Selective beta5c or beta5i Proteasome Inhibitors., Groll M, Korotkov VS, Huber EM, de Meijere A, Ludwig A, Angew Chem Int Ed Engl. 2015 May 14. doi: 10.1002/anie.201502931. PMID: 25973989

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