3CIR:
reference: A threonine on the active site loop controls transition state formation in Escherichia coli respiratory complex II., Tomasiak TM, Maklashina E, Cecchini G, Iverson TM, J Biol Chem. 2008 May 30;283(22):15460-8. Epub 2008 Apr 2. PMID: 18385138

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