3KGQ:
reference: Structural and Functional Analysis of the Complex between Citrate and the Zinc Peptidase Carboxypeptidase A., Fernandez D, Boix E, Pallares I, Aviles FX, Vendrell J, Enzyme Res. 2011;2011:128676. doi: 10.4061/2011/128676. Epub 2011 Jul 25. PMID: 21804935

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