4DCX:
reference: The structure of the periplasmic nickel-binding protein NikA provides insights for artificial metalloenzyme design., Cherrier MV, Girgenti E, Amara P, Iannello M, Marchi-Delapierre C, Fontecilla-Camps JC, Menage S, Cavazza C, J Biol Inorg Chem. 2012 Apr 21. PMID: 22526565

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