1RTX:
reference: The crystal structure of synechocystis hemoglobin with a covalent heme linkage., Hoy JA, Kundu S, Trent JT 3rd, Ramaswamy S, Hargrove MS, J Biol Chem 2004 Apr 16;279(16):16535-42. Epub 2004 Jan 21. PMID: 14736872

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