1QKS:
reference: The anatomy of a bifunctional enzyme: structural basis for reduction of oxygen to water and synthesis of nitric oxide by cytochrome cd1., Fulop V, Moir JW, Ferguson SJ, Hajdu J, Cell 1995 May 5;81(3):369-77. PMID: 7736589

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