4CX9:
reference: Conformational control of the binding of diatomic gases to cytochrome c'., Manole A, Kekilli D, Svistunenko DA, Wilson MT, Dobbin PS, Hough MA, J Biol Inorg Chem. 2015 Mar 20. PMID: 25792378

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