3IQB:
reference: Determinants of Ligand Affinity and Heme Reactivity in H-NOX Domains., Weinert EE, Plate L, Whited CA, Olea C Jr, Marletta MA, Angew Chem Int Ed Engl. 2009 Dec 16;49(4):720-723. PMID: 20017169

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