3HEP:
reference: The distal pocket histidine residue in horse heart myoglobin directs the O-binding mode of nitrite to the heme iron., Yi J, Heinecke J, Tan H, Ford PC, Richter-Addo GB, J Am Chem Soc. 2009 Dec 23;131(50):18119-28. PMID: 19924902

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