5CCP:
reference: Histidine 52 is a critical residue for rapid formation of cytochrome c peroxidase compound I., Erman JE, Vitello LB, Miller MA, Shaw A, Brown KA, Kraut J, Biochemistry 1993 Sep 21;32(37):9798-806. PMID: 8396972

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