3MD9:
reference: Two Stacked Heme Molecules in the Binding Pocket of the Periplasmic Heme-Binding Protein HmuT from Yersinia pestis., Mattle D, Zeltina A, Woo JS, Goetz BA, Locher KP, J Mol Biol. 2010 Oct 1. PMID: 20888343

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