Crystal structure of MbCO taken from the Protein Data Bank (1A6G). The heme group and three residues are indicated explicitly. The heme iron is covalently bound to the protein via His93. His64 is thought to play a major role in the conformational substates of the protein. Secondary structure is color-coded: helix A, blue; helix B, red; helix C, purple; helix D, green; helix E, orange; helix F, brown; helix G, pink; helix H, gray. All loops are colored cyan.