Figure 4. Structure of Heme a site in bCcO.

(A) shows the H-bonding network between the propionate groups of heme a and the amino acids/water surrounding it in fully oxidized bCcO (PDB: 3ABL). The red spheres indicate water molecules. (B) shows the comparison of heme a in the fully oxidized enzyme (PDB: 3ABL) and fully reduced enzyme (PDB: 2EIJ), which are highlighted in green and yellow, respectively. The dotted lines indicate the H-bonds present in the fully oxidized state; the red arrows indicate the structural changes induced by heme reduction. The structural alignment was made by superimposing the heme iron atoms. The structures were drawn with the PyMol Molecular Graphics System, Schrodinger, LLC.