Figure 10. The structures of the P. gingivalis holo- and apo-HmuY monomers.

In the apo-protein (yellow), the loop containing the axial heme ligand His¹⁶⁶ is no longer constrained by binding to the heme group and adopts a new position, rotating and reorienting the His¹⁶⁶ side chain externally toward the surface of the protein. This results in an opening up of the heme-binding pocket compared with the holo-protein (pink; PDB ID: 3H8T). A smaller shift away from the heme-binding site occurs for the β-strand containing the axial ligand His¹³⁴. This part of the structure is more rigid. The rms difference in the two aligned structures is 1.8 Å, calculated for 179 residues using the cealign command in PyMol (The PyMOL Molecular Graphics System, version 1.8 Schrödinger, LLC).