The calcium-binding sites and structural comparison around the axial ligands between the form 1 and form 2 structures. (a) The calcium-binding sites in the oxidized form 1 structure. Ca2+ ions are shown as green spheres and water molecules involved in calcium coordination are shown as red spheres. The C atoms of the neighbouring molecule in the crystal are coloured grey. 2Fo − Fc maps are shown as grey and pink meshes contoured at the 2.0σ and 10.0σ levels, respectively. (b) The protein surface around the haem propionate groups is shown as a space-filling model. Acidic residues (Asp and Glu) and basic residues (Arg, His and Lys) are shown in red and blue, respectively. Other residues are shown in white and the haem group is shown in yellow. Ca2+ ions are shown in green. (c) Structure comparison around the axial ligand His44 between the form 1 (blue) and form 2 (pink) structures of the oxidized form. Main-chain atoms are shown as tubes and side-chain atoms are depicted in stick representation. Ca2+ ions are shown as green spheres. (d) Structure comparison around the axial ligand His68 between the form 1 (blue) and form 2 (pink) structures of the oxidized form. The shifts of Val66 and Gly67 are shown as double-headed arrows. (c) and (d) show the superposition of the atoms in the porphyrin rings.