The metal ion in the active site of the metalloprotein myoglobin is iron Fe , which is part of a heme group. The coordination number of the iron ion in myoglobin is 6, and its geometry is octahedral.In the coordination environment, four of the six coordination sites are occupied by nitrogen atoms from the porphyrin ring of the heme group, forming a planar square. The fifth coordination site is occupied by a nitrogen atom from a histidine residue called the proximal histidine in the protein chain. This histidine residue helps to anchor the heme group within the protein structure. The sixth coordination site is available for binding to a ligand, which is typically oxygen O2 in the case of myoglobin.The octahedral coordination geometry of the iron ion in myoglobin allows for the reversible binding of oxygen. When oxygen binds to the iron ion, it causes a slight change in the geometry, moving the iron ion into the plane of the porphyrin ring. This change in geometry is essential for the protein's function as an oxygen carrier, as it allows myoglobin to bind oxygen in the lungs and release it in the tissues where it is needed.The coordination environment also helps to prevent the iron ion from being oxidized from the ferrous Fe2+ state to the ferric Fe3+ state, which would render it unable to bind oxygen. The histidine residue and the surrounding protein structure help to protect the iron ion from oxidation, ensuring that myoglobin remains functional as an oxygen carrier.