The metal center in the metalloprotein myoglobin is an iron Fe atom, which is part of a heme group. The coordination geometry of the iron atom in myoglobin is an octahedral geometry. In this geometry, the iron atom is coordinated to six ligands.Four of these ligands are nitrogen atoms from the porphyrin ring of the heme group, which form a square planar arrangement around the iron atom. The fifth ligand is a nitrogen atom from a histidine residue in the protein, which is coordinated to the iron atom from above the plane of the porphyrin ring. This histidine residue is often referred to as the proximal histidine.The sixth coordination site is where the oxygen molecule O2 binds to the iron atom. When oxygen is not bound, this site is typically occupied by a water molecule or is vacant. When oxygen binds to the iron atom, it does so at an angle, causing the iron atom to move slightly out of the plane of the porphyrin ring. This movement is transmitted through the proximal histidine to the protein, leading to subtle conformational changes that facilitate oxygen storage and release.The octahedral coordination geometry of the iron atom in myoglobin allows for the reversible binding of oxygen, which is crucial for its function as an oxygen storage protein. The geometry also helps to stabilize the iron atom in the Fe II state, which is necessary for oxygen binding. Additionally, the coordination of the proximal histidine residue to the iron atom helps to prevent the irreversible oxidation of the iron to the Fe III state, which would render the protein nonfunctional for oxygen storage and release.