The metal center in the enzyme myoglobin is an iron Fe atom, which is part of a heme group. The coordination geometry of the iron atom in myoglobin is octahedral. In this geometry, the iron atom is coordinated to six ligands arranged at the vertices of an octahedron.Four of the ligands are nitrogen atoms from the porphyrin ring of the heme group, which form a square plane around the iron atom. The fifth ligand is a nitrogen atom from a histidine residue called the proximal histidine in the protein chain of myoglobin. The sixth coordination site is where the oxygen molecule O2 binds.The octahedral geometry plays a crucial role in the binding and release of oxygen in myoglobin. When the iron atom is in the ferrous state Fe2+ , it can bind to an oxygen molecule at the sixth coordination site. The binding of oxygen causes a small change in the geometry of the iron atom, pulling it slightly into the plane of the porphyrin ring. This change in geometry is stabilized by the surrounding protein structure, which helps to facilitate the binding of oxygen.When the oxygen is released, the iron atom returns to its original position, and the coordination site becomes available for another oxygen molecule to bind. This reversible change in geometry allows myoglobin to function as an oxygen storage protein, binding oxygen when it is abundant e.g., in the lungs and releasing it when it is needed e.g., in the muscles .In summary, the octahedral coordination geometry of the iron atom in myoglobin is essential for its function as an oxygen storage protein. The geometry allows for the reversible binding and release of oxygen, enabling myoglobin to supply oxygen to muscles during periods of high demand.